Image guidance consists of five basic parts—planning, targeting, monitoring, controlling, and assessment of response (26). Computed tomography (CT) is the primary modality for planning prior to ablation of a renal tumor. The high spatial resolution and ability of CT to visualize adjacent structures, as well as to evaluate patients concurrently for evidence of metastatic disease, makes it superior to US for prepro-cedural evaluation. Magnetic resonance imaging (MRI) can perform a comparable role, particularly in patients with renal insufficiency.
US continues to play an important role for evaluating the malignant potential of indeterminate renal masses (27,28), and is the preferred imaging modality for tumor targeting during the ablation procedure in many institutions, including at the University of Wisconsin Clinical Science Center (UW CSC). This is not only because of the real-time feedback available with US, which allows rapid applicator positioning, but also because of its portability, excellent soft tissue resolution, and relatively low cost. The limitations of CT for targeting RCC as compared to US include the frequent difficulty of visualizing RCC on noncontrast CT, the comparatively limited approach path allowed by CT, the relatively thick imaging plane, which makes applicator positioning less precise, and the greater time (compared with US) required to position the applicator under CT guidance (29). CT fluoroscopy can speed the ablation procedure, but increases the radiation dose to the patient and physician (30). MRI has many advantages for use as a targeting modality, including multiplanar imaging capabilities, exceptional soft tissue differentiation, real-time fluoroscopic capabilities, and the ability to directly measure tissue temperature with MR thermometry based on gradient echo imaging (31,32). Despite these theoretical advantages over both US and CT (33,34), access to interventional magnets continues to be very limited, even in tertiary care facilities, and as a result, MRI-guided tumor ablation is not currently a viable option for most radiologists.
Monitoring an ongoing ablation can be difficult regardless of whether CT or US is being utilized. This is particularly true for RF and the other heat-based ablation techniques. The high temperatures produced by these modalities result in the release of water vapor. The water vapor distorts the US beam, producing "dirty shadowing,'' often obscuring the tumor being targeted, as well as surrounding normal parenchyma (35,36) (Fig. 1). On CT, the water vapor is visualized as small gas bubbles, allowing for improved definition of the involved area, but unfortunately, the gas bubbles do not closely correlate with the area of tissue necrosis (36). Cryoablation is more easily monitored than the heat-based methods of ablation, because the iceball produced during the ablation is readily seen by US, CT, and MRI (Fig. 2A and B). The iceball is identified as a regular, well-defined, hyperechoic line with dense posterior acoustic shadowing by US and as a region of low attenuation [0 Houston units (HU)] on CT and low signal on T1-weighted MRI images (37-40). If there is an appropriate acoustic window with US, the iceball can be evaluated from multiple angles, allowing superb definition of the involved area. Unlike the heat-based methods, the imaging changes are closely correlated with the area of tissue necrosis. In fact, the iceball identified on US corresponds to the portion of the ablated tissue that reaches a temperature of 0° C. At a depth of 1 mm to this region is the transition point at which the ablated tissue reaches a temperature of —20° C or less. Complete cell death generally results at a temperature of —20°C (41). Controlling the ablation is intimately linked to monitoring the ablation and thus can be difficult, particularly with the heat-based mechanisms. Appropriate planning and knowledge of
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