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

the upper and lower extremities in some centers for certain situations.34 However, the widespread availability of CT angiography and rapid 3D postprocessing techniques have limited the use of MR angiography in the acute setting. MR angiography provides excellent definition of complex

vascular anatomy, which can be helpful for preoperative planning.

Gadolinium-enhanced MR angiography o ers clear definition of vasculature with short image acquisition times. Intravenous gadolinium shortens intravascular T1

■ MRI-Guided Interventions

The use of MRI for image-guided musculoskeletal interventions has increased in popularity over the past several years.

In many respects, open interventional MRI systems o er many of the advantages of ultrasound and CT, such as ultrasound’s real-time visualization of tissue and CT’s excellent visualization of bone.40

A

Fig. 16.16 Interventional MRI: cryotherapy. This 50-year-old man had renal cell carcinoma metastatic to the proximal right femur.

(A) Sagittal T1-weighted image shows a cryotherapy needle in the hypointense metastasis in the region of the right lesser trochanter.

B

(B) Sagittal T1-weighted image shows an elliptical signal void of the ice ball, representing the treatment zone. Critical structures avoided were the femoral nerve, femoral vessels, and iliopsoas tendon attachment.

MR-guided cryotherapy of soft-tissue and bone metastases has been shown to be a safe and e ective technique, and it can provide excellent local tumor control and pain relief in the appropriate clinical setting.39 The e cacy of MR-guided cryotherapy is based on the fact that MRI is sensitive to temperature changes within tissue and that cryoablated tissue is easily recognized (and approximates tissue death) on standard MRI pulse sequences. Cryoablated tissue produces a signal void on standard pulse sequences and has been referred to as an ice ball39 (Fig. 16.16). MRIguided percutaneous cryotherapy reduces morbidity by using a percutaneous approach, and it o ers the advantage of estimating the volume of tissue ablation during the procedure.39

MR-guided interventions have also been shown to be safe and e ective when used for numerous pain management techniques, including the following:

•Sacroiliac joint injections

•Discography

•Transforaminal epidural injection

•Selective nerve block

•Sympathetic block

•Celiac plexus block

•Facet joint cryotherapy neurotomies36,37

The excellent depiction of soft-tissue structures provides a distinct advantage when isolating nerves and facet joints for injections. MR is also useful for recognizing reactive edema adjacent to arthritic joints.

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3.Sahin G, Demirtaş M. An overview of MR arthrography with emphasis on the current technique and applicational hints and tips. Eur J Radiol 2006;58:416–430

4.Sciulli RL, Boutin RD, Brown RR, et al. Evaluation of the postoperative meniscus of the knee: a study comparing conventional arthrography, conventional MR imaging, MR arthrography with iodinated contrast material, and MR arthrography with gadolinium-based contrast material. Skeletal Radiol 1999;28:508–514

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6.Ramnath RR. 3T MR imaging of the musculoskeletal system (Part I): considerations, coils, and challenges. Magn Reson Imaging Clin N Am 2006;14:27–40

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12.Lewin JS, Nour SG, Meyers ML, et al. Intraoperative MRI with a rotating, tiltable surgical table: a time-use study and clinical results in 122 patients. AJR Am J Roentgenol 2007;189:1096–1103

13.Jolesz FA. Future perspectives for intraoperative MRI. Neurosurg Clin N Am 2005;16:201–213

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17.Shellock FG. Functional assessment of the joints using kinematic magnetic resonance imaging. Semin Musculoskelet Radiol 2003;7:249– 276

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20.Baur A, Stäbler A, Brüning R, et al. Di usion-weighted MR imaging of bone marrow: di erentiation of benign versus pathologic compression fractures. Radiology 1998;207:349–356

21.Aboagye EO, Bhujwalla ZM. Malignant transformation alters membrane choline phospholipid metabolism of human mammary epithelial cells. Cancer Res 1999;59:80–84

22.Fayad LM, Bluemke DA, McCarthy EF, Weber KL, Barker PB, Jacobs MA. Musculoskeletal tumors: use of proton MR spectroscopic imaging for characterization. J Magn Reson Imaging 2006;23:23–28

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24.Burtscher IM, Owman T, Romner B, Ståhlberg F, Holtås S. Aneurysm clip MR artifacts. Titanium versus stainless steel and influence of imaging parameters. Acta Radiol 1998;39:70–76

25.Wang JC, Yu WD, Sandhu HS, Tam V, Delamarter RB. A comparison of magnetic resonance and computed tomographic image quality after

the implantation of tantalum and titanium spinal instrumentation. Spine (Phila Pa 1976) 1998;23:1684–1688

26.Viano AM, Gronemeyer SA, Haliloglu M, Ho er FA. Improved MR imaging for patients with metallic implants. Magn Reson Imaging 2000;18:287–295

