Книги по МРТ КТ на английском языке / MRI for Orthopaedic Surgeons Khanna ed 2010

Fig. 16.5 3.0-T MRI. (A) An axial oblique multiplanar reconstruction image of the knee from a 3D isotropic image data set superbly shows the medial and lateral meniscal anatomy. (B) An axial T2*-weighted image (a T2-like image [bright fluid] created with a gradient-

strength MRI systems (higher signal-to-noise ratio), which facilitates a shift toward rapid, volumetric image acquisitions. Recently, major vendors of MRI scanners have implemented versions of parallel imaging in several commercial systems.9

High-Resolution Imaging: MR Microscopy

MRI with microscopy coils o er much higher signal-to- noise ratio, higher contrast-to-noise ratio, and higher spatial resolution than does a conventional small surface coil. For example, resolution of conventional MRI has not permitted detailed evaluation of the entire TFCC, but primarily the triangular fibrocartilage proper (disc). However, the triangular ligament (upper and lower lamina), meniscus homologue, and ulnotriquetral ligament are clearly seen with microscopy coils. High-resolution MRI with a microscopy coil is a promising method with which to diagnose TFCC abnormalities and other ligamentous lesions (Fig. 16.6).

Microscopy coils are limited by inadequate depiction of deeper structures. This issue may be addressed by combined positioning with a larger surface coil or a flexible coil. In addition, the limited sensitivity of microscopy coils may make accurate coil setting over a targeted structure or lesion difficult. Superconducting coils have also been used for small joint imaging. Overall, it is likely that advanced coil develop-

echo rather than an SE technique) at the C4-C5 level of the spine shows a high signal-to-noise ratio with high spatial resolution, providing excellent visualization of the ventral and dorsal roots in addition to a small central disc protrusion.

ment will lead to improved diagnostic performance of MRI because high-resolution imaging is paramount for the depiction of infrastructural features of the wrist and elbow when evaluating internal derangements.

Extremity Scanners

Low field strength open MR scanners and extremity scanners have been used for several years. Recently, a higher field 1.5-T extremity scanner (ORTHONE, ONI, Inc., Wilmington, MA) has become available clinically.10 This device can image the elbow, hand, wrist, knee, foot, and ankle, but it cannot acquire images of the shoulder, hip, or spine. Therefore, it is not considered as a stand-alone unit, but it may be useful as a supplement to a whole body scanner for a highvolume site with a backlog of musculoskeletal patients. Because there are fewer site requirements for this device than for a conventional high field strength system, an extremity scanner is often an economical imaging option. Additionally, extremity scanners may prove useful for imaging claustrophobic patients because the image quality of a 1.5-T extremity scanner is much better than that of a typical low field strength open MRI or a very low field strength extremity scanner. This type of device will likely fulfill a niche role, providing high-quality clinical images for designated body parts (Fig. 16.7).

Fig. 16.6 These FSE proton-density images (top row) and coronal T2*-weighted images (T2-like images [bright fluid] created with a gradient-echo rather than a SE technique) (bottom row) of the TFCC were obtained with a 1.5-T MRI scanner and a conventional 80-mm diameter surface coil (left column), a 47-mm diameter microscopy surface coil (middle column), and a 23-mm diameter microscopy sur-

RF coils for extremity scanners are available in two sizes. Both are quadrature types of volume transmit-receive coils, which yield improved image quality compared with that of traditional surface coils. Extremity scanners o er several advantages compared with high field strength closed MRI systems:11

•Images are high quality despite lower field strength.

•Artifacts from o -isocenter imaging and signal-to- noise ratio loss are minimized.

•Most pulse sequences (including spectral fat suppression) are available.

Intraoperative MRI

New operating suites have been designed that incorporate MRI scanners, which allow a patient undergoing surgery to

face coil (right column). MRI with microscopy coils provided higher signal-to-noise ratios, higher contrast, and improved spatial resolution and depiction of the infrastructural details of the TFCC. (Courtesy of Hiroshi Yoshioka, MD, Department of Radiology, University of Tsukuba, Tsukuba, Japan, and Harvard Medical School, Brigham and Women’s Hospital, Boston, MA.)

be imaged intraoperatively. Careful architectural planning and MRI-compatible surgical and anesthesia equipment allow for this new technique, which has been primarily used in neurosurgery. The value of such information has been shown to outweigh the cost of time necessary for image acquisition.12 Low field strength open MRI units are common because of their lower cost and more flexible integration into the operative suite.13 However, image quality is improved with high field strength closed magnets; such systems have been mounted on ceilings of neurosurgical operating rooms to image the brain.13 Intraoperative MRI has the capacity to detect residual tumor during a di cult resection, define complex anatomy during orthopaedic procedures, and assess for complications. Although the potential for this technique is great, additional studies are needed to define the role of this technology in an orthopedic setting.

