Steven Needell, MD provides an overview of basic MRI imaging sequences and MRI anatomy of the ankle. Dr Needell discusses the progression of tendon degeneration to tendon tearing and the spectrum of pathology seen with the various tendons and ligaments about the ankle.
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Steven Needell, MD
Director of Musculoskeletal Imaging
Boca Raton Regional Hospital
Boca Raton, Florida
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FOOT AND ANKLE MRI: BONE AND SOFT TISSUE
Welcome to my lecture on Foot And Ankle MRI emphasizing bone and soft tissue disorders. I am Dr. Steven D. Needell. I am the director of Musculoskeletal Imaging at Boca Radiology Group in Boca Raton, Florida.
MRI is the most sensitive and specific modality for detection and characterization of bone and soft tissue pathology. Bone marrow edema is an important indicator of disease. There are many etiologies of bone marrow edema ranging from trauma to arthritis, AVN, and osteonecrosis, inflammatory and infectious etiologies, and tumor.
Alternatives to MRI include x-ray and CT. Both of these use ionizing radiation and they excel in detecting abnormalities of cortical bone such as complex fractures and bone destruction but they are very limited in their ability to visualize changes within the medullary cavity of the bone as well as soft tissue processes.
Bone scanning is a commonly used modality for detection of bone disease. Scintigraphy works by detecting abnormal activity and the rate of turnover of bone. This can be related to fracture or bone destruction. The degree of anatomic detail with nuclear medicine is relatively poor and activity which is abnormal in bone scanning is often times nonspecific, particularly in articular weightbearing joint such as the ankle which can suffer from changes related to osteomyelitis, fractures, and arthropathies.
X-ray is usually the first line in detection of fractures in the posttraumatic setting. However, with negative radiographs, MRI is the most sensitive modality for detection of occult fractures, stress fractures, bone contusions, and findings related to altered biomechanics.
An occult fracture is a fracture which was unable to be seen prospectively on other modalities. It does not matter whether or not you are able to see the x-ray in retrospect had a fracture. What matters is that prospectively the fracture was not apparent. The MRI is by far the most sensitive and specific modality for detection of fractures. X-rays are relatively insensitive, particularly in the hindfoot and midfoot when there is so much overlap. Bone scanning is a sensitive modality for fracture detection but is nonspecific, particularly in the weightbearing bone.
MRI is unique in its ability to detect bone contusions. Bone contusions are represented by increased water signal on MRI. The image on the right is a fat-sat T2 weighted image in which normal bone marrow appears dark and abnormal bone marrow is white. Bone contusions represents trabecular disruptions without a true cortical break or fracture. They are extremely conspicuous on STIR sequence and they are less conspicuous on T1 weighted images. Bone contusions are not visible on other modalities including x-rays and bone scans. Bone contusions often can indicate a mechanism of injury. Note here, you have contusions in the medial malleolus and in the sustentaculum trapping the medial third of the talus in between indicating an
inversion injury with associated kissing contusions. Bone contusions typically resolve in about 8 weeks if treated conservatively.
Next, we have a case presentation of a 9-year-old little ______ with persistent heel pain after injuring himself playing football 3 weeks before.
The image on the left is a sagittal STIR sequence of the ankle and the image on the right is an axial T1 weight sequence of the ankle. Note that the most conspicuous finding here is this well circumscribed round lesion in the middle third of the calcaneus beneath the sulcus. This lesion is intermediate in intensity to dark on the T1 weighted image and is bright on the STIR sequence typical of a bone cyst. This is a red herring in this case and is not the area of interest. If you look more posteriorly to the cyst, you see there is abnormal signal in the os calcis. Corresponding signals noted but much more subtly on the T1 weighted image, and if you look very closely, you can see that there is a subtle linear region of cortical interruption within the medullary cavity of the os calcis.
This is a stress fracture. An MRI is the only modality able to really characterize this well. On other modalities, you might be able to see the bone cyst but the stress fracture would not be apparent. MRI is also useful in being able to exclude the possibility that there is a pathologic fracture of this bone cyst.
