Imaging of the foot and ankle - East of England Trauma and … · 2016-08-08 · Imaging of the...

IMAGING

Imaging of the foot and ankleOrla Doody

Melanie A Hopper

Abstract
There are a number of imaging modalities available to assist in assess-
ment of the foot and ankle. The variety of techniques will be described

with emphasis on the particular advantages and limitations of each.

Recent advances and variations relating to the individual modalities are

reviewed together with specific clinical scenarios.

Keywords ankle; diagnostic imaging; foot

Introduction

Imaging of the foot and ankle is commonly undertaken and there

is a wide range of modalities available for assessment of a variety

of abnormalities. Radiography remains the mainstay of imaging

but there are several more advanced techniques which can be

usefully applied. An understanding of these is critical for a

balanced approach to imaging.

Plain radiographs

The initial evaluation of many musculoskeletal conditions of the

foot and ankle is with plain radiographs. These are produced

through variations in the absorption of ionizing radiation by the

body’s tissues, resulting in excellent spatial resolution between

soft tissue and bone due to their differing attenuation values.

Typically two views of a body part are taken, conventionally

in the anteroposterior (AP) and lateral planes. Due to the com-

plex anatomy within the ankle and foot this is frequently modi-

fied depending on clinical concern. The use of weight-bearing

films, other than in trauma, allows for standardization of images

and can reveal subtle but important changes in alignment

(Figure 1).

A modified AP image with the foot and ankle in 15e20� of

internal rotation, the mortise view, provides unobstructed

assessment of the talar dome (Figure 1a), as a standard AP image

can obscure pathology here. In the foot, due to the overlapping

orientation of the tarsal arches, oblique images can provide

valuable supplementary views but do not replace the standard

radiological assessment.

Advantages

Radiographs are widely available and relatively inexpensive. For

the diagnosis of bony abnormalities plain radiographs are

particularly useful. The demonstration of a joint effusion or soft

tissue swelling is useful in cases of radiographically occult in-

juries (Figure 2).1

Orla Doody MRCPI FFRRCSI Consultant Radiologist, Department of

Radiology, Tallaght Hospital, Dublin, Ireland.

Melanie A Hopper MBChB MRCS FRCR Consultant Musculoskeletal

Radiologist, Cambridge University Hospitals NHS Trust, Cambridge, UK.

ORTHOPAEDICS AND TRAUMA 28:5 339

Disadvantages

The acquisition of plain radiographs involves ionizing radiation

and whilst the dose to the extremity is minimal, the potential

hazards of radiation should not be ignored. Whilst allowing a

limited soft tissue evaluation as alluded to above, a detailed re-

view of the soft tissues is not possible on radiographs due to the

narrow range of attenuation values between them. It is important

that additional imaging is performed if clinical concern persists.

Variations

Stress views: active or passive stress views may demonstrate

indirect evidence of a ligamentous injury. The combination of the

additional applied force and an underlying ligamentous disrup-

tion results in widening of the joint space. In the ankle, stress

views can evaluate disruption of the lateral ligament complex

(talar tilt), the medial ligament complex and the tibiofibular

syndesmosis.

Fluoroscopy: fluoroscopic techniques are typically used in or-

thopaedic surgery and radiological services to guide fracture

reduction or aid interventional procedures. Similar to standard

radiography this modality utilizes a X-ray source but produces

real time dynamic images and allows dynamic evaluation of the

joint.

Arthrography: arthrography involves the injection of a radio-

opaque contrast agent into a joint, typically under fluoroscopic

guidance, or alternatively with ultrasound. Indirect information

pertaining to the soft tissues can be deduced from the pattern of

distribution of the injected contrast medium. Both diagnostic and

therapeutic joint injections are frequently combined with

arthrographic procedures via injection of local anaesthetic or

steroid agents respectively.

In the foot and ankle, arthrography is typically performed in

conjunction with MRI or less frequently CT utilizing a suitable

contrast agent.