27.Guermazi A, Miaux Y, Zaim S, Peterfy CG, White D, Genant HK. Metallic artefacts in MR imaging: e ects of main field orientation and strength. Clin Radiol 2003;58:322–328

28.Lee MJ, Kim S, Lee SA, et al. Overcoming artifacts from metallic orthopedic implants at high-field-strength MR imaging and multi-detector CT. Radiographics 2007;27:791–803

29.Rudisch A, Kremser C, Peer S, Kathrein A, Judmaier W, Daniaux H. Metallic artifacts in magnetic resonance imaging of patients with spinal fusion. A comparison of implant materials and imaging sequences. Spine (Phila Pa 1976) 1998;23:692–699

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34.Leiner T. Magnetic resonance angiography of abdominal and lower extremity vasculature. Top Magn Reson Imaging 2005;16:21–66

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Although the preceding chapters have focused on MRI of the musculoskeletal system, it is important for the clinician to recognize that other imaging modalities play an important role in the evaluation of patients with musculoskeletal disorders. These other commonly used imaging modalities include conventional radiography, CT, nuclear scintigraphy studies, and positron emission tomography. When the clinician is evaluating the patient for a particular disease process, a basic understanding of the strengths and weaknesses of these modalities is important in deciding which imaging study to request and evaluate for the patient. For many patients, one or more imaging modalities will be superior to MRI alone. In addition, after the patient has been evaluated with an MRI study, the other imaging modalities may provide additional anatomic or physiologic information that can help narrow the di erential diagnosis and guide treatment.

■ MRI

MRI provides excellent visualization of the tissues in the musculoskeletal system in multiple planes because of its high contrast resolution compared with other modalities. One of its major advantages is that the multiple pulse sequences (see Chapter 1) enable the detection of soft-tissue and bone-marrow pathology with great sensitivity (Fig. 17.1). In general terms, an MRI study is often obtained to evaluate the water content of tissues because most acute and subacute pathology results in free extracellular fluid. MR pulse sequences use the magnetization and relaxation properties of protons to assess and di erentiate various tissue types. Tissues that contain larger amounts of water (e.g., CSF or joint fluid) are bright on fluid-sensitive sequences, and tissues that contain lesser amounts of water (e.g., cortical bone or physeal scar) are dark on all pulse sequences. Structures that contain little or no water are not well assessed by MRI. For example, the lungs have a large amount of air; therefore, although MRI is an advanced technique, it is rarely, if ever, used for the evaluation of tumors and other pathology of the lung parenchyma. Cortical bone also has a low water content, and therefore MRI is relatively limited (compared with CT, for example) for the evaluation of cortical osseous structures. The preceding chapters discuss specific indications for

MRI and the conditions that can be e ectively evaluated using this imaging modality.

■ Conventional Radiography

As most clinicians have learned throughout their training, conventional radiographs are often the initial step in the evaluation of a patient who has (or is suspected of having) a musculoskeletal disorder. Conventional radiographs are obtained via the use of ionizing irradiation. The radiation beam is attenuated by structures between the image intensifier and the cassette. Structures with high density (e.g., cortical bone, metallic implants) block a larger amount of the radiation and thus leave a white region on the film. Structures with lower density (e.g., air, subcutaneous tissues) block less radiation and thus leave a dark or gray region on the film.

Conventional radiographs are easily available and are good for the evaluation of osseous detail. This modality is also exceptionally timeand cost-e ective, and the large field of view it a ords allows for the study of global deformities, for example, in a patient with a spine deformity such as scoliosis or deformity or malalignment of the lower extremities. For these reasons, conventional radiography is frequently used in musculoskeletal imaging to evaluate the joints and spine (Fig. 17.2). On the other hand, conventional radiographs do not provide optimal visualization of the softtissue structures such as ligaments, menisci, and the spinal cord. Radiographs are also less sensitive than CT or MRI for the evaluation of osseous destruction and involvement of the bone marrow. As an example, a 30% decrease in bone mass is often required before osteoporosis can be appreciated on radiographs.1,2 Conventional radiography is the study of choice for the initial evaluation of a patient with a traumatic injury to a joint, a long bone, or the spine. Radiography is also required as an adjunct to MRI in the evaluation of skeletal masses (Fig. 17.3) and of arthritis. However, conventional radiographs are less valuable in the evaluation of a patient with low back pain, neck pain, or spinal stenosis.

As a first-line imaging modality, conventional radiographs help the clinician determine the need for radiographic studies in other anatomic locations or for more advanced imaging studies of the local region. For example, a patient

Fig. 17.1 MRI: excellent soft-tissue contrast. Axial T1-weighted (A) and fat-suppressed T2-weighted (B) images of the left distal femur show a large mass involving the bone marrow and adjacent soft tis-

sues. Note the exquisite contrast between the soft tissues provided by MRI that allows for an accurate and detailed assessment of the extracompartmental involvement of the tissues.