Stress fractures represent an imbalance between the muscle and the bone strength. When a patient exhibits the new or different repetitive activity, the muscles become stronger more quickly than the bones do resulting in a stress fracture. An MRI of the fracture line is seen only in about 1/3rd of patients; however, in the appropriate clinical setting with a large degree of abnormal T1 signal and STIR signal, the accurate diagnosis of a stress fracture can be made.
Stress fractures can occur in many different parts of the foot, the metatarsal bases usually of the second and third are common areas of stress fractures. These are usually relatively easy to detect on x-ray due to the significant periostitis.
The tarsal bones, however, lack a true periosteum and therefore do not exhibit significant periosteal new bone formation and are thus invisible on x-ray. MRI, however, is extremely sensitive in being able to detect tarsal fractures. This is a patient with a diffusely abnormal signal throughout the intermediate cuneiform and you can see there is a horizontal line in the proximal third of the cuneiform representing the fracture.
Avascular necrosis can occur secondary to systemic diseases but this is relatively unusual in the foot and ankle where most cases are related to fractures. Most commonly fractures that cause AVN include the Jones fracture in the base of the fifth metatarsal and talar neck fractures. Note here, you can see extensive signal abnormality on the T1 weighted image throughout the body of the talus. On the STIR sequence below, there is corresponding diffuse bone marrow signal abnormality. Note here, you can see that there is a fracture line through the talar neck which resulted in AVN of the talus posteriorly.
Another common site of AVN in the foot is the navicular bone. As noted here, we have diffuse low intensity signal replacing the navicular on the T1 weighted image and below on the STIR sequence we have corresponding diffuse bone marrow edema. Note the signal changes distally in subchondral bone of the navicular resulted from subchondral insufficiency fracture.
MRI is exquisite in its ability to characterize osteonecrosis. The double line sign is a characteristic finding seen in avascular necrosis. When the double line sign is present, one should suspect a systemic cause because most posttraumatic AVN lacks the classic double line sign. You can see the double line sign in this patient throughout the talar dome but there is a low intensity region of sclerosis surrounding a central region of bone marrow intensity signal. The central region is devitalized bone marrow. Note also that there is a dense region of low intensity subchondral sclerosis in the talar dome which represents the devascularized, devitalized subchondral defect.
This is the corresponding sagittal STIR sequence in this patient. Remember that on the STIR sequence usually pathology and bone marrow edema appear as white. This is because of hyperemic changes. However, in osteonecrosis there is devitalization of bone and thus the absence of blood supply means the STIR sequence is going to be dark because there is no hyperemia. Note the large region of low intensity STIR signal indicating the area of devitalization. Contrast that to the diffusely abnormal signal within the distal tibia and elsewhere in the talus which reflects active hyperemia.
Well, I do not typically advocate the usage of gadolinium or intravenous contrast in MRI for the diagnosis of avascular necrosis. It is useful in being able to evaluate the true extent of devitalization of bone. Note here on the sagittal fat-sat T1 weighted sequence, the bright is representing gadolinium enhancement related to increased blood supply. The dark signal noted in the dome of the talus relates to devitalized or dead necrotic bone. Note also it is conspicuous that in the head of a talus there is second region of osteonecrosis represented by dark signal on this fat-sat T1 weighted sequence surrounded by bright hyperemic gadolinium signal.
Let us take a look at it again at the original T1 weighted image without fat saturation on the left side showing that the bone marrow in the head of the talus looks to be about the same signal intensity as a relatively normal appearing bone marrow in the calcaneus and the navicular. It is very difficult to tell without the STIR sequence and without alternatively gadolinium sequence which bone marrow is truly necrotic. See you really need to do it with multiple different sequences to be able to accurately characterize the degree of AVN.
Now, let us talk a bit about the different variety of osteochondrosis.
