Chap

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23E. Scott Pretorius, MD

CT and MRI of The KIdney

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1. What computed tomography (CT) protocol should be used to characterize a focal renal mass?A dedicated renal CT scan includes precontrast and postcontrast images because it is by comparing the appearance of a lesion on these two sets of images that the radiologist determines whether or not a lesion displays enhancement. Most CT examinations of the abdomen include only postcontrast images and are not optimal for characterizing a renal mass.

2. What magnetic resonance imaging (MRI) protocol should be used to characterize a focal renal mass?Our preferred protocol for characterization of a renal mass is (1) breath-hold T1-weighted gradient-echo images, in-phase and out-of-phase; (2) axial fat-saturated fast spin-echo T2-weighted images; (3) coronal fat-saturated three-dimensional T1-weighted gradient-echo images obtained before, during, and after injection of intravenous gadolinium contrast agent; and (4) axial delayed phase T1-weighted gradient-echo images of the entire abdomen.

In the kidneys and elsewhere in the body, it is imperative that if intravenous contrast agent is administered, a set of precontrast images must be acquired with parameters (e.g., TR, TE, flip angle) that are exactly the same as those of the postcontrast acquisition. This is the only way that one can determine whether or not a lesion enhances on postcontrast images.

3. What is the normal enhancement pattern of the kidneys on CT and MRI?Because kidneys receive approximately 25% of cardiac output, the kidneys enhance avidly and rapidly after contrast agent administration. Early renal enhancement on CT and MRI is termed the corticomedullary phase, in which the peripheral renal cortex is seen to enhance more than the central renal medulla (Fig. 23-1A). The corticomedullary phase

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A

C

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Figure 23-1. A-C, Axial T1-weighted gradient-echo MR images with fat saturation in the corticomedullary (A), nephrographic (B), and excretory (C) phases after administration of intravenous gadolinium chelate contrast agent. Excreted gadolinium chelate (arrows) may appear either bright on T1 (if dilute urine) or, as in this case, dark on T1 (if concentrated urine).

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is followed by the nephrographic phase (Fig. 23-1B), in which the renal cortex and medulla have enhanced to roughly the same degree. This is generally the best phase in which to detect renal neoplasms. Finally, in the excretory phase, a functioning kidney excretes the injected contrast material. This phase is bright in the renal collecting system on CT. The T1 appearance of excreted gadolinium chelates depends on whether the kidney is making a dilute or concentrated urine. More dilute gadolinium chelates in urine are bright on T1, owing to T1 shortening effects. At higher concentrations, gadolinium chelates may appear very dark on T1-weighted images because T2 effects predominate (Fig. 23-1C).

4. What are the expected CT and MRI findings of renal cell carcinoma (RCC)?Because of the presence of internal necrosis or hemorrhage, the appearance of RCC is variable. The common finding on CT and MRI of RCCs is that they enhance after the administration of intravenous contrast agents, and, in contrast to benign angiomyolipomas, they do not contain macroscopic fat. A solid renal lesion that enhances and that does not contain macroscopic fat is RCC until proved otherwise (Figs. 23-2 and 23-3).

5. What are risk factors for RCC?Known risk factors for RCC include cigarette smoking, exposure to petroleum products or asbestos, hypertension, and obesity. It is approximately twice as common in men as women, and slightly more common in blacks than in whites of European descent. Most patients are older than 40 years, and individuals in the seventh and eighth decades of life are most commonly affected. Patients with acquired cystic kidney disease, von Hippel-Lindau disease, hereditary papillary renal cancer, and possibly tuberous sclerosis are also at increased risk.

A

Figure 23-3. A and B, Axial T2-weighted image (A) and postcontrast Tretroperitoneal mass that arose from the left kidney. This lesion, which di

Figure 23-2. Axial postcontrast CT scan shows large, solid, enhancingrenal lesion (arrow) that does not contain macroscopic fat. Such lesions should be removed. This lesion was RCC.