Tomosynthesis: the conventional radiographic technique can be

modified to acquire numerous low dose images of specific body

part at differing focal depths. Digital tomosynthesis is established

in breast imaging and in the evaluation of pulmonary nodules

but has expanded into musculoskeletal imaging.2 The radiation

dose is greater than conventional radiography but is less than CT

and this modality shows promise in the evaluation of post-

operative patients with potential reduction in the extent of

streak artefact. Studies have demonstrated the value of tomo-

synthesis in relation to wrist fractures but there is potential to

investigate for occult bone injury in any area where complex

anatomy or soft tissue overlay limits evaluation or where ab-

normalities are radiographically occult (Figure 3).3

Ultrasound (US)

US plays a key role in the diagnosis and management of

musculoskeletal disease. For the evaluation of superficial

musculoskeletal structures a high frequency probe is necessary,

typically a linear array probe of at least 7 MHZ and ideally 10

MHz or greater. This enables greater spatial resolution at the

expense of limited depth penetration. A small footprint probe if

Crown Copyright � 2014 Published by Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.mporth.2014.02.003

Figure 1 Diabetic patient with distal neuropathy. (a) Weight-bearing mortise radiograph demonstrates talar tilt not appreciated on (b) reformatted coronal

CT of the same patient.

IMAGING

available can be a usual adjunct particularly in the foot. Whilst

evaluation of the bone is not possible with US the periosteum is

well visualized and occult stress fractures of the ankle or meta-

tarsals can be detected.4

Advantages

When compared to other imaging modalities, US offers the

unique advantage of dynamic assessment. It is a high resolution,

Figure 2 Lateral radiograph demonstrates an ankle joint effusion (arrow-

heads) but no fracture following trauma.


rapid real time examination which can be focused on the exact

site of clinical symptoms and involves no radiation. Tendons and

ligaments can be evaluated during active or passive movement.

For example, dynamic US can elucidate peroneal subluxation not

evident on static imaging.

Figure 3 Digital tomosynthesis ankle mortise radiograph shows a mini-

mally displaced lateral malleolus fracture (arrow) not evident on standard

radiography.



IMAGING

Doppler evaluation for vascularity is a valuable tool used in

the imaging of joints for active inflammation, tendons for neo-

vascularity and assessment of blood flow within soft tissue le-

sions (Figure 4). Both colour and power Doppler techniques can

be used but the latter is preferred in the foot and ankle as it is

more sensitive to blood flow. Information on the direction of flow

provided by colour Doppler is less important in musculoskeletal

US.4 Due to the superficial location of the ankle and foot tendons,

US is an ideal modality to evaluate these structures.

Ultrasound is widely used for image guided musculoskeletal

procedures, and allows excellent needle visualization.

Figure 5 EFOV sonogram of the calf demonstrates normal appearance of
Disadvantages
the medial gastrocnemius and soleus.
The main limitation of US is its operator dependence. In addition,
musculoskeletal US has a number of specific artefacts that can

influence image quality, the most frequently encountered is

anisotropy. When the US beam is perpendicular to a tendon, the

normal tendon has a characteristic hyperechoic, fibrillar

appearance. If the beam is at an oblique angle, the tendon be-

comes more hypoechoic, with this artefact known as anisotropy.

This is an important pitfall in the imaging of tendons, ligaments

and muscle, as pathology can also cause these structures to

appear hypoechoic. A combination of manual angulation and

electronic tilting of the beam can reduce anisotropic artefact.

Variations

Extended field of view imaging (EFOV): panoramic scanning or

extended field of view imaging can be used to demonstrate an

abnormality that is greater than the width of the ultrasound

probe. While the diagnostic quality of the ultrasound is not

improved, this technique produces a continuous image, which

can be a useful overview of the relevant finding for the referring

clinician (Figure 5).

US elastography: traditional US, termed B-mode, relies on

morphological changes to indicate an underlying pathological

process. The addition of elastography provides a measure of

tissue stiffness by gentle manual compression. In foot and ankle

imaging this has potential for detection of tissue softening

Figure 4 Longitudinal sonogram of the anterior ankle at the level of the

joint (*) demonstrates severe tendinosis within the tibialis anterior

tendon with tendon thickening and abnormal power Doppler indicating

neovascularity.


occurring for example as part of Achilles tendinopathy (Figure 6).