Osteochondral defects of the foot and ankle are most commonly seen in a talar dome. OCD laterally is typically seen at or slightly anterior to the equator of the talus and medially they are seen posterior to the equator of the talus. Osteochondral defects have variable signal intensity on imaging sequences but if there is low signal on T1 and T2 weighted images, there is probably necrosis in response to the OCD. The staging of OCD is determined by whether the cartilage layer overlying lesion is intact. Berndt and Harty classification are typically used where the type I lesion is noted here representing a small bruise in the bone. The type II lesions represent partially detached lesions.
The Berndt and Harty classification when used with MRI is based on whether there is a rim of high T2W signal around the lesion. If the high signal rim is partial, it indicates that the OCD lesion is partially loose. If the rim of high signal is complete, then it is a loose osteochondral lesion. This rim, although bright on T2 weighted images, may represent granulation tissue and not fluid. Note that in this patient, there is a completely detached but in situ osteochondral lesion in the medial third of the dome of the talus. Also, noticed that the subchondral bone is very low intensity indicating that it is necrotic.
The stage IV are the most advanced lesions which result in detachment and displacement of the osteochondral fragment leaving behind a crater as noted in this patient in the middle third of the talar dome of the sagittal T1 weighted image.
Here is the corresponding sagittal STIR sequence in this patient. Note the extensive degree of bone marrow edema beneath the osteochondral lesion. The crater is well seen, as are small cystic changes beneath the lesion. Usually, a loose body results from this unstable stage IV lesion but it is not demonstrated on this image.
Computed tomography and MRI are invaluable for assessment of tarsal coalition because they allow for differentiation of osseous from nonosseous coalitions and because they are able to depict the extent of joint involvement as well as secondary degenerative changes which are features of vital importance in surgical planning. One of the obvious features noted on the radiographs can be seen readily on MRI as seen here, the talar beak. The talar beak occurs because of impaired subtalar joint motion resulting in navicular bone overriding the talus. Periosteal elevation occurs at the insertion of the talonavicular ligament and ultimately a cycle of osseous repair results in formation of the talar beak. Note, the image on the right, the coronal T1 weighted image, shows the abnormal appearance of the sustentaculum which is abnormally broadened and downsloping. This is an example of a patient who has fibrocartilaginous coalition. There is not complete osseous union as there is an area noted in the middle of the coalition site which is not united.
Diagnosis of foot infection is a difficult clinical problem, especially in diabetic patients. Unlike other areas of the body, where hematogenous spread predominates, in the foot, direct continuous spread of infection is the rule occurring in over 90% of cases. In order to treat foot infection most efficiently and cost effectively, accurate and rapid diagnosis is essential. In order to accomplish this goal, accurate diagnosis of extent of involvement of osseous and soft tissue infection is necessary.
Radiographs are of low cost, easily accessible, and can be obtained rapidly but this modality is relatively insensitive for detection of osteomyelitis in the early stages. The earliest radiographic findings of pedal osteomyelitis is soft tissue swelling. Other osseous changes including periostitis, focal osteopenia, cortical interruption, and frank bone destruction may not be seen for another 2 weeks when the progression of an infection has resulted. CAT scanning improves the rate of detection of periostitis and cortical interruption and bone destruction over x-rays; however, there is also limited sensitivity and information regarding the extent of involvement of computed tomography and at a higher cost and thus CT is rarely used in the imaging algorithm.
The triple phase bone scan is the most commonly used scintigraphic test for evaluation of osteomyelitis. The radionuclide substance utilized technetium 99 MMDP is absorbed in two regions of rapid bone turnover resulting in hot spots. Although this test is very sensitive for the diagnosis of osteomyelitis, 90-100%, its specificity is limited since rapid bone turnover is seeing other conditions including trauma, arthritis, and tumors.
Following the initial angiographic phase, which requires the images immediately upon injection of the radionuclide and can assist in detecting the degree of blood flow, a blood pool study is performed between 5 and 15 minutes which allows for detection of the accumulation of radionuclide within the soft tissues.
