Applicators for magnetic resonance-guided ultrasonic ablation of benign prostatic hyperplasia

Abstract

Objectives: The aims of this study were to evaluate in a canine model applicators designed for ablation of human benign prostatic hyperplasia (BPH) in vivo under magnetic resonance imaging (MRI) guidance, including magnetic resonance thermal imaging (MRTI), determine the ability of MRI techniques to visualize ablative changes in prostate, and evaluate the acute and longer term histologic appearances of prostate tissue ablated during these studies.

Materials and methods: An MRI-compatible transurethral device incorporating a tubular transducer array with dual 120° sectors was used to ablate canine prostate tissue in vivo, in zones similar to regions of human BPH (enlarged transition zones). Magnetic resonance thermal imaging was used for monitoring of ablation in a 3-T environment, and postablation MRIs were performed to determine the visibility of ablated regions. Three canine prostates were ablated in acute studies, and 2 animals were rescanned before killing at 31 days postablation. Acute and chronic appearances of ablated prostate tissue were evaluated histologically and were correlated with the MRTI and postablation MRI scans.

Results: It was possible to ablate regions similar in size to enlarged transition zone in human BPH in 6 to 18 minutes. Regions of acute ablation showed a central "heat-fixed" region surrounded by a region of more obvious necrosis with complete disruption of tissue architecture. After 31 days, ablated regions demonstrated complete apparent resorption of ablated tissue with formation of cystic regions containing fluid. The inherent cooling of the urethra using the technique resulted in complete urethral preservation in all cases.

Conclusions: Prostatic ablation of zones of size and shape corresponding to human BPH is possible using appropriate transurethral applicators using MRTI, and ablated tissue may be depicted clearly in contrast-enhanced magnetic resonance images. The ability accurately to monitor prostate tissue heating, the apparent resorption of ablated regions over 1 month, and the inherent urethral preservation suggest that the magnetic resonance-guided techniques described are highly promising for the in vivo ablation of symptomatic human BPH.

Figures

Figure 1

Common appearance of benign prostatic hyperplasia (BPH) shown diagramatically in axial plane through mid gland. A) in the normal prostate, the transition zone lobes abut the prostatic urethra at its anterolateral aspect. Other major glandular regions (peripheral and central zones) are also present as indicated, at this level. B) in a typical prostate involved with moderate BPH, the enlarged transition zone lobes compress the prostatic urethra significantly.

Figure 2

Transurethral catheter designed to have capability for ablation of human BPH. Major features include: the retention balloon which is filled with a 1% solution of gadolinium contrast agent after catheter insertion, to maintain positioning of the device within the prostatic urethra and MRI visibility; the urethral balloon which is inflated within the prostatic urethra and through which water circulates during ablation; and the flexible catheter containing electronic connections. Within the urethral balloon are the transducers, two 10 mm long cylindrical transducers with dual 120 degree active sectors.

Figure 3

Treatment simulation of human BPH using MRI and RThawk treatment control software A) T2-weighted axial image through the mid-portion of a human prostate with BPH and enlarged transition zones (TZ) anterolateral to the urethra (U). B) BPH treatment plan with targeted TZ tissue indicated by green regions, and control points for therapy delivery monitored by MRTI indicated by 4 small gray circles (CP). Applicator (A) is present in the urethra.

Figure 4

MR Images obtained during acute study of ablation of a canine prostate in vivo. A) MRTI showing the spatial distribution of maximum temperatures recorded during the ablation B) MRTI map showing thermal dosimetry in equivalent minutes at 43°C during the ablation C) Contrast enhanced T1-weighted image obtained following the ablation shows two anterolateral regions in the prostate (large arrows) which have been ablated and appear dark due to lack of blood flow. The applicator in the prostatic urethra (U) is visible in this image. In A) and B), the applicator contains some artifactual color due to circulating water in the urethral cooling balloon.

Figure 5

Histologic analysis of the prostate ablated in the acute study illustrated in Figure 4. H and E stained images at the illustrated ablation level are shown: A) Whole mount section of the prostate demonstrates 2 prominent regions of ablation anterolaterally corresponding to the nonperfused regions seen in Figure 4C. The ablated region contains 2 distinct regions, a central heat-fixed region (HF) and a peripheral transition zone (TZ). The markedly disrupted tissue architecture of the TZ (B) contrasts strikingly with the near normal appearance of the heat-fixed region (C) which was rapidly heated to a higher temperature than the TZ. The boundary between the HF and TZ zones is abrupt and distinct, as shown in (D).

Figure 6

Images of enlarged canine prostate involved with extensive BPH, with two regions of ablation created under MRI guidance anterolaterally, and studied 31 days later (chronic study). A) Contrast enhanced T1-weighted image shows 2 large avascular regions anterolaterally to the urethra (U) with some enhancement at margins. B) Correlative trichrome stained histologic image shows the two large cystic spaces with some collagenous tissue at their margins. There is complete preservation of the prostatic urethra (U). C) and D) Two T2-weighted images show both the extensive cystic change throughout the gland due to BPH, and the two large residual cystic spaces on either side of the intact urethra (U). In C), the bladder neck (B) is visible cranial to the enlarged prostate gland.

Figure 7

Two examples of human BPH as depicted in fat suppressed axial T2-weighted MRI through the midportions of the prostate gland A) MRI of a patient with moderate BPH. The two transition zone (TZ) lobes are seen anterolateral to the prostatic urethra (U), and there is clear differentiation of the TZ from the more posterior peripheral zone (PZ), which has a higher signal intensity. Critical structures at risk for damage during ablation include the bilateral neurovascular bundles (NVB), and the rectum. B) Prostate MRI of a patient with more severe BPH shows assymetrical enlargement of TZ lobes, with the left being significantly larger. Compressed urethra (U) is seen between enlarged TZ lobes. The anterolateral aspects of the prostate are surrounded by slow flowing blood in the periprostatic venous plexus (PPV)