Elastography is well established in other radiological sub speci-

alities such as breast imaging and research is ongoing to quantify

the benefit of sonoelastography over conventional techniques for

musculoskeletal diseases.5

Contrast enhanced US (CEUS): microbubble contrast agents,

administered intravenously, can be detected with standard US

equipment. The microbubbles are extremely echogenic and can

be used to evaluate microcirculation. The technique has not yet

moved into the clinical arena for musculoskeletal assessment but

CEUS is emerging as a promising adjunct in the research forum

particularly in rheumatological conditions where neovascularity

is an important early finding in many disease processes.6

Computed tomography (CT)

Technique

Multiple parallel images are produced through an array of X-ray

detectors that move circumferentially around a patient, while the

patient is moved through a CT scanner. The spatial resolution of

CT renders it an ideal modality to evaluate bone and soft tissue

calcification. Whilst acquired axially, images can subsequently

be reconstructed in multiple planes, typically coronal and

sagittal. For surgical planning a 3-D surface rendered image can

be produced from the 2-D data (Figure 7).

For musculoskeletal extremity imaging, depending on the

clinical scenario iodinated contrast can be administered to

evaluate peripheral vascularity for example in trauma with

suspected vascular compromise or to further evaluate a soft

tissue mass.

Advantages

The process of acquiring a CT takes seconds and is well tolerated

by most patients. In the preoperative planning of fractures, in

particular of complex intra-articular fractures, cross-sectional

imaging with CT offers a detailed evaluation of fracture

complexity and greater detection of loose bodies than plain

radiographs.

Disadvantages

Similar to conventional radiography, CT involves ionizing ra-

diation but at a higher dose. The predominant limitation of CT



Figure 6 (a) Standard B-mode sonogram. (b) Elastogram of achilles tendinosis demonstrates focal softening corresponding to a partial thickness tear

(arrows).

IMAGING

is in its poor evaluation of soft tissue structures as when

injured they cannot easily be delineated from the adjacent

normal soft tissues. Whilst more problematic with MRI,

metallic artefact from a surgical prosthesis can obscure detail

on CT.

Advances

Dual energy CT: in dual energy CT two X-ray tubes at different

kilovoltages simultaneously acquire data sets of the desired re-

gion. A comparison between attenuation values at these two

acquisitions allows differentiation between uric acid and cal-

cium, and hence can be used to image for uric acid crystals in

tophaceous gout.7 Dual energy CT also has potential in the

evaluation of traumatic bony injuries and detecting acute

marrow oedema. Dose reduction techniques both in standard

and dual energy CT are being utilized without compromising the

diagnostic ability of the study.8

Figure 7 Surface rendered CT demonstrates a minimally displaced fracture

of the talar neck (arrow).


Magnetic resonance imaging (MRI)

Technical factors

MRI has revolutionized musculoskeletal imaging, and offers

excellent spatial and contrast resolution. An MR image is pro-

duced by the effect of a strong homogeneous magnetic field on

the body’s hydrogen nuclei in water molecules, hence avoiding

ionizing radiation and the potential associated risks.

Part of the complexity of MRI is centred on the vast variety of

sequences available, with inconsistency in terminology between

different manufactures. For general musculoskeletal imaging,

MR sequences can be simplified into three main groups. T1

weighted sequences show fat as bright or high signal and fluid as

dark or low signal. They are particularly useful for anatomical

assessment. T2 weighted sequences are fluid sensitive and show

fluid or oedema as high signal. Fat is also bright on T2 weighted

images and so to increase the conspicuity of fluid, the signal from

fat can be suppressed (T2fs). A frequently used alternative to fat

saturation is the short tau inversion recovery (STIR) sequence.

Proton density (PD) sequences have been optimized for hyaline

cartilage assessment; they are also fluid sensitive and like T2

weighted sequences, are often combined with fat suppression

(PDfs).

Advantages

MRI provides excellent spatial and contrast resolution of the

ankle and foot without reliance on ionizing radiation. MRI is

widely utilized in the evaluation of tendon and ligament pa-

thology as well as providing detailed review of bone and joint

abnormalities (Figure 8).

