is Professor of Radiology at the University of Virginia. Dr. Dee received his medical degree from the University of Newcastle upon Tyne, England and is a Fellow of the Royal College of Radiologists, London. Present interests include research on po- rous bone cement and the viability of vas- cularized bone transplants. is a Clinical Associate Professor of Medi- cine at the University of Virginia. He re- ceived his medical degree from Emory University and his cardiology training at Stanford University. Until recently Dr. Martin was Director of the Noninvasive Cardiology Laboratory at the University of Virginia Hospital. Dr. Martin is now in private practice in Charlottesville. is Assistant Professor of Radiology at the University Hospital of Leiden, The Neth- erlands. He received his M.D. degree from Leiden University. During his residency at Leiden University Dr. Oudkerk completed his Ph.D. thesis, which appeared in No- vember 1981, entitled Infusion Rate in En- teroclysis Examination. He is presently en- gaged in research on aortic dissections and small bowel disease. 8
Transcript
is Professor of Radiology at the University of Virginia. Dr. Dee received his medical degree from the University of Newcastle upon Tyne, England and is a Fellow of the Royal College of Radiologists, London. Present interests include research on po- rous bone cement and the viability of vas- cularized bone transplants.
is a Clinical Associate Professor of Medi- cine at the University of Virginia. He re- ceived his medical degree from Emory University and his cardiology training at Stanford University. Until recently Dr. Martin was Director of the Noninvasive Cardiology Laboratory at the University of Virginia Hospital. Dr. Martin is now in private practice in Charlottesville.
is Assistant Professor of Radiology at the University Hospital of Leiden, The Neth- erlands. He received his M.D. degree from Leiden University. During his residency at Leiden University Dr. Oudkerk completed his Ph.D. thesis, which appeared in No- vember 1981, entitled Infusion Rate in En- teroclysis Examination. He is presently en- gaged in research on aortic dissections and small bowel disease.
8
is Assistant Professor of Radiology at the University Hospital of Leiden, The Neth- erlands. Dr. Overbosch received his medi- cal degree at the University of Leiden, where he also trained as a resident in di- agnostic radiology. His present interests include renovascular hypertension, sub- periosteal osteosarcoma, and selective liver perfusion.
THE INCIDENCE of acute aortic dissection in the United States has been estimated to be between 5 and 10 cases per million of the population per annum. ’ Thus, assuming the higher figure, up to 2,500 new cases might be anticipated annually in this country. Centers with thoracic surgical units will encounter a disproportionate number of patients with acute aortic dissection because of referrals from other hospitals. Acute aortic dissection may, however, be encountered in any hospital, and it is impor- tant for radiologists to be conversant with the full scope of in- vasive and noninvasive methods of investigation now available and the rationale for their use.
TERMINOLOGY
Aortic dissection is a condition characterized by irregular lon- gitudinal dissection by hemorrhage of the medial layer of the aortic wall. The process may or may not be accompanied by aneurysmal dilation of the aorta. The classic designation of this condition was dissecting aneurysm of the aorta. Generally, how- ever, the term aortic dissection is now preferred and will be used in this presentation.
Aortic dissection varies in its point of initiation and its longi- tudinal extent, and these features have a very considerable bearing on prognosis and treatment. Aortic dissections may be classified according to site of origin and extent. The DeBakey classification (Fig 1) has been the one most widely accepted.’ Type II aortic dissections, however, are small in number com- pared with the other two types. Mills et a1.,3 in an analysis of 74 cases of aortic dissection, found the relative incidences to be as follows: type I, 45%; type II, 14%; and type III, 42%. Further- more, type II aortic dissections are generally grouped with type I dissections for treatment purposes. Therefore, aortic dissec-
9
TYPE I TYPE II TYPEIU
Fig 1 .-The DeBakey classification of aortic dissections.
tions increasingly are being classified into two groups rather than three: proximal or type A dissections and distal or type B dissections. A proximal or type A aortic dissection is any dissec- tion that involves the ascending aorta antegradely or retro- gradely. A distal or type B dissection involves only the aorta distal to the origin of the arch vessels; the ascending aorta is left intact. This classification into proximal (type A) and distal (type B) aortic dissections has the advantage of being more geared to treatment and the newer imaging modalities and will be used in the rest of our discussion.
HISTORY
On the morning of October 25, 1760, the aged George II of England had retired to the closet stool after partaking of his usual cup of chocolate when his German valet de chambre heard noises from within, which he described with Teutonic precision as being “louder than the Royal wind.” Upon entering the cham- ber he found the king lying unconscious, and in spite of the min- istrations of his attendant physicians, the king died without re- gaining consciousness.4 The autopsy performed by Dr. Nichols revealed approximately one pint of blood in the pericardium and a 1%inch rent on the inner side of the ascending aorta, through which blood had passed to form a large ecchymosis under the external layers of the aorta.5 This was one of the first, if not the first, pathologic descriptions of acute aortic dissection.
Morgagni in 1761 cited the case of George II in a presentation of several additional cases of aortic dissection.6 Morgagni’s de- scription leaves no doubt that he recognized aortic dissection as a distinct mechanism of disease although possibly but a stage in the development of the more usual type of saccular aortic aneu- rysm. In 1802 Maunoir7 distinguished aortic dissection as a dis- tinct and separate entity, to which the term dissecting aneurysm was applied by Rem5 Laennec in 1826.8
In 1822 Shekelton of Dublin9 reported cases of chronic aortic 10
dissection in which points of reentry were found distally, and the concept of a continuous “double-barreled” aorta was clearly ap- preciated. Otto in 18241o reported the first case of aortic dissec- tion in a patient with coarctation of the aorta and a biscuspid valve, an important association. Parenthetically, it might be mentioned that it was not until 1943 that Taussig established the association of Marfan’s syndrome and aortic dissection.‘i Eliotson in 1830 addressed the question of the role of hyperten- sion in the development of aortic dissection-in fact, he tended to minimize the importance of hypertension.” The first contri- bution from the United States was by Pennock in 1839.13 He localized the plane of dissection to the medial coat and showed that injections of fluid into normal arteries resulted in dissection in the axis of the vessel. In the same year Rokitansky postulated that impaired nutrition of the inner layers of the aorta resulted in structural changes predisposing to aortic dissection.i4 In 1864 von Recklinghausen argued the importance of the breakdown of elastic tissue in the aorta and postulated that this was due to “molecular changes.“15 The importance of Erdheim’s cystic me- dial necrosisi was perhaps anticipated in theory.