Fig. 16.7 Images obtained with an open high field strength extremity scanner. (A) This coronal oblique STIR image shows a characteristic focal signal abnormality along the ulnar aspect of the lunate, round fluid region (reflecting cystic change), and surrounding marrow edema (arrow), consistent with a wrist ulnolunate abutment.

(B) This sagittal T2-weighted image with spectral fat suppression shows tendinosis, intrasubstance linear fluid signal (arrow) (reflect-

■Positional, Load-Bearing, or Dynamic (Functional) Imaging

Positional, load-bearing, or dynamic (functional) imaging of the spine has proven useful because imaging in the supine position may not fully reveal relevant pathology. Available options for imaging the spine include the supine, supine with axial loading (simulated weight bearing), seated, and standing upright positions.

Simulated weight bearing is performed by applying an axial load during supine imaging, which may be accomplished by having the patient wear a hardened plastic vest (DynaWell L-Spine, DynaWell, Billdal, Sweden).14 This device, which is free of material that would disturb the magnetic field in the MRI scanner, is strapped over the patient’s shoulders and upper chest. The patient’s feet are placed against the footplate of a compression device, which is attached to two adjustable cords, one on each side of the vest. By tightening the cords to a desired measured load (up to 50% of body weight), it is pos-

ing interstitial tearing), and associated retrocalcaneal bursitis (arrowhead), consistent with Achilles tendinopathy. These images show high signal-to-noise ratio, good contrast resolution, and spatial resolution similar to that of high field strength closed systems. Adequate and uniform fat suppression is obtained because the region of interest is located at the magnet’s isocenter, avoiding inadvertent water suppression.

sible to provide compression that is similar to that of upright posture. This axially loaded position may reveal pathologies such as spondylolisthesis (see Fig. 13.42), kyphosis, and disc herniations that are otherwise not seen in a supine position (Fig. 16.8).

MRI in the seated position is possible in a specially designed vertically open intraoperative 0.5-T magnet (Signa SP, General Electric Medical Systems, Milwaukee, WI) configured as a double bore with a 60-cm gap, which has been likened to a “double donut.”15 A seat for the patient may be placed between the “donuts” in the center of the bore, allowing imaging of the lumbar spine in neutral, flexion, and extension positions for a seated patient, providing a form of dynamic imaging. The seated position provides an axial load that may reproduce back pain symptoms for some patients. This type of imaging has shown physiologic changes of the spinal canal; for example, the cross-sectional area of the spinal canal and neural foramina is smallest in the extended position (Fig. 16.9). Conventional MRI may show abnormalities that may take on greater importance because

Fig. 16.8 Load-bearing spine imaging: simulated weight-bearing paradigm. These axial T1-weighted images were obtained at the level of the intervertebral disc in a conventional horizontal-bore 1.5- T MRI scanner with the use of a specially designed compression device (DynaWell Corp.) in a patient with neurogenic claudication. (A) A

of encroachment or neural impingement on dynamic loadbearing (seated) sequences.

Another option for functional musculoskeletal imaging is upright, weight-bearing, dynamic-kinetic MRI of the spine. A specially designed 0.6-T magnet (Stand-Up MRI, Fonar Corporation, Melville, NY)16 allows imaging of the standing patient. This system is applicable for imaging of the cervical, thoracic, and lumbar spine. This scanner is useful for normal positional and kinetic images, for showing position-related disc protrusions in the spine worsening with extension, and for showing fluctuating positional foraminal and central spinal stenosis in the spine between recumbent and upright neutral positions. This type of imaging has led to a concept of fluctuating kinetic central spinal stenosis (fluctuating fluid disc herniation) that can be shown only by imaging in various upright positions (Fig. 16.10). Although a cervical spine MRI in the recumbent position may show posterior osteophytes, an upright MRI with cervical extension may reveal cord compression.

Back pain often occurs in weight-bearing situations, so positional imaging has great potential for showing pathology that may be subtle or inconspicuous on conventional supine MRI. However, the role of imaging the hip, knee, and ankle under an axial load has not been fully investigated. If supine-simulated weight-bearing techniques become validated, traditional magnets can be used without having to

view without axial loading shows a mild disc bulge without substantial stenosis. (B) A view with axial loading shows a focal protrusion type disc herniation and increasing stenosis. (Courtesy of Per Lennart Westesson, MD, PhD, DDS, University of Rochester Medical Center, Rochester, NY.)

deploy new, costly, space-occupying devices.17 Additional studies comparing simulated weight-bearing and upright imaging are needed to determine if new magnets are required for this purpose. Load-bearing and dynamic imaging may not be necessary to show relevant pathology for all patients; patient selection criteria need to be developed so that this technology can be applied appropriately.