6. What is the Bosniak system for cystic renal lesions?

This important classification system describes features of cystic renal lesions and indicates recommendations for management. Although developed for CT, the morphologic features can be applied to MRI as well. Calcifications within a lesion are likely to be best seen on unenhanced CT. Internal soft tissue architecture within a cystic lesion is likely to be best depicted on multiplanar MRI.

• Bosniak class 1 lesions are simple cysts. On CT or MRI, they have a thin or imperceptible wall, have no internal architecture, and do not enhance after contrast agent administration.

• Bosniak class 2 lesions may have a single thin septation or fine mural calcification. Also in class 2 are CT “hyperdense” cysts, which contain internal protein or hemorrhage, and which are hyperintense to normal renal parenchyma on T1-weighted images. These benign cysts do not enhance after contrast agent administration and have variable T2 signal intensity depending on their protein content.

B

1-weighted gradient-echo image (B) show large enhancing left d not contain macroscopic fat, was RCC.

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Figure 23-4. Coronal postcontrast T1-weighted gradient-echo image with fat saturation shows a cystic left renal lesion with an enhancing mural nodule (arrow). This is a Bosniak class 4 cystic RCC.

Figure 23-5. Axial unenhanced CT image shows a fat-density mass (arrow) in the left anterior interpolar region. Because this renal lesion contains material that is of fat attenuation, it is a benign angiomyolipoma.

• Bosniak class 3 lesions have thicker septations, multiple septations, or bulky mural calcifications. These lesions should be excised.

• Bosniak class 4 lesions display enhancing mural solid nodules that should be excised. Most of these lesions are cystic RCCs (Fig. 23-4).

7. What if I cannot tell whether a septation is thin or thick, or if the mural calcification is fine or bulky?Deciding whether a cystic renal lesion is class 2 or class 3 can often hinge on whether an internal septation is judged to be thin (class 2) or thick (class 3), or on whether mural calcification is fine (class 2) or bulky (class 3). For the rare lesions that are truly indeterminate, class 2F may be assigned. These lesions are followed with CT or MRI, usually at an interval of 4 to 6 months.

8. Is CT or MRI better for characterizing cystic renal lesions?This is a question without a simple answer. Both modalities have proven to be very good at characterizing renal masses. A high-quality MRI performed on a high field (1.5-Tesla) scanner is, overall, probably better than CT for characterizing renal masses because of the high soft tissue contrast of MRI. High-quality MRI is difficult to perform, however, because it requires considerable experience on the part of the technologist and radiologist. A good MRI examination also requires a cooperative patient because the examination requires that the patient remain motionless during imaging, and requires that the patient perform several breath-holds. If a patient is incapable of remaining motionless or of performing multiple breath-holds, a CT examination is likely to be much better than a motion-filled MRI.

9. What is an angiomyolipoma?An angiomyolipoma is a benign hamartoma that, as its name suggests, contains elements of blood vessels (angio-), muscle (myo-), and fat (lipoma). Because of their solid nature and enhancement, they may be confused

with enhancing RCCs. A CT (Fig. 23-5) or MRI (Fig. 23-6) diagnosis of angiomyolipoma is made by identifying macroscopic fat (a region of negative Hounsfield values on CT, or tissue that follows the signal intensity of body wall fat when the same MRI sequence is performed with and without frequency-selective fat saturation).

10. What syndrome is associated with angiomyolipomas?Tuberous sclerosis is associated with angiomyolipomas. This syndrome can cause renal cysts or renal angiomyolipomas. Tuberous sclerosis probably also puts a patient at a slightly higher risk of RCC. Angiomyolipomas can also occur sporadically, in patients without tuberous sclerosis.

11. How are angiomyolipomas treated?They are usually embolized—deprived of their blood supply—when they grow to be 4 cm or greater. Embolization is performed because larger lesions are at greater risk of hemorrhage.

12. I see an enhancing renal mass that does not have fat attenuation on CT, and that does not have fat signal on MRI. What is the differential diagnosis?This lesion would be presumed to be RCC until proved otherwise. Other lesions that could have this appearance include oncocytoma, transitional cell carcinoma (TCC), angiomyolipoma without detectable fat, or renal lymphoma.