By 4 hours soft tissue activity will have washed out and residual bone activity is considered abnormal. In the setting of an infection, abnormal activity on the triple phase of the bone scan is considered to be highly suggestive of osteomyelitis. However, these findings are nonspecific; arthropathy, trauma, and neoplasm can all have an identical appearance. There is little information regarding anatomic detail and extent of involvement with bone scanning. However, the test is relatively cheap and remains a cost effective method for diagnosis if a positive or negative diagnosis is the only information required.
The low specificity of bone scintography has led to the development of other radionuclide agents. Labeled white blood cell scanning is useful in conjunction with 3-phase bone scintography for increase in specificity in the diagnosis of acute osteomyelitis. However, white blood cell scanning is expensive and again provides limited anatomic detail.
Gallium-67 citrate imaging can be used for the diagnosis of infection but this also is a relatively nonspecific indicator of inflammation. Note, in this patient, abnormal activity is present in the region of second digit. This is the same patient as we saw on the x-ray two slides earlier and will also be seen in the subsequent slide with the MRI.
This is the same patient who had been taken to MRI for a better evaluation of the degree of osteomyelitis. Note the improved detail on MRI relative to bone scanning and x-ray. There is diffusely abnormal signal replacing the head of the proximal phalanx. In normal bone marrow, you should have a nice, well defined, low intensity cortex which is lost in this patient. Note also overlying the head of the proximal phalanx, you can see there is an ulcer with skin thickening and abnormal signal within the subcutaneous soft tissues. The STIR sequence below shows corresponding abnormal signal throughout the head of the first proximal phalanx and in addition abnormal signal is noted in the base of the middle phalanx.
The diagnosis of osteomyelitis and MRI relies on evaluation of primary and secondary sites. This is the first patient that we saw which had the bone scan which was diffusely abnormal at both sites of the first metatarsal phalangeal joint. Note here, on MRI this is a on top a T1 weighted short axis transverse view which shows not only skin thickening surrounding the digit with ulceration but more importantly there is diffuse replacement of the bone marrow of the medullary cavity of the first proximal phalanx. Note the normal bone marrow in the subjacent phalanges is going to be white centrally but here in the first digit, it is diffusely abnormal and infiltrative related to influx of white blood cells. On the STIR sequence below, there is corresponding bone marrow edema, which results in diffuse white signal. The medullary infiltration is the critical finding in the primary diagnosis of osteomyelitis on MRI. Contrast enhancement can be utilized to further evaluate the extent of an involvement of osteomyelitis and in particular is helpful to evaluate overlying soft tissue changes.
There is a spectrum of disease with osteomyelitis and not all abnormal signal within the bone marrow on MRI is related to osteomyelitis. Note that I emphasized on the previous slide medullary infiltration is the key to diagnosing osteomyelitis on MRI. In this particular patient, the bone marrow itself in the medullary cavity is relatively preserved. However, there is a thin rim of abnormal signal as noted on the STIR sequence below indicated by the arrows which is abnormal periosteal signal related to periostitis. This does not represent osteomyelitis but is in early stage which may progress to it.
Just as periostitis is related to overlying hyperemic changes, so is osteitis. Osteitis represents a thin linear region of subcortical edema with no evidence of cortical interruption or medullary infiltration. Note, on this patient that the low intensity cortex of the bone is relatively maintained. However, there are small regions of abnormal signal on the STIR sequence which are related to hyperemia but on T1 weighted sequence the bone marrow appears relatively preserved. So there is not overt medullary infiltration, therefore no overt osteomyelitis is present.
Contrast with this patient who had resection of the first metatarsal. The patient has lost the expected low intensity cortex at the region of the operative site and there is diffuse medullary infiltration and replacement on a T1 weighted sequence above and on the STIR sequence below. Note also the significant continuous overlying soft tissue changes delineated on the MRI. This patient has overt osteomyelitis at the postoperative site. It is difficult in the immediate postoperative setting to be able to differential postoperative changes from osteomyelitis. Usually, I recommended the patient to be at least 6 weeks out after surgery when the typical postoperative changes should have resolved.