In 1843 Peacock presented his first series of 19 cases, which increased to 38 cases by 1849 and to the astonishing number of 80 cases by 1863. 17, is Peacock not only gave a remarkably com- plete description of aortic dissection, he also carried out experi- mental work which demonstrated the tendency for reentry of in- jected fluids into the original vessel rather than external rupture. Peacock’s writings undoubtedly enabled Swaine and Latham in 1855 to make the first diagnosis during life.lg
A little known but important observation was made in 1910 by Babes and Mironescu, who noted that aortic dissection could occur in the absence of intimal rupture,” a finding confirmed in 1920 by Krukenberg.21 Krukenberg postulated rupture of the vasa vasorum in the aortic wall as the initial event in aortic dissection, the intimal rupture being secondary. Erb in 1905 had already developed an animal model for aortic dissection by epi- nephrine infusions in the rabbit.22 The effect of epinephrine on the vasa vasorum could have been the trigger in this model. Aortic dissection without intimal rupture was observed in 4% of 505 cases of aortic dissection studied by Hirst et a1.23
Nearly 200 years of clinical and pathologic documentation was analyzed in a monumental review of over 300 cases by Shennan in 1935.24 In this series the rate of diagnosis during life was a mere 1.5%.
THE MODERN ERA
Coincident with Sherman’s review came the first report, of sur- gical intervention in a case of aortic dissection. The operation
11
was an attempt to restore circulation to an ischemic lower ex- tremity and could in no way be described as radical surgical treatment of aortic dissection.25 The first published report of a definitive attempt to treat aortic dissection surgically was that of Johns in 1953.26 The patient underwent continuous suture of a leaking abdominal aneurysm, although the fact that it was aneurysmal as a result of aortic dissection was not actually rec- ognized at the time of operation. In 1955 Shaw reported an un- successful attempt to fashion a reentry.27 The milestone report, however, came that same year, when DeBakey, Cooley, and Creech described direct transthoracic treatment of aortic dissec- tion.28 Six patients were described, of whom four survived the surgery, which entailed elimination of the false aneurysm space and reestablishment of aorto-aortic continuity either directly or by means of a graft. The surgical approach outlined in that re- port was adopted with enthusiasm by surgeons, although admit- tedly with widely varying success. Perhaps it was some disillu- sionment with the surgical results that led Wheat, himself a surgeon, to develop nonsurgical treatment of selected cases of aortic dissection.2g Wheat’s treatment was derived from obser- vations on the prophylactic value of reserpine in preventing aor- tic dissection in turkeys (hitherto a significant cause of loss of stock in the industry). Wheat’s article of 1965 was as significant as the article of DeBakey et al. 10 years earlier.
MODERN TREATMENT OF ACUTE AORTIC DISSECTION
It is incumbent upon the radiologist to have a grasp of the present-day medical and surgical management of acute aortic dissection. Modern treatment of this condition demands not only confirmation of the diagnosis by radiologic techniques but also precise knowledge of the distribution and extent of involvement of the aorta.
A consensus of opinion regarding the management of acute aortic dissection has evolved from collective experience using the surgical approach of DeBakey et al. and the conservative approach of Wheat et al. Propagation of the aortic dissection is diminished by reduction of the arterial blood pressure and by reduction in the rate of rise of pressure with the cardiac impulse (dp/dt). To this end, treatment with hypotensive agents (e.g., ni- troprusside) and negative inotropic agents (e.g., propranolol) should be instituted immediately after a clinical diagnosis of aortic dissection is made. This diagnosis can in most instances be made on the basis of the history, the physical findings, the ECG, and the chest radiograph. If the hospital lacks facilities to manage these problems surgically the patient should be trans- ferred to one that does.
During the initial few hours following presentation the pa- 12
tient is stabilized medically and the diagnosis is confirmed by aortography or by noninvasive modalities such as computerized tomography (CT) and ultrasonography. It is crucial to manage- ment that a diagnosis of either proximal (type A) or distal (type B) aortic dissection is established definitively. In large part the management of the patient depends on the physician making this distinction with all reasonable certainty.
The majority of patients with proximal dissections involving the ascending aorta have operations that entail at least the in- sertion of an ascending aortic graft and obliteration of an adja- cent false lumen. Exceptions are made for patients with medical contraindications, such as anoxic brain death or unrelated ter- minal illness. This policy has resulted in a significant improve- ment in survival compared with the natural history of the un- treated condition. Hospital mortality of less than 25% has been reported in at least one series of patients treated surgically for acute proximal dissections.30 Sutton et al. report a 5-year sur- vival rate of over 40% in this grou~.~l This figure compares fa- vorably with the figures established by Hirst et al. for the nat- ural mortality, namely, 50% at 4 days, 75% at 2 weeks, and 90% at 3 months.
Dissections confined to the descending aorta, on the other hand, are treated by a continuation of the hypotensive, negative inotropic drug regimen in an intensive care unit. Operative intervention may be indicated if there is failure to control hypertension, continued pain, development of evidence of isch- emia in the distribution of a major vessel, development of neu- rologic deficit, or evidence of impending rupture. Eventually a number of patients with dissections involving the descending aorta will require surgery, usually grafting for aneurysmal di- lation of the aorta. Such surgery is associated with a much low- er operative mortality if it can be delayed until some fibrosis has developed and the tissues are less friable. However, Cooley and his group have advocated a more aggressive surgical ap- proach to distal dissections with medical treatment being ad- junctive.32
Medical treatment in the acute phase involves reducing the systolic blood pressure to loo-120 mm Hg as well as reducing the cardiac impulse (dp/dt max) with negative inotropic agents. Close monitoring of the ECG, blood pressure, pulmonary artery wedge pressure, pulmonary artery pressure, and urinary output is essential. The treatment protocol is summarized in Table 1. Chest radiographs are also essential to monitor progress, in par- ticular to observe the size of the aorta. Medical treatment is con- tinued on a long-term outpatient basis in all cases treated sur- gically or otherwise. Chest radiographs form an integral part of this follow-up process. Increasingly CT and ultrasound are also being used for this purpose.