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A B

C D

Figure 23-6. A-D, Axial T1-weighted gradient-echo images obtained in-phase (A), out-of-phase (B), with explicit fat saturation (C), and with explicit water saturation (D). The right-sided renal lesion (arrows) follows the signal of the body wall fat on all pulse sequences, indicating the

13. How is RCC staged by CT and MRI?RCC is most commonly staged using the Robson classification. Stage 1 includes tumors that are confined entirely to the kidney. Stage 2 lesions invade the perinephric fat or the adrenal gland on the same side. Stage 3a has tumor thrombus in the renal vein or inferior vena cava; stage 3b has tumor in regional lymph nodes; stage 3c involves regional lymph nodes and the renal vein, inferior vena cava, or both. Stage 4 RCCs involve distant metastases, such as the lung, liver, or bones.

presence of macroscopic fat within the lesion. This is diagnostic of benign angiomyolipoma.

Key Points: CT and MRI of the Kidney

1. An enhancing renal mass that does not contain macroscopic fat is RCC until proved otherwise.

2. An enhancing renal lesion with macroscopic fat is a benign angiomyolipoma.

3. Cystic renal lesions that contain thick internal septations, thick mural calcification, or enhancing mural nodules are suspicious for cystic RCCs and should be excised.

14. What does renal infarction look like on CT or MRI?Patients who have embolic disease, or who have injury to the aorta, renal artery, or a renal artery branch can develop segmental renal infarctions. These are wedge-shaped areas of hypoenhancement, broader at the periphery of the kidney and narrow near the center of the kidney. In segmental renal infarction, there is often a thin rim of preserved enhancement at the edge of the kidney because this region has a separate arterial supply via the renal capsular artery.

15. What does renal laceration look like on CT?Laceration of the kidney complicates 10% to 25% of blunt abdominal trauma cases. In patients with renal cortical laceration, CT shows interruption of the renal cortex, with associated subcapsular hematoma.

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16. What findings on a trauma CT scan indicate that a kidney would need to be surgically repaired?Superficial renal lacerations in stable patients can be managed conservatively, without surgery. Surgical repair is indicated for deep lacerations with disruption of the collecting system, for comminuted renal fractures, and for vascular pedicle injuries. Vascular pedicle injuries may include arterial/venous avulsion, intimal flap dissection, or traumatic vessel occlusion.

17. What is autosomal dominant polycystic kidney disease (ADPKD)? Is it associated with RCC?As the name implies, ADPKD is a slowly progressive, single-gene inherited condition that leads to formation of multiple renal cysts in the kidneys. Although many individuals present as adults with symptoms of renal enlargement and renal insufficiency (with associated hypertension and anemia), affected individuals may begin to develop cysts in childhood. Patients with ADPKD often also develop multiple liver cysts, and are at increased risk for development of intracerebral aneurysms. There is no increased risk for RCC.

18. What is autosomal recessive polycystic kidney disease (ARPKD)? Is it associated with RCC?ARPKD usually manifests on prenatal sonography, or in early childhood. The disease may be fatal in very early childhood, but some patients survive in to adolescence. ARPKD is associated with congenital hepatic fibrosis. There is no increased risk for RCC.

19. What are two genetically inherited syndromes that are associated with RCC?Von Hippel-Lindau disease and the very rare hereditary papillary RCC syndrome are autosomal dominantly inherited and associated with development of multiple RCCs. A diagnosis of one of these syndromes often leads to screening of first-degree relatives because they may also carry the disease gene.

20. What is acquired cystic renal disease? Is it associated with RCC?Acquired cystic renal disease is the term applied to patients who have end-stage renal disease, are on dialysis, and have very small kidneys with multiple cysts. These patients are at approximately six to seven times normal risk of developing RCC.

21. Should patients on dialysis be screened for RCC?This is controversial. RCCs found in patients with the acquired cystic renal disease are often low-grade neoplasms, and are of relatively little clinical significance in patients with short expected life spans and multiple comorbid diseases. If screening is performed, it is probably best reserved for younger patients or for patients who, other than their renal disease, are in good health.

