Preoperative Evaluation Choice of Imaging Modality for Guidance

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Although the basic technique for imaging-guided bone and soft-tissue biopsy has not undergone substantial changes, the proliferation of imaging modalities has added new emphasis on preoperative planning.

Often, patients are referred for a biopsy with imaging studies obtained at different institutions, with reports generated by radiologists not directly connected with the receiving hospital. Therefore, before even attempting to schedule the patient, all images and reports should be collected, and a decision made as to the appropriateness of the indication for biopsy. Sometimes, additional imaging can reveal a more accessible lesion than the one presented for biopsy, minimizing risks and procedure time.

End plate Modic changes can be dramatic (Fig. 1), and after the intravenous administration of contrast material, can exhibit florid enhancement, leading to a heightened concern for osteomyelitis. Although there is no single rule to exclude spine infection based on imaging techniques, there is a defined pattern of false-positive interpretations, including patients with no fever or changes in their white blood cell counts, erythrocyte sedimentation rate (ESR), or other markers of infection. A more critical analysis may forgo the need for a biopsy if the adjacent end plates are spared and there is no fluid-like signal in the disc. Careful evaluation of the facet joints may reveal marked degenerative changes, and flexion and extension views may be indicated in this context to diagnose underlying instability. Sometimes, scrutiny of gradient echo images, including scannograms, will exclude the possibility of infection if a vacuum phenomenon is identified. In addition, marked involvement of the facet joints implies neuropathic spine.

Once the review of available imaging studies is complete, and the lesion has been identified and deemed amenable to percutaneous biopsy, several technical decisions must be made, starting with the imaging modality that will be utilized. Operator preference is a major determinant factor, as well as scheduling flexibility and availability of different modalities within the department of radiology.

Fluoroscopy offers some well-defined advantages, particularly if a C-arm or biplane is available (Fig. 2). Patients can be positioned comfortably, and the approach to the lesion selected without restrictions related to the limited craniocaudal tilting of a computed tomography (CT)-scanner gantry. Real-time guidance is only matched by CT-fluoroscopy at the expense of a substantial increase in the amount of radiation delivered to the patient and operator. The working space between the patient and the image intensifier, a unique advantage, allows for comfort and ease during the procedure.

Some limitations are intrinsic to the projectional nature of fluoroscopy and its lack of low-contrast resolution. Hence, many lesions are inadequately visualized on conventional radiographs, making fluoroscopy inadequate as a guiding modality. Similarly, lesions close to a neurovascular bundle may pose an increased technical risk. Fluoroscopy also fails to provide accurate measurements of the distance between the tip of the needle and the lesion, and stroke margins cannot be calculated when using spring-loaded guns. Therefore, fluoroscopic guidance is most suitable for use with fine-needle aspiration and coring needles.

FIGURE 1 End plate changes can assume an extremely aggressive appearance on magnetic resonance. In this case, the biopsy showed reactive bone without acute inflammatory signs. The marked changes at L4-L5 were likely due to instability.

Ultimately, fluoroscopy is more operator dependant than CT scan, and requires familiarity with osseous landmarks. Systematic use of fluoroscopy for spine procedures such as diskog-raphy, vertebroplasty, or selective nerve root injections has a natural extension in its application to percutaneous spine biopsies.

CT offers improved low-contrast resolution, allowing increased lesion detectability. Likewise, vital structures adjacent to the lesion can be avoided. Therefore, CT scan is an excellent modality in the cervical spine because of its intrinsic anatomical difficulty. In the thoracic spine, CT scan will help minimize the chances of creating a pneumothorax. Small osseous lesions inconspicuous to conventional radiography and soft-tissue masses are prime indications for CT guidance (Fig. 3). The needle tip can be clearly identified due to the conspicuous streak artifact cast distally. This allows accurate estimates of the stroke margin, enabling the use of spring-loaded guns for soft tissue and destructive osseous lesions.

The advent of CT flouroscopy has reduced the length of procedures. However, CT continues to be limited by its initial conception as a cross-sectional technique. In this regard, it offers limited flexibility for angled approaches.

Ultrasound guidance presents an alternative approach to lesions with a soft-tissue component. Naturally, cystic lesions are exquisitely depicted with ultrasound. Ultrasound

FIGURE 3 Prostate carcinoma metastatic to the scapula. Although primarily a bone lesion, there is a large soft-tissue mass displacing the infraspinatus (A), amenable to a soft-tissue biopsy with a spring-loaded gun (B).

flexibility in needle placement is virtually unlimited, and its real-time feedback as to the needle position is unmatched by any other modality. It is almost a platitude to mention its additional advantage of avoiding ionizing radiation. Naturally, it is not a suitable technique to target strictly intraosseous lesions, and as with any ultrasound procedure, operator expertise is fundamental to the success.

The introduction of nonferromagnetic magnetic resonance (MR)-compatible biopsy needles has opened the door to the use of MR as a guiding technique for needle placement, with great potential based on the superb soft-tissue contrast afforded by MR, which enables visualization of lesions otherwise inconspicuous or poorly defined under alternative imaging methods. Open and closed systems have been used for this purpose albeit this technique is of less use in the musculoskeletal system than the brain (1,2).

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