Differentiating Charcot changes from osteomyelitis presents unusually difficult diagnostic dilemmas.
Neuropathic changes of the foot present very similarly to osteomyelitis. There are similar soft tissue changes and bone marrow edema and fluid collections. However, the location with respect to contiguous ulcers is critical in being able to differentiate osteomyelitis from a Charcot foot. Since 90% of osteomyelitis is a contiguous disease, if these changes are remote from soft tissue ulcers, it is less likely to be osteomyelitis and more likely to be neuropathic changes. With neuroarthropathy, you see deformity, disorganization, dislocation, and debris, relatively little with regard to the overlying soft tissue changes related to cellulitis when compared with osteomyelitis.
For Charcot foot, it is a juxta-articular process, most commonly involving the Charcot in the Chopart articulations followed by the ankle. With osteomyelitis, it is more typically a weightbearing or friction related location, commonly involved sites include the phalanges, the distal metatarsals, and the calcaneus. Note this patient has a large plantar ulcer beneath the calcaneocuboid articulation and there is contiguous soft tissue disease with bone disruption involving the cuboid. The bone marrow elsewhere in the foot is relatively preserved, although you can see there is significant deformity related to longstanding diabetic neuroarthropathy. However, the presence of the ulcer and contiguous bone marrow changes indicates presence of osteomyelitis involving the cuboid in this patient.
Here is a corresponding STIR sequence, which is icing on the cake. You can detect that there is diffusely abnormal signal throughout the cuboid but the remainder of the foot while significantly chronically deformed shows relatively no abnormal STIR signal.
Many imaging modalities are available for evaluation of bone and soft tissue tumors about the foot and ankle. The first modality that should always be utilized are plain radiographs. Radiographs have been used for many decades and the most literature has been written about the radiographic appearance of bone and soft tissue tumors. X-rays are exquisitely capable of evaluating for the presence of matrix within a lesion. They are also excellent at being able to evaluate whether the lesions are blastic or lytic. For improved resolution radiographically, CAT scanning is utilized. CAT scanning is able to evaluate the extent of bone destruction better than any other modality. MRI, however, is better able to evaluate the presence of marrow infiltration in advance of bone and cortical destructive changes. So MRI is actually the most sensitive modality because it is able to evaluate the true extent of tumor within the marrow cavity. In addition, MRI is best able to evaluate for accompanying soft tissue changes in masses. Bone scintography is primarily utilized to evaluate the activity of a lesion and to evaluate for the presence of diffuse metastatic disease and for screening of the osseous skeleton.
In the foot and ankle when evaluating soft tissue tumors, it is easiest to know the differential diagnosis based on the location of the mass. If the mass involves tendon sheaths, it is most likely to represent a fibroma, rheumatoid nodule, giant cell tumor of tendon sheath, PVNS, or ganglion cyst. If the mass lies along the plantar aspect of the foot, it is most likely related to foreign body granuloma, plantar fibroma, or rheumatoid nodule. Distally in the toes and forefoot, inclusion cysts are most common, as well as glomus tumors and changes related to psoriatic arthritis.
Giant cell tumor or pigmented villonodular synovitis are a spectrum of disease quite similar to one another. These processes arise from the synovial lining of tendon sheaths, bursae, or diarthrodial joints, and usually occur in young adults. Microscopically, the most characteristic feature are hemosiderin-laden macrophages. The presence of hemosiderin results in dark signal on both T1 weighted images and STIR and T2 weighted images. Note here on the top is a sagittal T1 weighted image showing a large lobulated mass, which is dumbbell shaped around the first metatarsal phalangeal joint. On the bottom is a STIR sequence at the same level. Note on the T1 weighted image how the mass is dark and on the STIR image how it is also dark in signal intensity. These features indicate that it is a very highly cellular tumor and the presence of macrophages diminishes the signal intensity, which is a characteristic feature of giant cell tumor or PVNS.