13
TABL
E l.-
TREA
TMEN
T PR
OTOC
OL
FOR
ACUT
E AO
RTIC
DI
SSEC
TION
S
Diag
nosis
su
spec
ted
(clin
ical
hist
ory
and
findi
ngs,
EC
G, c
hest
rad
iogra
ph)
I 4 In
stitu
te
med
ical
treat
men
t de
signe
d to
red
uce:
(a
) ar
teria
l blo
od p
ress
ure
(b)
dp/d
t m
ax i
Defin
itive
radio
logic
and/
or
ultra
sono
grap
hic
stud
ies
TvDe
A
r( L
Type
B (p
roxim
aij
diss
ectio
ns
1
Surg
ical
treat
men
t t I
Failu
re
to c
ontro
l blo
od
pres
sure
En
largin
g ao
rta
Cont
inued
ble
eding
Pu
lmon
ary
arte
ry
obst
ruct
ion
Deve
loping
CN
S sig
ns
Com
prom
ise
of a
majo
r ao
rtic
bran
ch
Unco
ntro
llable
ch
est
pain
(dist
aljd
issec
tions
1
c Co
ntinu
ed
med
ical
treat
men
t
r(
L Lo
ng-te
rm
med
ical
treat
men
t /
THE CHEST RADIOGRAPH IN THE DIAGNOSIS OF ACUTE AORTIC DISSECTION
The standard chest radiograph still has a role in both the ini- tial diagnosis of aortic dissection and in follow-up after the event. Apart from the value of the chest radiograph in excluding other causes of acute chest pain, such as spontaneous esophageal rupture, one may obtain some indication that one is dealing with an aortic dissection from the radiograph. Salient indica- tions are listed below.
1. Alteration of the configuration of the aorta on successive radiographs. Increased tortuosity and slight ectasia of the aorta are not uncommon findings in the age group most susceptible to aortic dissection. However, if comparison with previous radio- graphs indicates that these changes have supervened more rap- idly than would be expected with aging, then in the appropriate clinical setting an aortic dissection is suggested (Fig 2).
2. Disparity in size between the ascending aorta and the de- scending aorta. Ordinarily the ascending aorta is wider than the descending aorta. However, in aortic dissection, particularly of the distal type, this size relationship may be reversed (Fig 3). It should be emphasized that this sign, like all the other plain film findings, must be interpreted in light of the clinical circum- stances.
3. Aortic undulations. The outer margin of the dissected aorta may show an undulant contour. This is usually best appreciated in the lateral projection (Fig 41, but the undulations may also produce a variety of double contour abnormalities around the aortic arch in the frontal projection (Fig 5).
4. Displacement of intimal calcification. Thickening of the aor- tic wall is a subsidiary angiographic sign of aortic dissection. The only possible marker of such thickening on plain films is displacement of calcified intimal plaques away from the outer margin of the aorta (Fig 6). This sign must be interpreted with caution, particularly in the arch region, on frontal radiographs. A calcified plaque seen over the aortic arch in this projection may in fact be positioned several centimeters anterior to the arch and give a spurious appearance of displacement away from the outer margin. On the other hand, a tangentially visualized plaque may not show any displacement if the irregularly spiral- ing dissection does not happen to involve the wall directly under the plaque. In only a distinct minority of cases is this sign of help. Eyler and Clark found plaque displacements of 6 mm or more in only 8 of 46 patients, even though conventional tomog- raphy was also used in many of the patients.33
5. Localized aneurysm formation. Aortic dissection is fre- quently associated with aneurysmal dilation of the aorta. Usu- ally this dilation involves a relatively long segment of the aorta,
15
Fig 2.-A, normal chest radiograph 2 years prior to admission. B, interval wid- ening of the aortic silhouette as a result of aortic dissection.
but on occasion focal bulges may develop and be apparent on the plain films. In Figure 7 a focal bulge in the region of the aortic isthmus is producing a deformity of the adjacent bronchial tree, while in Figure 8 a focal aneurysm of the aortic arch is seen.
6. Mediastinal, pleural, and lung changes. Changes secondary to leakage from the aorta may be seen in adjacent structures.
16
Fig Z-Frontal (A) and lateral (B) chest radiographs showing disproportionate enlargement of the descending aorta in a patient with a distal aortic dissection.
Fig 4.-Lateral chest radiograph showina an aortic dissection producing an un- dulat? co - ntour to the aorta.
Fig S-Frontal chest radiograph showing a widened aortic arch with a contour in a proximal aortic dissection.
Fig 6.-Detailed view of the lower descending aorta showing a calcified plaque kmall arrow) displaced away from the outer margin of the aorta (large arrow IS).
double
18
Fig 7.-A, baseline, normal radiograph showing region of the aortic isthmus in detail. 6, comparison view at the time of dissection showing a focal aneurysm dis- torting the trachea and left bronchus (arrows).
The changes include mediastinal widening (Fig 9), hemorrhage into the lung root (Fig lo), an apical pleural cap representing tracking of blood from the mediastinum, and pleural effusions, most commonly on the left side (Figs 11 and 12).
The chest radiograph may, however, be normal or may deviate only very sli
5.4 htly from normal in a patient with aortic dissec-
tion (Fig 13). Sutton et al., in their analysis of 113 patients
Fig E.-Large focal aortic arch aneurysm in a patient with a proximal aortic dis- section.
19
” 1 ‘,*Y i
Fig 9.-Upright chest radiograph in a patient with a known aortic dissection show- ing diffuse mediastinal widening.
Fig lO.-Central pulmonary shadowing, shown at autopsy to represent hemor- rhage into the lung roots in a patient with a proximal aortic dissection. The heart was enlarged as a result of long-standing severe hypertension.
20
Fig Il.-A patient with a proximal aortic dissection. Slight blunting costophrenic sulcus is seen.
Fig 12.4nterval development of a large left hemothorax in the same in Figure 11.
of the lefl
patient as
21
Fig 13.-A, normal chest radiograph prior to aortography. B, ascending aorto- gram demonstrating a proximal dissection. Arrows delineate the intimal flap.
with aortic dissection, found the chest radioyaph to be of diag- nostic assistance in less than half the cases.3 Patients with aor- tic dissection confined to the ascending aorta may have an en- tirely normal aortic arch and descending aorta (Fig 14).
The chest radiograph is of particular significance and impor- tance in the follow-up of patients, especially those being treated conservatively. Progressive enlargement of the aorta is a fre- quent indication for operative intervention in distal aortic dis- sections (Fig 15). The development and progression of medias- tinal, pleural, or lung changes may indicate continuing leakage and be a pointer to early surgery.