Disadvantages

Due to the strong magnetic field many implantable devices such

as pacemakers are not MR compatible. The development of wide,

short bore MRI scanners has increased compliance in claustro-

phobic patients. Each sequence of a diagnostic study requires the

patient to remain completely still, with a much longer scanning

time compared to CT. Unlike CT the majority of sequences in

common usage must be acquired in the plane they are to be

viewed in and it is not possible to manipulate the images to

reformat in an alternative plane.



Figure 9 Bone scintigram shows increased joint based radiotracer uptake

in right foot Charcot.

Figure 8 Sagittal PDfs MR demonstrates abnormal bone marrow signal in

the talar dome with a classical geographic distribution in avascular ne-

crosis (arrow) due to systemic steroids. Normal marrow in the adjacent

calcaneum and tibia.

IMAGING

Susceptibility artefact from ferrous material impacts on image

quality due to image distortion and signal voids. The newer

generation orthopaedic titanium and non-ferrous prostheses are

less problematic; however imaging of the older generation

prostheses is a challenge with development of metal artefact

reduction sequences (MARS) to minimize artefactual distortion.9

While MRI is highly sensitive, it is not always specific and

study findings need to be interpreted in the context of the clinical

scenario and the appearance on other imaging modalities. For

example an MRI examination will detect increased fluid or

oedema, but cannot always differentiate between various aeti-

ologies including trauma, infection or malignancy.

Advances

Figure 10 SPECT. Fused scintigram and low resolution CT shows increased

radiotracer uptake in a 2nd metatarsal stress fracture.

MR arthrography: MR arthrography can be performed as a

conventional injection of contrast into the joint (direct arthrog-

raphy) which has the benefit of joint distension but also has

potential risks such as infection. Alternatively, indirect arthrog-

raphy can be performed by intravenous injection of contrast

medium with delayed imaging of the joint although the lack of

distension can be problematic. In the ankle joint both direct and

indirect techniques can have a role in the evaluation of a range of

pathologies including ligamentous injuries, impingement syn-

dromes, cartilage lesions, loose bodies, osteochondral lesions of

the talus, and synovial joint disorders but with MRI advances

including the use of 3 T scanners, many units no longer use

arthrography routinely.

Cartilage imaging: MRI provides an excellent non-invasive

evaluation of articular cartilage. As the treatment of chondral

injury evolves there has been increased focus on the develop-

ment of accurate cartilage specific sequences which focus both

on the biochemical alterations and on the morphological changes


including fissuring, thinning and cartilage loss. Alterations in the

water or sodium content of cartilage or in the proteoglycan

composition and distribution can predict cartilage damage.

Evaluation with T2 mapping or sequences such as delayed

gadolinium-enhanced MR imaging cartilage (dGEMRIC) can

identify an irregularity of chondral make-up which precedes any

morphological abnormality.10

Nuclear medicine

A standard bone scintigram is the most common nuclear medi-

cine technique used for the evaluation of musculoskeletal dis-

orders and in the foot is particularly useful in the evaluation of

stress fractures of the metatarsals. A radioactive substance,

typically technetium-99m labelled methylene disphosphonate

(99mTcMDP) is injected into the patient. As this undergoes

radioactive decay it emits gamma rays which are detected by a

gamma camera. Three phase imaging to include an arterial

phase, blood pool phase and bone scan image can be performed

to improve differentiation between bone and soft tissue. The



Figure 11 (a) Sagittal T2fs. (b) Sagittal T1 MR images demonstrate a calcaneal stress fracture in a runner. Low signal fracture line with surrounding bone

marrow oedema.

IMAGING

tracer detects increased osteoblastic activity and hence areas of

increased bone turnover (Figure 9).

Advantages

Bone scintigraphy is widely available, is highly sensitive for a

range of osseous conditions and is generally well tolerated by

patients.

Disadvantages

While bone scintigraphy is highly sensitive, it has a low speci-

ficity. A variety of bone disorders including trauma, degenera-

tion, malignancy and infection can cause increased tracer uptake.

Processes without increased osteoblastic activity such as multiple

myeloma or lytic metastases may be occult. Lack of anatomical

differentiation can limit detail in complex anatomical areas. The

radiation dose is greater than standard radiography.