Localized proliferation of a fibrous tissue arising from the plantar aponeurosis is a common finding known as plantar fibromas. They present as hard, fixed masses and frequently are tender. The role of trauma in its development is unclear; however, the incidences increase in patients who have diabetes and seizure disorders. On MRI, fibroma is present as well defined low intensity masses along the plantar aponeurosis. The low intensity in nature of this process is characteristic due to its highly cellular nature.
The multiplanar capabilities of MRI render it particularly useful in evaluation of soft tissue tumors in evaluating their extent. Plantar fibromas can invade deep musculature. Notice here on the T1 weighted transverse short axis view, on the left side, there is fusiform expansion of the plantar aponeurosis overlying flexor digitorum brevis muscle belly. On the right side is the corresponding STIR sequence revealing the aponeurosis to be enlarged and relatively low intensity indicating the highly cellular nature of this tumor. There is subjacent soft tissue edema but no evidence of deep invasion.
Here is a case presentation of a 54-year-old with unremitting, persistent burning pain in the sole of the foot which radiated down to the toes and up into the leg. The pain was worsened with activity and reduced by rest.
This is a typical presentation of a patient with posterior tibial nerve entrapment or compression. An MRI was obtained. On the left is a sagittal T1 weighted sequence and on the right there is a sagittal STIR sequence. Notice that there is a mass behind the sustentaculum which is intermediate in intensity on the T1 weighted image and exhibits bright signal with a small central region of low intensity on the STIR sequence. The signal characteristics here suggest this represents a lesion which has relatively low cellular component. Often times, these lesions can represent ganglions but a more aggressive tumor such as a sarcoma cannot be excluded because of the small region of low intensity STIR signal which suggests it may contain a solid component as well.
Here is an axial T2 weighted sequence showing interpose between flexor digitorum and flexor halluces longus tendons is a well circumscribed fluid intensity lesion consistent with a ganglion cyst. Note that this lesion is contacting and compressing the posterior tibial nerve resulting in plantar pain and paraesthesias. Tarsal tunnel masses are commonly related to ganglion cysts, neuromas, and varicosities. They can be related to synovial hypertrophy and scar tissue, which can fibrose along the posterior tibial nerve as well.
Note on this patient is an axial fat-sat T12 weighted sequence. There are large dilated venous varicosities within the tarsal tunnel resulting in posterior tibial nerve compression.
In the foot and ankle, compressive and entrapment neuropathies can effect the posterior tibial nerve as just discussed with tarsal tunnel syndrome. Masses can impinge upon the sural nerve. Trauma most commonly results in impingement and symptoms of the deep peroneal nerve known as anterior tarsal tunnel syndrome and masses such as Morton neuromas can impinge upon the plantar nerve resulting in significant symptoms in the forefoot.
Here is a case of a 36-year-old woman with pain in the ball of the foot with weightbearing, painful catching sensation when walking, and sharp pains radiating to the third digit.
An MRI was performed of the forefoot in this patient to evaluate for plantar nerve impingement. Note, on the left is a longitudinal or long axis T1 weighted image of the forefoot and on the right there is a STIR sequence. There is a mass as indicated by the arrows in the third interspace which is low intensity on the T1 weighted sequence and relatively low intensity on the STIR sequence as well indicating it contains a highly cellular elements.
The clinical presentation and the MRI findings are classic for a Morton neuroma.
A Morton neuroma is not truly a neuroma but actually it results from perineural fibrosis impinging upon the plantar interdigital nerve. Most commonly, neuromas occur in the 3rd interspace and less commonly in the 2nd and 4th interspaces. You can see on this transverse T1 weighted image, there is a bulbar low intensity mass projecting between the 3rd and 4th metatarsal heads, which represents the Morton neuroma. Differential diagnosis of interspace symptoms can result from bursitis, capsulitis, stress fracture, and ganglion cyst. Obtaining MRI can be very useful in differentiating these entities.
While not routinely necessary, gadolinium can be particularly helpful in differentiating postoperative fibrosis from Morton neuroma. Note on this patient, gadolinium was administered and on the fat-sat T1 weighted image below you can see that there is abnormal enhancement between the 3rd interspace corresponding to the region of abnormal mass on the T1 weighted sequence above. This patient had a recurrent or stump neuroma in this location.