22. What are some features that suggest oncocytoma on CT and MRI?Oncocytomas are solid, enhancing lesions. Oncocytomas may display a central, hypoenhancing scar, and they tend to be relatively homogeneous for lesion size. Because these findings can also be seen in RCC, oncocytomas cannot be reliably differentiated from RCCs on CT or MRI. Although most exhibit benign behavior, there have been reports of rare metastases from oncocytomas. They are perhaps best thought of as lesions of low malignant potential, and most can be excised by partial nephrectomy.

23. What are the findings of pyelonephritis on CT or MRI?The affected kidney is usually enlarged, and there may be stranding in the fat surrounding the kidney. The enhancement of the kidney often is delayed, relative to the unaffected contralateral kidney. A “striated nephrogram” can usually be seen, especially on delayed phase images, where the kidney displays alternating bands of high and low CT attenuation or MRI signal intensity (see Fig. 20-7).

24. A multiloculated cystic lesion looks like it has herniated into the renal collecting system. What could this be?This is likely a multilocular cystic nephroma. This unusual lesion is found in young boys and in older women. As its name suggests, it appears as an agglomeration of multiple cystic spaces. In approximately one third of cases, the lesion can herniate into the renal collecting system. Although considered benign, multilocular cystic nephroma undergoes sarcomatous degeneration in approximately 7% of cases. These lesions are excised.

25. Are CT and MRI useful in imaging TCC?TCC may arise in the epithelium of the renal collecting systems, the ureters, the urinary bladder, and portions of the urethra. CT and MRI can detect large masses, but both modalities have difficulty identifying small, flat tumors that do not result in proximal hydroureteronephrosis. Intravenous urogram (IVU) remains superior to CT and

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MRI for detecting this type of lesion, although multidetector CT urography and gadolinium-enhanced magnetic resonance urography (MRU) are rapidly gaining acceptance as viable alternatives to IVU in imaging patients for suspected TCC.

26. What are the MRI signal characteristics typical of TCC?TCC is hyperintense on T1-weighted and hypointense on T2-weighted images compared with surrounding urine.

27. What does renal lymphoma look like on CT and MRI?Renal lymphoma is usually B-cell lymphoma and is generally a manifestation of widespread systemic disease. The most common initial presentation is that of a larger retroperitoneal mass, surrounding one or both kidneys and possibly the inferior vena cava and aorta. It is of low intermediate signal on T1-weighted and T2-weighted images, similar to the spleen or lymph nodes. It enhances weakly after contrast agent administration. After treatment for lymphoma, renal involvement by the disease is often seen as multiple, focal rounded intraparenchymal masses.

28. Does metastatic disease go to the kidneys? What does it look like on CT and MRI?Yes, metastatic disease can go to the kidneys, although these lesions are usually very small and not clinically significant. They are seen often on autopsy, but usually do not cause clinical morbidity on their own. On CT or MRI, they are seen as multiple bilateral renal lesions with infiltrative margins.

29. What protocol should be used for performing CT or MRI on a patient who is a potential renal donor?Either CT or MRI may be used to evaluate a potential renal donor. On CT, a precontrast topogram may be used to evaluate for the presence of renal stones. Multidetector thin-section imaging through the kidneys is performed to detect and evaluate the renal arteries. Delayed phase imaging may be performed to evaluate venous morphology. Finally, excretory phase topogram can be performed to visualize the renal collecting system and ureters.

On MRI, precontrast T1-weighted gradient-echo and T2-weighted images are acquired, followed by coronal, three-dimensional T1-weighted gradient-echo images acquired in the arterial and venous phases of enhancement. Furosemide, 10 mg, may be given with the gadolinium chelate, and MRU may be obtained approximately 5 minutes later.