Variations

Multi-planar data can be acquired using single photon emission

computed tomography (SPECT) utilizing similar radiopharma-

ceuticals to traditional scintigraphy. Multi-planar SPECT-CT

combines both SPECT and CT imaging resulting in greater

anatomical evaluation of many musculoskeletal conditions

(Figure 10).11

Specific clinical scenarios

Fractures

Figure 12 DP radiograph of the mid and forefoot demonstrates a Lisfranc

fracture at the base of the 2nd metatarsal with normal alignment.

Stress and occult fractures: whilst the majority of fractures will

be diagnosed with plain radiographs, MRI, CT and scintigraphy

all play an important role in the diagnosis of stress fractures and

in the detection of clinically suspected but radiographically

occult fractures in the setting of trauma. In the ankle and foot

stress fractures are common, frequently involving the second

metatarsal, calcaneus and less commonly the talus or navicular.

A fracture line on MR is depicted as a very low signal intensity

line with associated changes within the adjacent bone consistent

with oedema and haemorrhage (Figure 11). A bone bruise is


radiographically occult but at MRI is depicted as an ill-defined

area of low signal on T1 and high signal on T2 or STIR, which

is confined to the medullary cavity. Stress fractures in nuclear

medicine studies are revealed as focal areas of increased radio-

tracer uptake (Figure 10).

Osteochondral fractures: in the ankle an osteochondral fracture

typically involves the talar dome most commonly occurring in



Figure 13 (a) Longitudinal Doppler sonogram demonstrates neovascularity and fusiform tendon thickening in Achilles tendinosis. (b) Sagittal T2fs MR

demonstrates insertional Achilles tendinosis (arrowhead) with a Haglund’s deformity and calcaneal oedema (arrow). There is oedema in Kager’s fat pad

(*) and a small retrocalcaneal bursa.

IMAGING

the middle third of the lateral border and in the posterior third of

the medial border.12 An osteochondral fragment partially or

completely detaches as a result of single or multiple traumatic

insults. The staging of these lesions is based on the condition of

the subchondral bone and the integrity of the articular cartilage.

Both plain radiographs and CT can detect these lesions, however

MRI provides important information regarding condition of the

articular cartilage, the viability and stability of the bone fragment

and the extent of any healing.13

Lisfranc injury: conventional radiographs can indicate a fracture

or malalignment in the case of Lisfranc injury but changes can be

subtle and may be missed (Figure 12). It is possible to optimize

radiographs14 but in the majority of cases additional imaging

with CT or MRI is required.

Tendon disease

Figure 14 Axial PDfs MRI demonstrates thickening and oedema within the

tibialis posterior tendon (arrow) in a patient with tendinosis. There is

excess fluid within the tendon sheath.

The posterior, medial and lateral tendons of the ankle and foot

are particularly prone to acute and chronic injury. Acute teno-

synovitis manifests as increased fluid in the tendon sheath with a

normal appearance of the tendon. In chronic tenosynovitis the

tendon may appear nodular or diffusely thickened. Tendinosis

depending on its severity will cause mild to severe thickening

and heterogeneity to the tendon and can make assessment for a

partial tear more difficult.15

Achilles tendon: both MRI and ultrasound are widely used in the

evaluation of Achilles tendon pathology and both imaging mo-

dalities can readily detect all aspects of tendon disease (Figures

6a and 13). In the acute setting the dynamic nature of ultrasound

gives it an advantage over MRI in the assessment of Achilles tears

as it is possible to calculate separation of the tear in plantar

flexion and thus guide treatment. The greater resolution of US

also allows confident assessment of alternative causes of calf

pain, in particular plantaris injury.

Insertional Achilles tendinosis may be associated with a

Haglund deformity, retrocalcaneal or Achilles bursitis, focal

thickening of the tendon at its insertion, readily diagnosed with

ultrasound and MRI. Intrasubstance calcification is more readily

assessed using US and/or plain film. At MRI, calcaneal marrow


oedema and increased signal within the distal tendon may be

demonstrated (Figure 13b).15

Tibialis posterior tendon dysfunction: chronic tendon rupture

typically occurs at the level of the medial malleolus affecting

women in the 5th and 6th decades who develop a progressive flat

foot deformity.16 In contrast, in younger athletic patients tears

either partial or complete occur at the navicular insertion.17

Acute tenosynovitis is also seen in the young athletic patient

secondary to overuse. Tibialis posterior abnormalities can be

appreciated using US or MRI (Figure 14).



