Ankle and Foot 47 - Department of Radiology - University of ...

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2238 VII Imaging of the Musculoskeletal System flexion and dorsiflexion. US is used to evaluate small superficial structures that are sometimes difficult to see on MRI, such as Morton’s neuromas or the plantar plate. US can also be used to characterize soft tissue masses, particularly to assess the degree of vascularity or to determine if the mass is cystic or solid. US is also extremely sensitive for the detection of subcutaneous foreign bodies in the extremities, particularly wooden splinters, which can be difficult to detect with radiographs, CT, or MRI. (A discussion of US of the ankle and foot is beyond the scope of this chapter.) Nuclear medicine (NM) plays a limited role when it comes to imaging the ankle and foot, although in certain circumstances a bone scan can be helpful. Fractures of the sesamoid bones of the great toe tend to be less conspicuous on MRI than on bone scans, especially when the both-feeton-detector view is used. In neuropathic feet in which radiographs show Charcot changes of collapse and bone fragmentation, a bone scan combined with a white blood cell scan can be as sensitive and more specific for the detection of osteomyelitis than MRI. (A discussion of nuclear medicine is beyond the scope of this chapter.) • Radiography Because radiographs are a necessary first step in the workup of the ankle or foot, let us now briefly review how these should be obtained. • Ankle Radiography Ankle radiographs can be either weight bearing or non– weight bearing, depending on the preference of the ordering clinician. A standard radiographic ankle series consists of three projectional views: anteroposterior (AP), mortise, and lateral (Fig. 47-41). The mortise view is similar to the AP view, with the leg internally rotated 15 degrees to obtain a better profile of the ankle mortise. When obtaining radiographs of the ankle, it is important that the technologist include the base of the fifth metatarsal on at least one view. Patients with fractures of the base of the fifth metatarsal clinically present complaining of lateral ankle pain, and this can cause the clinician to request radiographs of the ankle rather than of the foot. Figure 47-41 is such a case, where the Jones* fracture can be seen at the edge of the lateral view. *Sir Robert Jones (1857-1933) was the father of orthopedic surgery in England and revolutionized the care of wounded soldiers during World War I. An early proponent of x-rays, Jones imaged the transverse extra-articular fracture across the proximal diaphysis of the fifth metatarsal just a few months after Röntgen published “On a New Kind of Rays” (December 28, 1895). Jones first described this fracture after having sustained such an injury himself “whilst dancing.” (This was not ballroom dancing; rather it was “dancing in a circle round the tent pole” with his military colleagues. There was no mention as to whether alcohol was involved.) He subsequently identified this fracture on radiographs of two other patients and published his series of three in the Annals of Surgery in 1902, “Fracture of the Base of the Fifth Metatarsal Bone by Indirect Violence.” • Foot Radiography It is preferable to obtain radiographs of the foot with the patient standing to visualize the bones in their weightbearing alignment. 5 AP and oblique views (Fig. 47-42A and B) can be obtained by placing the x-ray cassette on the floor and having the patient stand on the cassette while the x-ray beam is pointed downward. It is important to closely scrutinize the alignment of the tarsometatarsal joints on both of these views when assessing for a Lisfranc fracturedislocation. Normally, the first metatarsal should line up perfectly with the first (medial) cuneiform, the second metatarsal with the middle cuneiform, the third metatarsal with the third cuneiform, and the fourth and fifth metatarsals with the cuboid. The standing lateral view of the foot (Fig. 47-42C) is somewhat more difficult to obtain because it is usually not possible to lower the x-ray tube all the way down to the floor. Consequently, we use a set of wooden steps (Fig. 47- 43). This elevates the feet to a level where the x-ray beam can be oriented horizontally while the cassette is held between the feet. Figure 47-44 is an example of differences that can be seen between standing and non–weight-bearing views. • Computed Tomography • Overview Bone CT protocols have evolved as scanner technology has progressed. In the broadest terms, a CT gantry consists of a spinning ring on which is mounted an x-ray tube. The tube emits a fan-shaped x-ray beam, aimed through the center of the ring to an array of x-ray detectors mounted on the other side. The patient lies on a padded table that moves through the spinning gantry. With early generations of single-slice CT scanners the gantry would spin clockwise one rotation, then stop and spin counter-clockwise one rotation to prevent tangling of the power cables supplying the x-ray tube. The patient table would be stationary during each of the scanning rotations while the gantry was spinning and the tube was emitting x-rays. The table would move between gantry rotations and stop at each slice position. These were the days of true CAT, in which “A” stood for “axial,” and scans consisted only of a series of axial slides. Scans were relatively slow because time was lost stopping the gantry’s clockwise momentum to reverse its rotation. With the innovation of slip-ring technology, tangled power cables were eliminated and the gantry could spin in one direction continuously while the table moved continuously through it. Thus, helical CT (also known as spiral CT, analogous to a spiral-sliced ham) was born. Now the data stream coming out of the x-ray detectors no longer represents individual axial slices, but rather a continuous volume of patient imaging information. The raw data are then reconstructed into a series of axial slices that we refer to as Ch047-A05375.indd 2238 9/9/2008 5:34:14 PM
47 Ankle and Foot 2239 47 Figure 47-41. Non–weight-bearing radiographic ankle series in a 37-year-old with lateral ankle pain after an acute inversion injury. Anteroposterior view (A) and mortise view (B) demonstrate a normal appearance of the ankle joint. C, The lateral ankle view reveals no abnormalities of the hindfoot. (The region outlined by the dashed rectangle is magnified and displayed to the right). Close inspection of the base of the fifth metatarsal on the lateral view of the ankle reveals a proximal diaphyseal fracture, a Jones fracture (arrow). Fractures of the base of the fifth metatarsal often present clinically as lateral ankle pain. The technologist must be careful always to include the base of the fifth metatarsal on at least one view of all ankle radiographic series. A B C the source images. Because of its volumetric nature, helical data can be reconstructed at any slice width, and with any interval spacing between slices. These axial source images can then be reformatted into two-dimensional slices in any desired plane and of any desired width, or into threedimensional volume-rendered images. In the past decade, single-slice helical scanners have evolved into multislice scanners, able to acquire larger volumes of patient data with each gantry rotation. This technology has largely been driven by the desire to scan the entire chest within a single breath-hold and the coronary arteries in a single heartbeat. Although a multislice CT scanner is not absolutely required for bone CT, covering the desired volume faster minimizes artifacts related to patient motion as well as minimizing the amount of time the patient has to lie still on the scanner table. Thus, the modern bone CT scan consists of the acquisition of three sets of imaging data. The raw data tend not to be archived; they are temporarily stored on the scanner’s hard drive and are overwritten as the hard drive becomes full (often after 24 hours). The source images are reconstructed from the raw data using a variety of filtered backprojection algorithms. These images are oriented in a plane axial to the scanner gantry. Once the raw data are overwritten, no additional source images can be reconstructed; thus, it behooves the CT technologist to create whichever source image data sets are needed for future reformats. These source images can be viewed by the radiologist as desired and can be sent to the picture archiving and communications system (PACS) for short- or long-term storage. However, the multiplanar two- and three-dimensional images reformatted from the source images are the ones primarily used for diagnostic and planning purposes and ultimately sent to the PACS for archiving. Achieving the highest-resolution two-dimensional reformatted images requires the source images to be Ch047-A05375.indd 2239 9/9/2008 5:34:15 PM
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