ANGIOGRAPHY IN THE DIAGNOSIS OF ACUTE AORTIC DISSECTION
Angiography is widely recognized as the “gold standard” in the diagnosis of acute aortic dissection, against which the new noninvasive modalities-CT and ultrasound-must be mea- sured. Statistical analyses of the accuracy of angiography are difficult to obtain. Ideally the diagnosis should be confirmed in all positive cases by surgery or autopsy and very stringent cri- teria established before cases are accepted as truly negative. Earnest et al., in an analysis of the angiographic findings in 52 patients, indicated that “there were no false positive and no false negative examinations in patients who subsequently un- derwent thoracotomy for other reasons or who died and under- went postmortem examinations.“35 Sutton et al., in a review of more than 100 angiograms obtained from patients with aortic dissection, found there were no false negative diagnoses but that three patients had been incorrectly classified as having a distal dissection instead of a proximal dissection.31 Other authors,36, 37
22
Fig 14.-A, normal aortic arch and descending aorta in a dissection confined to the ascending aorta. 6, ascending aottogram demonstrating the aortic dissection (arrows).
however, have dealt with the problems of diagnosing aortic dis- section, and one is intuitively opposed to the idea that any test, radiologic or otherwise, could be 100% accurate. Shuford et al., in an analysis of some of the problems of angiographic diagnosis in 44 patients with suspected aortic dissection, encountered ma- jor diagnostic problems in 10 of these patients. There were ap- parently 4 false negative studies, 2 false positive studies, and 4 equivocal or nondiagnostic studies.38 Keene et al., in a study of 10 patients with aortic root aneurysms, found that aortography had failed to diagnose dissection in any of the 4 patients in whom dissection was found at surgery.36
The angiographic diagnosis presents very little problem if an intimal flap separating the true and false lumina can be dem- onstrated (Fig 16). The diagnosis is then absolute, and it re- mains only to delineate the extent of the dissection. Earnest et al. indicated that an intimal flap may be visible in as many as 79% of studies.35 Of almost the same value is the demonstration of the entry or reentry points (Fig 17). Earnest et al. defined the intimal tear in 56% of their series, a remarkably high figure. The false lumen may be demonstrated without the intimal flap actually being profiled (Fig 18) and it is not uncommon that the true and false lumina are catheterized separately and selectively (Fig 19).
The angiographic signs described above allow a definitive di- agnosis of aortic dissection. There are also secondary signs of aortic dissection that permit correct diagnosis even when the false lumen completely fails to opacify, as occurred in 13% of the patients in the series of Earnest et al. Compression of the true lumen by the false lumen produces narrowing with angular
23
Fig 15.-A, normal chest radiograph obtained in 1966. B, radiograph obtained in 1972 shows distal aortic dissection. C, radiograph obtained in 1974 shows pro- gressive enlargement of the aortic arch and distal aorta.
zones of flattening of the aortic contour, sometimes likened to a twisted ribbon (Fig 20). The changes may be very extensive along the axis of the aorta and instantaneously recognizable. On occasion the change is localized and much more subtle (Fig 21). Thickening of the aortic wall may indicate the presence of a nonopacified false lumen (Fig 221, but this sign must be inter- preted with caution.3g Aortic thickening may also result from such causes as thrombus lining the aorta, aortitis, periaortic fat, and malignant infiltration. Aortic regurgitation is a common finding in dissection involving the ascending aorta and may be sufficiently severe to lead to congestive cardiac failure. Clearly, however, a finding of aortic regurgitation is by no means specific for aortic dissection. Occlusions of branch vessels from the aorta occur with some frequency and may lead to serious conse- quences. The false lumen may compress the intact intima, caus- ing the occlusion, or, alternatively, the intima may rupture at
24
Fig lb-Distal aortic dissection showing the lucency of the dissected intimal flap (arrows).
the origin of the vessel, allowing the false lumen to be the chan- nel of blood supply (Fig 23). Whatever the mechanism, one may commonly encounter nonfilling of the great vessels from the arch, the intercostal arteries, or the visceral arteries. The occlu- sion may also predate the dissection, as is suggested by the well- established collateral pathways in Figure 24. The occlusion may be more apparent than real, as is seen in Figure 25. Failure of the coronary arteries in an aortic root aneurysm to fill usually means that the injection was made into the false lumen. The coronary arteries rarely originate from the false lumen, and this may be an important pointer to the diagnosis, as seen in Figure 26. An unusual angiographic sign of aortic dissection is the pres- ence of ulcer-like projections from the aortic contour (Fig 27). These are variously thought to represent the partially throm- bosed sites of intimal tearing32P 38 or the sites of detachment and occlusion of vessels.40
Common causes of possible false negative examinations are simultaneous and equal opacification of the true and false lu-
25
Fig 17.-Arch aortogram showing contrast medium exiting from the true lumen into the false lumen (arrow).
mina, separation of the intimal flap in a plane that is not tan- gential to any of the beam directions used, acute external rup- ture with decompression of the false lumen, and failure of the false channel to opacify because of delayed flow or thrombus for- mation. When simultaneous and equal opacification of the true and false lumina occurs, the position of the catheter may yet indicate the presence of aortic dissection. During the pressure injection the catheter should recoil to the upper margin of the aortic arch; an intimal flap, though not visualized, may prevent this recoil.41 Visualization of the intimal flap itself is of course more likely if several projections are employed.
False positive examinations may be caused by preexisting branch occlusions, thickening of the aortic wall from other causes, and layering of contrast material on the dependent side of the descending aorta. The latter cause is particularly likely to occur in patients with aortic regurgitation and may simulate de- layed washout from a false channel.
Venous aortography as formerly practiced has no place in the diagnosis of aortic dissection, but a recent report of computer- enhanced intravenous angiography holds out some hope for the future.42 Contrast material injections are ordinarily made di- rectly into the aorta by the transfemoral or transbrachial routes, although success with transeptal left atria1 injections has been reported.43 Advantages have been claimed for the use of cinean- giography over conventional filming.44P 45 It is claimed that the
26
Fig l&-A and B, aortograms showing a distal aortic dissection with a “double- barreled” descending aorta.
examinations can be performed more expeditiously, that the morphology and function of the aortic valve are better assessed, and that the intimal flaps are more readily visualized, particu- larly in the region of the aortic root. Intimal flaps can be shown by cineangiography to have rapid movement in the aortic valve region, and flaps may even prolapse into the left ventricle in diastole (personal observation). It is further claimed that the en- try points are better defined because of the dynamic nature of the study. The authors of the articles cited claim that these ad- vantages more than offset the slight loss of resolution and field size involved. Theoretically at least, selective coronary angiog- raphy could also be performed fairly readily in certain cases us- ing cineangiographic equipment.
