CT and MRI Guidance for Ablative Therapies

The principles described for ultrasound guidance for laparoscopically directed ablative therapies can also be used for percutaneous ablative therapies. A percutaneous approach may be used for accessible tumors: those that can be accessed without traversing the colon, small bowel, blood vessels, or lung. The percutaneous approach offers the added benefit of no postoperative recovery time. Most patients are discharged the same day.

CT and MRI are both used to guide percutaneous cryoablation and radiofre-quency ablation. The relative advantages of CT are its greater availability, ease of use, and the lack of need for specialized, MRI-compatible, nonferromagnetic equipment. The primary advantage of MRI is that it provides a more accurate determination of the extent of the ablation during the procedure.

Of these two ablation techniques, there has been greater experience with radio-frequency ablation using CT guidance. In most hospitals, patients undergo deep sedation for CT-guided ablation procedures. After an initial preparatory scan, a skin site affording a safe and direct route to the mass is chosen. After skin cleansing and local anesthesia, the probe is advanced into the mass using CT fluoroscopy

Figure 19 CT guidance of percutaneous RFA. An enhanced CT scan shows the RFA probe deployed in small peripheral renal cell carcinoma. Abbreviations: CT, computed tomography; RFA, radiofrequency ablation.

(Fig. 19) (23,24). The main role of CT during radiofrequency ablation is to direct the probe into the mass, because the location and amount of ablated tissue are not readily distinguished on unenhanced images. Postprocedural scans with intravenous contrast are necessary.

MRI has been used to guide cryoablation and radiofrequency ablation (25,26,46). It allows better evaluation than CT of the progress of ongoing ablation. MRI sequences afford a clear depiction of the ablation site, and temperature-sensitive sequences are available that can monitor ablation progress (47). In comparison to other modalities, MRI offers the advantage of better contrast resolution to discriminate between ablated and nonablated tissue. However, the specialized MRI scanners, expertise, and equipment needed are currently available in only a few centers.

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