■ Novel Pulse Sequences

Although often useful, conventional musculoskeletal MRI pulse sequences may yield nonspecific findings. Several new imaging techniques may help narrow a di erential diagnosis and further characterize a disease entity. For example, inand out-of-phase chemical-shift imaging uses an MR artifact to determine if there is a loss of normal marrow fat by an infiltrative process. Normal marrow and benign entities (such as vertebral fractures, hemangiomas, or Schmorl nodes) show loss of signal on out-of-phase images, which is not seen in neoplastic processes (Fig. 16.11). Chemical-shift sequences are therefore useful for determining whether heterogeneous marrow signal on T1-weighted images reflects tumor involvement or benign processes in the axial skeleton.18,19

Di usion-weighted imaging is based on Brownian motion of water molecules in tissues. Pathologic processes typically

Fig. 16.10 Load-bearing spine imaging: standing positional paradigm. These sagittal T2-weighted views were obtained in a specially designed open 0.6-T magnet that allows for horizontal (recumbent) and vertical (upright) imaging (Fonar Corp.). This patient had recurrent radiculopathy 8 months after partial discectomy, with symptoms only when upright. (A) A supine image shows no focal contour

method for evaluating the malignant potential of tumors (Fig. 16.12).

■MRI in the Presence of Metallic Implants

Susceptibility artifacts from metallic orthopaedic implants have historically limited the usefulness of MRI in the postoperative setting. Although radiography, conventional arthrography, and nuclear medicine studies are accessible and cost-e ective, these imaging modalities have limited sensitivity and specificity for common postoperative clinical questions. With minor adjustments to routine pulse sequences, MRI can be a highly accurate tool for the evaluation of postoperative conditions, with excellent visualization of periprosthetic tissues.

Metallic objects cause local magnetic field distortions, which lead to varying degrees of misregistration and characteristic artifacts that are worsened at higher magnetic field strengths. Titanium and tantalum implants create sub-

abnormality at L5-S1. (B) An upright-neutral image reveals a posterior disc herniation at the L5-S1 level elevating the posterior longitudinal ligament (arrow), which was not visible on the supine image. (Courtesy of J. Randy Jinkins, MD, FACR, FEC, Downstate Medical Center, State University of New York, Brooklyn, NY.)

stantially less artifact than does stainless steel,24,25 and such artifacts are reduced when the long axis of a metallic object is parallel to the long axis of the magnet. Appropriate patient positioning and imaging protocols can dramatically reduce such artifacts. Metallic interference with the local magnetic field limits the usefulness of fat-suppression techniques; STIR sequences avoid this problem and are preferred for generating T2-weighted images.26 Additionally, FSE/ turbo-spin sequences are typically less susceptible to metallic artifacts than are conventional SE techniques.27 Smaller fields of view also help to limit the influence of metallic objects.28

The diagnostic accuracy of metallic artifact reduction sequences compared with that of conventional MRI techniques is well established. Artifact reduction sequences are particularly important for imaging the spine, where key findings are routinely missed without appropriate imaging protocols.29,30 MRI of hip, knee, and shoulder arthroplasties can be of more diagnostic value than CT, conventional radiography, or nuclear scintigraphy, particularly for the evaluation of common pathology (such as mechanical loosening, infection,

C

Fig. 16.11 Chemical-shift imaging: biopsy-proven benign vertebral compression. These lumbar spine images were obtained with a 1.5- T system. (A) A sagittal T1-weighted SE image. (B) A sagittal STIR image. (C) A sagittal gradient-echo in-phase (TE = 4.2 ms) image.

(D) A sagittal gradient-echo out-of-phase (TE = 2.1 ms) image. The

D

T11 vertebra shows a di use compression deformity and bone marrow edema (arrow on B). There is also a compression deformity at L1 with only minimal superior end-plate edema. The T11 vertebra signal intensity measured 185 on the in-phase image and 132 on the out-of- phase image, corresponding to a proportional decrease of –29.8%.

Fig. 16.13 MRI in the presence of metallic implants. This patient had posterior spinal fixation at the T1 vertebral level. (A) An axial gradi- ent-echo T2*-weighted image (a T2-like image [bright fluid] created with a gradient-echo rather than a SE technique) shows a large signal void obscuring the spinal canal. (B) An axial T2-weighted image with

FSE/turbo-spin technique at the same level in the same patient during the same imaging examination minimizes the susceptibility artifact from the metallic implants and substantially reduces the signal void so that the spinal canal contents are visible.

A

Fig. 16.14 MR angiography: vascular malformation. Frontal projection views of a left-thigh MR angiogram shows (A) filling of the superficial femoral artery to the level of the popliteal fossa accompanied

B

by early filling of the profunda femoris artery and (B) an enhancing vascular malformation (arrow) only after the profunda femoris artery is filled, thus establishing it as the feeding vessel.