CT IN THE DIAGNOSIS OF ACUTE AORTIC DISSECTION
CT has been promoted in recent years as a reliable noninva- sive method of diagnosing aortic dissection. Developments in technology over the past few years have resulted in markedly
27
Fig 19.-A and B, aortography with separate injections into the true and false lumina.
28
Fig 20.-Arch aortogram deformity of the true lumen an aottic dissection. The fals not outlined.
showing the produced by ;e lumen \I was
improved images with relatively short scan times. Considering the reports of workers using modern scanners with scan times of 5 seconds or less, the results of studies of the value of CT in the diagnosis of aortic dissection are indeed encouraging. Mon- cada et al. studied 16 patients in whom CT and angiography were performed within a 24-hour period and found that CT was as accurate as angiography in the diagnosis and typing of aortic dissection.46 Codwin et al. studied 6 patients with aortic dissec- tion and found that CT not only diagnosed all dissections but also identified a proximal dissection which was only identified distally by angiography.4’ Lard6 et al. studied 14 patients with suspected aortic dissection and correctly diagnosed and typed the dissection in 7 of the 8 cases in which a dissection was pres- ent.48 The single false negative examination was attributed to failure to inject sufficient contrast material.
CT has also been used in very interesting work on patients who have been operated on for aortic dissection.4g, 5o In spite of the fact that attempts have ordinarily been made to obliterate the false lumen by oversewing in the region of the graft, these
29
Fig 21 .-Ascending aortogram showing slight effacement of the aorta produced by a localized dissection (arrows).
reports have shown that in the majority of cases, the false lumen remains open. The CT work by these groups confirms the find- ings of Guthaner et a1.51 using angiography. It appears that the open false lumen in these cases forms an essential conduit for blood perfusing major arteries, which have been separated from the true lumen. Such results from CT studies seem bound to have an impact on surgical techniques in the future.
CT scanning is capable of detecting aortic dissection without contrast enhancement, based on increased or decreased density of coagulating blood in the false lumen and displacement of cal- cified intimal plaques. However, contrast enhancement is nec- essary if CT is to yield the best results. The findings indicating aortic dissection on CT scans are:
1. Visualization of the dissected intimal flap. The axial nature of the scans and the high-density discrimination of CT is highly advantageous for the detection of intimal flaps. The intimal flap is usually imaged as a curvilinear lucency within the opacified aorta (Fig 28). In the arch it may take a more serpiginous out- line (Fig 29). Aortic dissections do not usually involve the aorta circumferentially along the length of the dissected segment, and
30
. . ..a . I
Fig PP.-Proved proximal aortic dissection showing diffuse aortic wal! thickening as the only sign of the dissection. Arrows indicate the outer margins of the aortic wall.
Fig 23.-A, normal appearance of aorta with side branch. 8, branch occlusion by extension of the dissection into the branch. C, branch occlusion by pressure from the expanding false lumen. 0, rupture of the intimal flap at the branch origin with perfusion of the branch from both lumina. E, rupture of the intimal flap at the branch origin with perfusion from the false lumen only. F, extension of the dissection along the branch with perfusion of the branch from both lumina.
C
‘\ \
e,
: \
\ \ I’
. ’ .-*
F
31
Fig 24.-A, acute aortic dissection with associated occlusion of the left subcla- vian artery. Note the well-developed collateral circulation. B, the left subclavian and vertebral arteries are outlined on the delayed film.
in cross section the outline of the dissected flaps may be complex (Fig 30). Frequently plaques of calcification can be identified on the flap (Fig 311, and it is these plaques that may indicate the position of the intima on unenhanced scans.
2. Demonstration of the true and false lumina. The false lu- men fills and empties in a delayed fashion compared with the true lumen, a finding that can be properly appreciated only on dynamic scans (Fig 32). The true lumen is to a variable extent compressed by the false lumen. In keeping with observations at angiography, the compression of the true lumen may on occasion be marked, the false lumen being considerably larger (Fig 33). Thrombosis within the false lumen is common and has been touted as a prognostically favorable sign.52 This has recently
Fig 25.-A, aortogram in aortic dissection outlining the true lumen. Note the ab- sence of filling of the renal arteries. B, renal function is well preserved, indicating continued adequate perfusion.
32
Fig 26.-Ascending aortogram in a proximal aortic dissection. Note the e of COI ‘onary artery filling-the injection was made into the false lumen.
Fig 27.-Abdominal aortogram showing an ulcer-like projection from the men L If an aortic dissection (arrow).
true
rice
Iu-
I -ig 28.-CT scan showing an intir ing aorta.
nal flap extending sagittally across the descend-
Fig 29.-CT scan through the top of the aortic arch
showing a serpiginous IUqncy representing an intimal: flap.
34
Fig 30.-CT scan demonstrating the complexity of certain irregularly spiraling dis- sections. This patient also has a partially thrombosed aneurysm of the aortic arch.
been questioned by Sanderson et a1.53 Be that as it may, throm- bosis may occur throughout the false lumen or only in part of it (Fig 34). Cle ar y, 1 complete thrombosis of the false lumen with- out markers such as intimal calcification could cause problems in distinguishing a dissection from an aneurysm lined by lami- nated thrombus. Such laminations are, however, rare in nonsac- cular aneurysms and the longitudinal extent of a dissection on
Flg 31.-CT scan demonstrating displacement of intimal calcification (arrow) into the lumen by an aortic dissection.
35
Fig 32.-Aottic dissection demonstrated by dynamic CT scanning. Interval be- tween scans was 4-5 seconds.
sequential scans should be a helpful clue. Careful study of a scan may show that some contrast is percolating into the false lumen even though it is largely thrombosed (Fig 35). As previously in- dicated, thrombus may have a different density from blood in the true lumen, allowing the dissection to be identified on unenhanced scans.
3. Increased diameter of the ascending or descending aorta. Aortic dissections very commonly become aneurysmal, and a
36
Fig 33.-CT scan showing marked compression of the true lumen (T) by the false lumen (F).
greater than normal disparity between the sizes of the ascending and descending aortas is of particular significance (Fig 36). Clearly, such findings can not be taken in isolation but may serve to draw attention to more subtle findings.