30. What kinds of things should be included in the CT or MRI report on a patient who is a potential renal donor?Important information includes renal size; the presence or absence of renal mass or other renal abnormality; and the number, size, and position of renal arteries, renal veins, and ureters for each kidney. Renal artery duplication is common and is found in approximately 30% of all kidneys. The presence or absence of prehilar arterial branching should be noted; in particular, the distance between the aorta and first arterial branch is surgically relevant. The morphology of the left renal vein is more variable than the right. The left renal vein usually passes anterior to the aorta before joining the inferior vena cava. It may pass posterior to the aorta (retroaortic left renal vein) however, or there may be two left renal veins—one anterior to the aorta and one posterior—which is called a circumaortic left renal vein.

31. I have an emergency department patient with flank pain. What is the best radiologic test to find renal and ureteral calculi?Unenhanced multidetector CT has replaced IVU as the examination of choice for identifying renal and ureteral stones (Fig. 23-7). If a cause for the patient’s pain is not found, or if a calcific density is identified, and it is

A B

Figure 23-7. A and B, Axial unenhanced CT images of the kidneys (A) and pelvis (B) show the presence of a 1-mm right renal calculus (arrow). The patient’s flank pain was due to the right distal ureteral calculus (arrow in B).

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uncertain whether that density lies within the ureter, contrast agent may be given, and the abdomen and pelvis may be scanned again.

32. What is post-transplant lymphoproliferative disorder (PTLD)?PTLD is a spectrum of diseases ranging from benign lymphoid proliferation to malignant lymphoma. PTLD is caused by Epstein-Barr virus and is seen in transplant patients receiving immunosuppressive medications, such as azathioprine (Imuran), muromonab-CD3 (Orthoclone OKT3), and cyclosporine. Greater amounts of immunosuppression are associated with higher rates of incidence of PTLD. Liver transplant recipients are at greater risk than renal transplant recipients.

33. Other than PTLD, what are some things that can go wrong with a renal transplant?Things can go wrong with any of the surgical anastomoses—arterial, venous, and ureteral. Complications include renal artery stenosis, renal artery thrombosis, renal vein thrombosis, and ureteral leak. Patients may also develop pelvic “masses,” including urinoma, hematoma, and lymphocele. Finally, the transplant may not function well and may develop acute tubular necrosis or rejection.

34. What is a horseshoe kidney?When the lower poles of the left kidney and right kidney are attached to one another, this is termed a horseshoe kidney. The region of connection, termed the isthmus, may be composed of fibrous tissue or functioning renal parenchyma. Horseshoe kidney is more common in patients with Turner syndrome.

35. For what diseases are individuals with horseshoe kidneys at increased risk?Individuals with horseshoe kidney are at increased risk for ureteropelvic junction obstruction, stone formation, vesicoureteral reflux, infection, trauma, Wilms tumor, TCC, and RCC.

Figure 23-8. Coronal, excretory-phase MRU performed 8 minutes after administration of intravenous contrast agent shows a filling defect in the lower pole of the left kidney, representing TCC (arrow). The right renal collecting system is normal.

36. What is CT urography, and how is it performed?CT urography is an examination that is generally performed for hematuria. It involves acquisition of unenhanced images (for identification of renal or ureteral calculi), nephrographic phase images (at approximately 110 seconds postinjection, for renal masses), and excretory phase images (at approximately 6 to 8 minutes postinjection, for evaluation of renal collecting systems and ureters). Reformatted images of the excretory data are generated in multiple projections, to create images of the renal collecting systems and ureters that are analogous to conventional urograms.

37. What is MRU? What are the relative strengths and weakness of CT urography and MRU?MRU is performed similarly to CT urography, using precontrast images, nephrographic images, and excretory phase images. Coronal, three-dimensional, T1-weighted gradient-echo images are used for all acquisitions, and gadolinium chelates are injected rather than iodine-based compounds (Fig. 23-8).

Two major advantages of MRU versus CT urography are the lack of ionizing radiation in MRU and the ability to acquire multiple excretory phase

datasets rather than just one, which is especially useful if renal excretion is delayed. Advantages of CT urography over MRU include the much greater sensitivity of CT for detection of renal calculi, and the superior spatial resolution of multidetector row CT relative to MRI.

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