This patient had prior surgery related to os trigonum and presented with numbness in the dorsal aspect of the lateral side of the foot. MRI revealed a large mass compressing and entrapping the sural nerve. Sural nerve entrapment can result from ganglion cysts, fibrosis secondary to ankle sprains and prior surgery, fractures, particularly involving calcaneus and the fifth metatarsal, as well as Achilles tendon ruptures.
Ganglion cysts are thin walled cysts filled with clear mucinous fluid which generally occurs in periarticular locations. They are believed to represent either synovial herniation of mucinous degeneration of a dense fibrous connective tissue. Occasionally, they may erode into bone and become interosseous. Synovial ganglion cyst demonstrate low signal intensity on T1 weighted images and bright signal on T2 weighted images. They may be septated and outlined by bright signal intensity of fluid on T2 weighted images. With hemorrhage, ganglion may appear similar to malignancies; however, they should not enhance following gadolinium administration.
Another frequently encountered fluid intensity lesion is the epidermal inclusion cyst. Epidermal inclusion cyst often result from the implantation of epithelial elements in the dermis. They are generally slow growing purely cystic lesions and should be low intensity on T1 and homogenously bright on T2 weighted images. Epidermal inclusion cysts do not enhance; however, they may become infected. Often times there is a history of trauma and this may be related to foreign body reaction. While on the topic of foreign bodies, let me touch on the subject of which modality I feel is best suited to evaluate for foreign bodies. Radiographs are only sensitive in a small number of cases for foreign bodies. Metallic foreign bodies can certainly be detected on x-rays as can some sorts of glass which contain lead, but more commonly foreign bodies such as wood, toothpicks cannot be detected on x-rays nor can they be detected on MRI because they do not have any mobile protons. They do not have any fat. They do not have any water elements. So ultrasound is actually the best modality to detect foreign bodies. Ultrasound is extremely sensitive at being able to detect foreign bodies; however, it is very operator dependent. Here is an example of a patient who stepped on a toothpick which was radiolucent and unable to be detected on any other modality. On ultrasound, the lesion is conspicuous, presents as bright echogenic on the ultrasound and exhibits distal acoustic shadowing. Ultrasound is very useful to be able to localize and size foreign bodies, particularly in the preoperative setting.
In summary, I offer you the following guidelines to determine which modalities should be used in general for particular pathologic presentation. When you are looking for a fracture that you are not sure exists, MRI is the best modality, it is basically able to detect exactly where the fracture is and also it is excellent at being able to detect the extent of the fracture. For characterization of soft tissue masses, MRI excels at being able to differentiate cystic from solid lesions, as well as getting closer to be able to diagnose the pathology such as whether or not it has a highly cellular or a low cellular nature. For osteomyelitis and neoplasm, MRI best determines the extent of marrow infiltration. To evaluate for the presence of osteomyelitis in a nonarticular bone, for instance the distal tibia, bone scintography is the best way to go, but when you are looking in articular bone where you can have the trauma and arthritis confounding the clinical presentation, MRI is the best way to go. In general, if you are not sure what is the causing the problem and you basically want a nice evaluation of the anatomy looking for pathology that may be unsuspected, MRI is going to be the best thing to be able to evaluate for tendons, ligaments, and bone pathology, as well as cartilaginous pathology. If you have a patient with a complex fracture particular distal tibia pilon fractures and calcaneal fractures, CAT scanning is by far the best modality to be able to show you exactly where all the bone fragments lie. If you are looking to see whether or not there is an occult foreign body, ultrasound is the best way to evaluate. MRI is insensitive to most foreign bodies because they do not contain mobile protons, and foreign bodies that are not radiographically evident would not show up on a CAT scan. So ultrasound is the best way to evaluate for foreign body.
I hope you have enjoyed this lecture. Please also be sure to check out the lecture on MRI in the tendons and ligaments.