4. Evidence of leakage, localized aneurysm formation, and pressure on adjacent structures. Leakage from an aortic dissec-
Flg 34.-CT scan showing thrombus formation in a
, component of the false lumen. ! The false lumen remains patent
anteromedially.
37
Flg S.-Partial thrombosis in the false lumen demonstrated by CT scanning. Arrow indicates some contrast percolating into the false lumen.
tion may occur into the pericardium (Fig 371, the mediastinum (Fig 38), or the pleural space (Fig 39). Localized aneurysms may form (see Fig 30) and may be a particularly important feature on postoperative scans. Pressure effects may develop on adjacent structures such as the trachea (Fig 401, the superior vena cava (Fig 41), or bone.
CT does not produce the same range of diagnostic information
Fig 36.-A, unenhanced CT scan showing marked disparity in size between the ascending and descending aortas. 0, enhanced CT scan demonstrates the obliquely running intimal flap in the ascending aorta.
38
Fig 37.-CT scan demonstrates fluid (blood) in the pericardial sac in a patient with an aortic dissection.
Fig B&-Bleeding into the periaortic soft tissues demonstrated by CT scanning.
39
Fig 39.-A proximal aortic dissection associated with a right pleural effusion shown by CT scanning.
Fig 40.-CT scan showing deviation and narrowing of the trachea by an aneul mal aortic dissection.
as
W-
40
Fig 41 .-Severe compression of the superior vena cava (arrows) by an aneurys- mal ascending aortic dissection. The right pulmonary venous return was also com- promised between the aorta and the spine.
as angiography. Aortic regurgitation cannot be detected by CT, nor can CT be used to arrive at any estimate of the state of the coronary arteries. Parenthetically, it might be stated that con- ventional aortography gives at best only a very moderate delin- eation of the coronary arteries. Angiography provides some in- formation concerning occlusion of major vessels which CT can not possibly provide at present. Whether the information pro- vided by angiography is of practical clinical significance is a moot point. Occlusions of major vessels may predate the dissec- tion (see Fig 25), and on the other hand, one may completely fail to demonstrate major visceral vessels without untoward effects developing clinically in the vascular territory in question (see Fig 26). CT can be used to determine extension of aortic dissec- tions below the diaphragm and should be capable of visualizing the rare dissections that are confined to the abdominal aorta. CT can crudely estimate renal function, as indicated by Figure 42, although in practice, information on renal perfusion scarcely af- fects treatment. The postsurgical CT and angiographic studies already referred to4g-51 suggest that renal and other arteries may continue to perfuse from the false lumen in spite of the best efforts of surgeons to obliterate the false lumen.
CT cannot indicate the actual point of intimal tearing, even though the locus of origin of tears is fairly small and usually situated either close to the aortic valve or in the aortic isthmus
41
--- -- Fig 42.-CT scan of the upper abdomen showing compromised perfusion of the
left kidney in a patient with aortic dissection.
region. According to the data of Earnest et al., the site of tearin may be detected angiographically in no less then 56% of cases. dz The site of tearing is situated close to the point of impending rupture, and the essence of surgical treatment is to excise these two points with graft replacement. Conceivably, therefore, the ability of angiography to pinpoint the site of the tear might be construed as a definite advantage for angiography. However, 56% represents barely more than half of all cases and in any event does not influence the basic surgical approach. In the final analysis the clinician’s fundamental need is to distinguish prox- imal from distal dissections.
The small series of Godwin et a1.47 included one case in which CT indicated involvement of a portion of the ascending aorta whereas the angiogram demonstrated dissection only distal to the origin of the left subclavian artery. A somewhat similar case is illustrated in Figures 43 and 44. The involvement of the as- cending aorta in all likelihood represented retrograde dissection from a tear in the aortic isthmus. An excellent quality angio- gram in several projections failed to detect this. Another proba- ble example of retrograde dissection is illustrated in Figures 45 and 46. In this case a slight alteration in the mediastinal con- tours was noted on the chest radiograph of a patient being fol- lowed up for a known distal dissection. A CT scan showed clear evidence of involvement of the arch and the distal ascending aorta, a finding later confirmed surgically. Another example of
42
143.-Thoracic aortogram jnstrating a distal dissection. iding aorta appeared normal
Flg 44.-Same patient as Figure 43. The aortic dissec can be seen to involve the ascending aorta.
The
in tion
43
Fig 45-A and B, routine follow-up series of chest radiographs in a patient with a known distal aortic dissection. An aneurysmal dilation of the upper ascending aorta (arrow) developed between the two examinations.
the use of CT in follow-up is shown in Figure 47. In this case of catheter-induced dissection, several tracks can be seen to fuse into one and eventually resolve.
CT interpretation is not without its pitfalls, as has been em- phasized by Godwin et al. 54 Contrast enhancement is of course essential, and we concur with Godwin et al. about the value of dynamic scanning in aortic dissection. Possibly the most trou-
Fig 46.-Same patient as in Figure 45. A and B, CT scan demonstrates retro- grade involvement of the ascending aorta (arrows).
blesome pitfall is the streak artifact, but the character of the artifact, the lack of correlation with other sections, and inability to reproduce the artifact on repeat examination should provide the essential clues for recognition. Other pitfalls mentioned by Godwin et al.-enhancing pericardial and pleural thickenings and overlapping venous structures-were also encountered in this series. The relatively long scan times of 2 seconds and longer do not appear to impair visualization of the intimal flap, suggesting that the integrated movement of the flap is not marked, at least distal to the aortic root. Cineaortography of dis- sections involving the aortic root indicate, however, that these very Froximal intimal flaps may show considerable mobil- ity,44, 5 and the flap may even prolapse into the left ventricle in diastole.
Theoretically such flaps should be difficult to demonstrate with CT. Two-dimensional echocardiography (2DE) appears likely to be the appropriate noninvasive mode of investigation to complement CT in examination of the aortic root.
In summary, CT scanning is almost certainly as accurate as 45
angiography in the diagnosis and typing of aortic dissections. It suffers from the possible drawbacks of not being able to detect aortic regurgitation, the actual site of intimal tearing, or occlu- sions of major vessels. CT is able to detect evidence of bleeding into the pericardium, mediastinum, and the pleural space and can detect thrombosis of the false lumen. CT is particularly suit- able for short- and long-term follow-up of cases, whether treated medically or surgically.