IMAGING

Peroneal tendons: in the younger athletic population peroneal

tears are typically secondary to overuse, with the older popula-

tion developing degenerative tears. Peroneus brevis has a

distinctive appearance at MRI in the presence of a longitudinal

intrasubstance tear, which classically originates in the fibular

groove and demonstrates an inverted v shape, enveloping the

peroneus longus tendon (Figure 15).18 On axial MRI images,

dislocation is readily diagnosed with the peroneal tendons

located anterior and lateral to the distal fibula. The dynamic

nature of ultrasound is of particular use in evaluating for inter-

mittent or internal peroneal tendon subluxation where it is su-

perior to a static MRI study.19

Flexor hallucis longus tendon: this tendon is vulnerable to

injury as it passes through the fibro-osseous tunnel bounded by

the lateral and medial talar tubercles, where the tendon may

develop chronic or stenosing tenosynovitis, tendinosis, partial or

complete tear secondary to chronic repetitive friction.15,20 Im-

aging features of tenosynovitis and tendinosis can also occur

distally at the knot of Henry.

Ligaments

Figure 16 Axial PDfs MRI demonstrates a complete anterior talofibular

ligament rupture with proximal and distal ligament stumps (arrowheads).

Medial and lateral ligament injuries: the normal peri-ankle

ligaments should be well defined low signal linear structures.

In the acute setting lateral collateral ligament and medial

collateral ligament injuries are well demonstrated on MRI.21,22

Given that acute ankle ligamentous injuries are rarely treated

surgically, US and MRI tend to be reserved for high level athletes

where surgical repair may be undertaken and in the evaluation of

chronic ankle instability.23 The lateral ligament complex injuries

follow a predictable pattern of injury with the anterior talofibular

ligament tearing first followed by the calcaneofibular ligament

Figure 15 Axial PDfs MRI demonstrates a longitudinal split within the

peroneus brevis tendon which appears as an inverted V (arrow).


and finally the posterior talofibular ligament.15 Acute rupture on

MRI is diagnosed by morphological and signal intensity alter-

ations including discontinuity, detachment, thinning, thickening

or irregularity of the ligament (Figure 16). Co-existent bony

oedema, soft tissue oedema and extravasation of joint fluid may

be present. In a chronic tear the ligament affected may appear

thinned, thickened, irregular in contour, wavy, or elongated with

adjacent scarring or synovial proliferation.15

Contusional injuries in particular of the tibiotalar component

of the deltoid ligament complex have a high association with

inversion sprains.24 These contusions result in loss of the normal

striations within the deltoid, with the ligament demonstrating

homogenous intermediate signal intensity.

Figure 17 Sonogram of plantar fasciitis with thickening of its calcaneal

origin (callipers).



Figure 18 (a) Oblique radiograph shows fibrous calcaneonavicular coalition with the anteater nose sign of the anterior process of the calcaneum. (b)

Lateral radiograph shows talar beaking (arrow) and the continuous C-sign (arrowheads) in talocalcaneal coalition. (c) Sagittal T2fs demonstrates fibrous

calcaneonavicular coalition with bone marrow oedema (arrow). Talar dome osteochondritis (arrowhead).

IMAGING

Compressive neuropathies

The two most common compressive neuropathies in the foot and

ankle are Morton’s neuroma and tarsal tunnel syndrome, with

less frequently encountered neuropathies such as sural nerve

entrapment syndrome, and deep and superficial peroneal nerve

entrapment syndromes.