Fig. 17.4 CT: excellent osseous detail. (A) A sagittal CT image of the elbow shows a posterior elbow dislocation and fracture fragment in the joint. (B) An AP radiograph of the right femur shows a varus deformity at the femoral neck that was noted on all views.

information about the status of the various tissues (except for limited cases in which an intravenous contrast is used), it does provide excellent spatial resolution. This modality is particularly useful for imaging the axial skeleton and for evaluating the extent of osseous lesions.4–6 With the development of multiplanar 3D reconstructions, CT imaging provides additional guidance in preoperative planning, fracture assessment and classification, and the evaluation of complex deformities.7,8 Although CT images provide excellent osseous detail and can precisely localize fracture extent and

(C) Given this deformity, CT was requested and more clearly showed a femoral neck fracture. (D) A 3D sagittal reconstructed CT image of the foot and ankle shows a complex calcaneal fracture.

fragmentation (information that can guide operative intervention), as a general rule, they do not provide high enough contrast resolution for optimal evaluation of the soft tissues, such as menisci and ligaments.9 With the recent introduction of 16 and 64 multidetector CT and the advent of isotropic data sets, 3D CT evaluation of the tendons has emerged as a new indication for CT imaging (in patients in whom MRI is not possible).

CT can also assist in preoperative planning for orthopaedic surgical procedures. For example, a patient

who presented with knee pain after direct trauma had normal-appearing conventional radiographs (Fig. 17.5A,B). Because the patient had a large knee e usion associated with severe pain, suspicion was high for a ligamentous injury

or an occult fracture. Coronal and sagittal CT images confirmed a lateral anterior femoral condyle comminuted fracture and an avulsion fracture of the inferior pole of the patella (Fig. 17.5C–E). MR images showed osseous edema of the lat-

Fig. 17.5 Value of multimodality imaging. Posteroanterior (A) and lateral (B) radiographs of a young female involved in a motor vehicle accident show a mild cortical irregularity at the left lateral femoral condyle (arrow on A). (C) A coronal reconstructed CT image toward the anterior aspect of the knee better shows a small fracture frag-

ment adjacent to the anterolateral femoral condyle (arrow). (D) A sagittal reconstructed CT image also shows the fracture (arrow) at the anterior aspect of the lateral femoral condyle. (E) A midline sagittal CT reconstructed image shows a small avulsion fracture at the inferior pole of the patella (arrow). (Continued on page 419)

eral anterior femoral condyle and the inferior pole of the patella, and disruption of the medial retinaculum (Fig. 17.5F,G).

■ Nuclear Scintigraphy

Although the imaging studies described above all provide excellent anatomic information, they are relatively limited in their abilities to provide high-quality physiologic information, specifically information related to bone turnover. In this regard, the imaging modality of choice is three-phase bone scintigraphy. For a bone scan, the patient is injected with technetium-99m phosphate or another radiopharmaceutical agent that emits gamma rays. Subsequently, the patient is placed under a scintillation camera to detect the presence and distribution of the radiotracer activity. The typical “three-phase” bone scan acquires the radiotracer activity information in the following phases:

•Blood-flow phase

•Soft-tissue phase

•Delayed or bone phase

Each one of these phases provides di erent information. In the blood-flow phase, increased uptake is found in areas of mature blood vessels. The second (soft-tissue) phase shows areas of neovascularity, such as with acute inflammation or vascular neoplasms. In the delayed or bone phase, the radio-

tracer has had time to adsorb newly formed crystals at sites of bone turnover.10,11

The advantages of bone scans are that they provide physiologic information regarding bone (more specifically, mineral turnover) and have the capability of surveying the entire body and detecting lesions early in their course. Their disadvantages are that the images are nonspecific and provide very poor spatial and anatomic resolution. Thus, the general thought is that with a bone scan, one is trading the anatomic detail provided by other imaging studies for physiologic information regarding the metabolic activity in the bone. Fortunately, in this era, systems have been developed that, in combination with CT scanning, allow scintigraphy and anatomic location to occur simultaneously. The indications for scintigraphy include evaluation of osseous lesions, usually metastatic disease, benign and malignant primary tumors, metabolic disorders, infection, and stress fractures.10,12–14 Specifically, di erent findings in the phases of a three-phase bone scan can help di erentiate a patient with osteomyelitis from one with soft-tissue infection without osseous involvement15,16 (Fig. 17.6).

In the setting of osteomyelitis, conventional radiographs are the initial study of choice, but additional imaging modalities are often required for a definitive diagnosis because osteomyelitis is not detectable by radiography until approximately 40% bone destruction has occurred. MRI and nuclear scintigraphy can provide greater diagnostic sensitivity and