TWO-DIMENSIONAL ECHOCARDIOGRAPHIC RECOGNITION OF AORTIC DISSECTIONS*
Improving the grave prognosis in aortic dissection depends on prompt diagnosis. Echocardiography has been applied to the rapid noninvasive diagnosis of aortic dissections. The first report of the use of single-dimensional or M-mode echocardiography in the diagnosis of an aortic root dissection was made in 1972 by Millward et a1.55 Subsequent to that initial description, multiple authors reported the use of M-mode echocardiograph
5K in the de-
tection of proximal aortic dissections. Nanda et al. developed M-mode criteria for the echocardiographic detection of aortic dis- sections, as follows: (1) enlargement of the proximal aortic root by 42 mm or more, (2) widening of the anterior aortic wall to greater than 16 mm at the proximal aortic root level and/or wide separation of the posterior aortic wall echoes, and (3) mainte- nance of a parallel motion between the separated walls. Follow- ing this report, other authors, including Brown et a1.,67 empha- sized the nonspecificity of these proposed M-mode criteria. It is not at all uncommon for the above-mentioned criteria to be met by patients without evidence of aortic dissection or aortic valve disease, and a pseudo-double lumen appearance of the aortic wall can be produced by visualization of the junction of the fi- brous aortic root and membranous ventricular septum or sinus of Valsalva walls.
Two-dimensional echocardiography has become a widespread noninvasive technique. A wide angle view allows tomographic interrogation of the cardiac structures and great vessels through many transducer positions in anatomical planes. Two-dimen- sional echocardiography remains a biologically safe, portable, noninvasive technique that can be readily applied to patients in critical care areas such as emergency rooms, coronary care units, and intensive care units. It is the only technique widely available today that permits complete tomographic interroga- tion of the heart. As such, the 2DE physician-operator can quickly assess valvular integrity and the size and function of ventricular chambers and can detect abnormalities of the peri-
*The section on 2DE was prepared by Dr. Martin. 46
cardium and of the proximal great vessels. By imaging the pa- tient from a parasternal long-axis transducer position, from the suprasternal notch, and from the subcostal area, the operator can visualize the aortic root, the ascending aorta, and the ante- rior portion of the aortic arch. The descending aorta cannot be consistently imaged, although the aorta can be picked up again in its abdominal portion. Multiple investigators have shown that 2DE is a sensitive technique for detecting proximal aortic root dissections.58-60 As these authors have noted, the ability of 2DE to tomographically interrogate the ascending aorta from a par- asternal long-axis position and the suprasternal notch often al- lows not only the detection of the proximal aortic root dilation but often the presence of an oscillating intimal flap. The report by Victor et al. of 42 patients referred for 2DE showed that the technique accurately detected a proximal aortic dissection in 12 of 15 patients in whom aortic dissection had been conflrmed.60 All 12 patients had an oscillating intimal flap and type A or proximal aortic dissections. The three false negative 2DE stud- ies in this report occurred in patients with type B or distal dis- sections.
Over a l-year period at the University of Virginia, we were able prospectively to detect five aortic dissections, all of the type A variety. In none of the five was the preechocardiographic di- agnosis considered to be an aortic dissection, based on the pa- tient’s presenting symptoms or clinical findings. As such, the 2DE technique was the first test that established the diagnosis, although it was performed to assess another clinical question. During this period, 21 additional patients were referred for echo- cardiographic evaluation specifically to detect or exclude a prox- imal aortic dissection. In none of these patients were the echo- cardiographic findings suggestive of a proximal aortic dissection, and no patient was later discovered to have aortic dissections. In all five patients in whom the diagnosis of type A dissection was made by 2DE, an oscillating intimal flap was clearly visu- alized. Two of the five patients were referred for the recent onset of aortic valve insufficiency thought possibly to be on the basis of bacterial endocarditis. In both of these patients a large inti- ma1 flap could easily be visualized (Fig 48). An additional pa- tient was imaged soon after admission from the emergency room in cardiogenic shock. The requesting physician had asked for a portable 2DE study to evaluate left ventricular function in this young patient. A classic type A aortic dissection could easily be visualized (Fig 49). This dissection had compromised the left main coronary artery osteum and resulted in an extensive myo- car-dial infarction. In each of these three patients not only did the 2DE technique establish the diagnosis, but the additional intbrmation provided by 2DE concerning valvular integrity and left ventricular size and function guided further therapeutic and
47
SYSTOLE DIASTOLE
Flg 48.-Parasternal long-axis views in systole and diastole demonstrating pro- lapse of intimal flaps (IF) through the aortic valve (AV) in diastole. LV, left ventricle, LA, left atrium; T, true aortic lumen; F, false lumen. Open white arrows on the sys- tolic view indicate an intimal flap.
diagnostic decisions. The final two patients also had unsuspected type A aortic dissections, and the 2DE studies provided crucial diagnostic information leading to the proper therapeutic inter- vention.
As with any new method, the limitations of the technique and the potential for false positive information must be emphasized. The major limitation of 2DE is its “artlike nature.” It is a tech- nique that is tremendously dependent on the skill of the physi- cian-technician performing the test and the skill of the physi- cian interpreting the results. If care is not taken to carefully interrogate the ascending aorta and to consider aortic dissection, the true diagnosis can easily be missed. The most common cause of a false positive diagnosis of aortic dissection by 2DE is a re- verberation artifact produced by the anterior aortic wall. When the ascending aorta is imaged from a high parasternal trans- ducer position, it is not uncommon, as the aorta begins to angle into the arch, to see a ringdown artifact within the aortic lumen. By visualizing the aorta from multiple transducer positions, in- cluding the suprasternal notch, the differentiation between a re- verberation artifact and a true intimal flap can usually be made. Additionally false positive diagnoses could be suspected in pa- tients who have Swan-Gantz catheters in the right ventricular outflow tract, unusual thickening of the aortic walls due to ath-
48
VA LVULAR SUPRAVALVULAR
Fig 49.--Short-axis views of the aortic root at valvular and supravalvular levels. The intimal flap separating the true aortic lumen (7) from the false lumen (F) is seen in the supravalvular position. N, noncoronary sinus; R, right coronary sinus; L, left coronary sinus. The left coronary artery was compromised, resulting in massive in- farction.
erosclerotic plaques or inflammatory arteritides (rheumatoid ar- thritis, lupus erythematosis, etc.), and dilation of the sinus of Valsalva. In our experience, the most common 2DE feature of a type A dissection is the actual visualization of the proximal aor- tic intimal flap and/or false lumen. This can often be seen throughout many transducer positions. Additionally, the aortic root is often dilated. Finally, there may be signs on the M-mode and 2DE studies of aortic insufficiency. The advent of cardiac Doppler, coupled with the anatomical information provided by 2DE, should make the diagnosis of a type A dissection even more accurate. Two-dimensional echocardiographic imaging of the ascending aorta allows accurate positioning of the Doppler sample cell. Differences in blood flow between the true and the false lumina provide additional supportive evidence for the di- agnosis of type A dissections, thereby further improving diag- nostic accuracy.