Morton neuroma: a Morton neuroma or interdigital neuroma is a

benign fibrosing process of an intermetatarsal nerve, with

resultant perineural thickening. These are often associated with

an intermetatarsal bursa and the diagnosis can be confirmed with

either MRI or ultrasound. On MRI a Morton neuroma is a

dumbbell shaped mass demonstrating low to intermediate signal

on T1 and T2 weighted imaging, often more conspicuous on T1

imaging due to the surrounding hyperintense fat. It may be

associated with an intermetatarsal bursa which presents as a

fluid signal mass.25 Ultrasound offers dynamic evaluation of the

Figure 19 (a) DP radiograph shows cortical destruction of the hallux distal phal

of a different patient shows 2nd metatarsal osteomyelitis (arrow) and a planta


hypoechoic mass with compression of the metatarsal heads to

cause plantar displacement of the mass and a palpable click (the

sonographic Mulder sign). At ultrasound the frequently associ-

ated intermetatarsal bursa presents as an anechoic fluid

collection.4

Tarsal tunnel syndrome: nerve compression or entrapment can

occur at any level of the posterior tibial nerve or its branches

(medial plantar nerve, lateral plantar nerve, medial calcaneal

nerve) with the resultant clinical symptoms varying according to

the level of compression.26 MRI can evaluate mass lesions

compressing these nerves within the tarsal tunnel including

ganglions, varicosities, accessory muscles, lipomas, neurogenic

tumours, scar tissue and synovial hypertrophy.15 MRI can also

demonstrate secondary signs of nerve compression such as at-

rophy of abductor digiti quinti in Baxter’s neuropathy. Entrap-

ment of the medial plantar nerve in the narrow space between

anx in osteomyelitis (white arrow). (b) Short axis T1fs post gadolinium MRI

r soft tissue abscess (arrowheads) contiguous with an ulcer (not shown).



Figure 20 Lateral radiograph of Charcot foot in diabetes shows midfoot

collapse, dislocation and deformity, sclerosis and bone fragmentation.

IMAGING

the abductor hallucis muscle and the knot of Henry results in

Jogger’s foot with this complex anatomy well evaluated with

MRI.27

Plantar fascia

The plantar fascia is readily evaluated with both ultrasound

and MRI. At MRI the normal fibrous fascia is a thin band

measuring 2e4 mm with low signal intensity on all sequences.28

On ultrasound the plantar aponeurosis is a uniform, fibrillar

structure, measuring 4 mm or less at its calcaneal origin.4

Plantar fasciitis from repetitive microtrauma or in association

with an enthesopathy has characteristic MR findings of thick-

ening and oedema of typically the proximal aspect of the medial

plantar fascia.28 Oedema may be present within the adjacent

calcaneum and heel fat pad. At US, plantar fasciitis manifests as

hypoechoic thickening (>4 mm) of the calcaneal origin

(Figure 17).4 US can guide steroid injection and dry needling as

part of management.

The fibrous proliferation of plantar fibromatosis can be

demonstrated as single or multiple fusiform nodules within the

plantar fascia which are hypoechoic or isoechoic on ultrasound

and low to intermediate signal intensity on T1 and T2 weighted

MRI. If larger the nodules may be heterogenous on MRI and may

be locally aggressive involving the plantar musculature.29

Coalition

Tarsal coalitions can be radiologically assessed with plain film,

CT and MRI. Even when there is a complete bony synostosis, the

radiographic features can be subtle but classic findings are

described in the more common types of coalition (Figure 18a and

b). Fibrous and cartilaginous coalition causes cortical irregularity

and is often associated with osseous oedema (Figure 18c).

Infection

Whilst established bone destruction can be detected on serial

plain radiographs (Figure 19a), MRI is frequently used in the

evaluation of osteomyelitis to differentiate soft tissue infection

from osteomyelitis and to establish the extent of involvement

(Figure 19b). Small locules of gas can be difficult to appreciate on

MRI but are evident both on plain film and CT.

The neuropathic foot

The chronic stage of neuroarthropathy, the Charcot joint, is well

recognized on plain film (Figure 20). The earlier stages are often


radiographically occult but MRI and scintigraphy can identify

neuropathy before deformity occurs and help differentiate it from

infection. In diabetes and other causes of neuropathy, infection is

almost always contiguous with a soft tissue ulcer.30 This and the

secondary signs of infection such as an abscess can help to

differentiate from abnormalities due to neuropathy where bone

marrow changes in MRI and increased radiotracer uptake in

scintigraphy tend to be periarticular and subchondral (Figure 9).

Conclusion

The foot and ankle are commonly imaged for a wide range of

abnormalities. An understanding of the array of techniques

available including their strengths and weaknesses allows a

rational approach to imaging. A

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