Two-dimensional echocardiography, which permits the tomo- graphic interrogation of cardiac and great vessel structures through many transducer planes, has proved to be a reliable technique in detecting type A aortic dissections. The most com- mon echocardiographic feature has been the visualization of an oscillating intimal flap. Problems in interpretation exist with re- verberation artifacts, and the lack of a suitable window makes
49
the diagnosis of type B dissections extremely problematic. In this connection, it should be mentioned that aortic dissections may be detected in the abdominal aorta (Fig 50). The presence of a normal aortic root in such a case could suggest the diagnosis of a type B dissection. However, aortic dissections may occasion- ally be confined to the abdominal aorta; clinical correlation would be essential in such instances.
Advances in 2DE instrumentation have led to small, high-res- olution, portable units. As such, 2DE can now readily be utilized in such critical care areas as the emergency room, the coronary care unit, and the intensive care unit. Patients who present with unusual physical findings and in whom the least suspicion of a type A aortic dissection exists can be quickly and easily studied by 2DE. Many investigators have now shown that 2DE is a sen- sitive and very specific technique for ruling in or ruling out the diagnosis of type A aortic dissection. As previously emphasized, problems do exist with the echocardiographic detection of a type B aortic dissection. Since a type A dissection remains a surgical emergency, the ability of 2DE accurately to detect or give strong indication of a type A aortic dissection can lead to the judicious rapid application of further diagnostic techniques, such as CT and aortography.
DIAGNOSTIC INVESTIGATIONS IN PATIENTS WITH SUSPECTED AORTIC DISSECTION
The foregoing presentation discussed the major advantages and drawbacks of the different modalities of investigation that are available for patients with suspected aortic dissection. It is clear that these modalities, which are expensive and time-con- suming and, in the case of angiography, invasive, must be used in a rational, goal-directed manner. The individual surgeon in particular, however, must be provided with all the information he or she requires before attempting surgical intervention. This information may vary from surgeon to surgeon. For example, one surgeon may not be interested in the angiographic delinea- tion of the infradiaphragmatic extent of an aortic dissection,
Fig 50.-Longitudinal (A) and cross-sectional (B) ultrasound images of the ab- dominal aorta showing an intimal flap.
whereas a colleague may deem this absolutely essential. The following discussion treats the algorithm proposed in Ta-
ble 2. The algorithm is based on the premise that any patient being evaluated for surgical intervention will require an angio- gram. Under certain circumstances patients are, in fact, being treated surgically on the basis of CT or ultrasonographic find- ings alone. The basic goal of the algorithm is to obviate the need for angiography in patients likely to be treated conservatively.
Few would dispute the need for a chest radiograph. Two-di- mensional echocardiography has the advantages of being speedy, noninvasive, and relatively cheap. The limitations of echocardiography have been dealt with and have been taken into account in the algorithm. The echocardiogram, it is pro- posed, could be used to triage patients to angiography or CT. The echocardiographic examination could be performed in the emergency room or other receiving station. If the echocardio- gram detects a suspected intimal flap, the patient is triaged to angiography. On the other hand, if the echocardiogram is nega- tive for aortic root dissection the patient is referred for a CT examination. There are then four possible outcomes. The first is that the patient is in fact found to have a proximal dissection, in which case the patient is rerouted to the angiography track, as is the patient with the second outcome, namely, an equivocal examination. The third outcome is a negative CT scan, which should terminate further investigation. This of course is based on acceptance of the idea that CT is as reliable as angiography in the actual diagnosis of aortic dissection. The fourth outcome is that the patient is found to have a distal dissection for which continued medical therapy but no further investigation is indi- cated. On occasion there may be indications for surgery in such a case, for example, a very large aneurysm or evidence of consid- erable leakage. In such a case angiography might be indicated as a prelude to surgery.
An important question arises in connection with performing angiography in close temporal sequence with a contrast-en- hanced CT scan: How does one manage the contrast doses? We handle this by allotting one third of the total permissible con- trast dose to the CT examination. The total dose is determined on the basis of the patient’s body weight, hydration, and esti- mated renal function. Usually about 100 ml of contrast medium is available for the scan, which suffices if attention is concen- trated on the strictly relevant areas. The cuts can be spaced to decrease the total number required to collect the data.
CONCLUSION
Modern treatment of acute aortic dissection is very dependent on accurate delineation of the site of origin of the dissection and
51
TABL
E 2.-
ALGO
RITH
M FO
R TH
E IN
VEST
IGAT
ION
OF S
USPE
CTED
AO
RTIC
DI
SSEC
TION
Ches
t ra
diogr
aph
L Tw
o-dim
ensio
nal
echo
card
iogra
phy
!/ L
Posit
ive
for
prox
imal
aorti
c di
ssec
tion
Nega
tive
for
prox
imal
aorti
c di
ssec
tion
d L
Angio
grap
hy
CT e
xam
inatio
n d
I I/
L -
Posit
ive
Nega
tive
Posit
ive
for
prox
imal
Eq&v
ocal
Nega
tive
Posit
ive
for
diss
ectio
n di
stal
di
ssec
tion
J Su
rger
y J
stop
i
Angio
grap
hy
i Su
rger
y
L i
Angio
grap
hy
stop
i
Med
ical
ther
apy
determination of the extent of involvement of the aorta. Ultra- sound and CT will undoubtedly play an increasing role in this process in the future. Treatment of aortic dissection should be instituted expeditiously, and rapid, noninvasive, and reliable methods of investigation clearly have great advantages in this regard. Nevertheless, individual clinicians, particularly sur- geons, will undoubtedly have different requirements, and radiol- ogists must perform in tune to these requirements, particularly because the reliability and place of the newer modalities remain under investigation.
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