MRI Findings After Prostatic Artery Embolization for Treatment of Benign Hyperplasia

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1 Genitourinary Imaging Original Research Frenk et al. MRI After Prostatic Artery Treatment for BPH Genitourinary Imaging Original Research Nathan E. Frenk 1 Ronaldo H. Baroni 1 Francisco C. Carnevale 1 Octavio M. G. Gonçalves 1 Alberto A. Antunes 2 Miguel Srougi 2 Giovanni G. Cerri 1 Frenk NE, Baroni RH, Carnevale FC, et al. Keywords: benign prostatic hyperplasia, prostate MRI, prostatic artery embolization, prostatic infarct DOI: /AJR Received August 5, 2013; accepted after revision January 18, Instituto de Radiologia, Hospital das Clínicas, Faculdade de Medicina da Universidade de São Paulo, Av Dr. Enéas Carvalho de Aguiar, s/n Rua 1, Cerqueira Césa, São Paulo, SP , Brazil. Address correspondence to R. H. Baroni (rbaroni@einstein.br). 2 Divisão de Urologia, Hospital das Clínicas, Faculdade de Medicina da Universidade de São Paulo, São Paulo, Brazil. Supplemental Data Available online at AJR 2014; 203: X/14/ American Roentgen Ray Society MRI Findings After Prostatic Artery for Treatment of Benign Hyperplasia OBJECTIVE. The purpose of this article is to assess and describe the MRI findings after prostatic artery embolization for treatment of benign prostatic hyperplasia. MATERIALS AND METHODS. We retrospectively evaluated 17 patients who underwent prostatic artery embolization as part of different prospective studies to evaluate this alternative treatment of benign prostatic hyperplasia. Clinical results were evaluated by assessment of urinary catheterization and International Prostate Symptom Score (IPSS). Serial MRI examinations were performed, and the prostatic central gland and peripheral zone were evaluated for signal intensity changes and the presence and characteristics of infarcted areas. Statistical analysis was performed with ANOVA for repeated measures and Student t test. RESULTS. All patients had clinical success, as defined by the removal of indwelling urinary catheter or decreased IPSS after embolization. Infarcts were seen in 70.6% of the subjects, exclusively in the central gland, were almost always characterized by hyperintensity on T1-weighted images and predominant hypointensity on T2-weighted images, and became smaller (mean reduction, p < 0.001) and isointense to the remaining of the central gland over time. Volume reduction of the prostate after embolization was significant (averaging 32.0% after months; p < 0.001) only in patients with infarcts. No statistically significant association was seen between the development of infarcts and IPSS. CONCLUSION. MRI can be used for assessing the development of infarcts and volume reduction in the prostate after embolization. Further studies are needed to correlate these findings to clinical outcome. B enign prostatic hyperplasia (BPH) is an extremely common disease, affecting more than 50% of men older than 60 years, up to 30% of whom will present with lower urinary tract symptoms [1]. Although surgery (open or transurethral resection) remains its most effective treatment [2, 3], prostatic artery embolization has recently been described as a safe and effective alternative treatment of BPH [4 12]. All of the patients participating in studies evaluating prostatic artery embolization for BPH in our institution are also undergoing serial imaging examinations, which we hope may help us understand its mechanisms of action and evaluate its effects. MRI has become the most powerful imaging technique for the prostate [13]; it is a minimally invasive multiplanar imaging method that involves no ionizing radiation, offers superb tissue contrast, and can evaluate contrast-enhancement features of the gland. In addition, it is much less user dependent than is transrectal ultrasound, another method available for imaging of the prostate. Because of all these qualities, MRI is arguably the best suited imaging method for evaluating the prostate after prostatic artery embolization. The purpose of this study was to assess and describe MRI of the prostate in a series of patients who underwent prostatic artery embolization. Materials and Methods Subjects Different prospective studies are currently being conducted in our institution by the Radiology and Urology Departments to evaluate prostatic artery embolization as an alternative to surgery for treatment of BPH. Patients diagnosed with BPH on clinical grounds, with estimated prostate volume greater than 30 cm 3, who presented with urinary obstruction or moderate-to-severe lower urinary tract symptoms (International Prostate Symptom Score [IPSS] higher than 18) refractory to medical AJR:203, October

2 Frenk et al. TABLE 1: Results of Prostatic Artery in 17 Patients Patient Age (y) Unilateral or Bilateral treatment, who were on the waiting list for prostate surgery, and who agreed to undergo prostatic artery embolization as an alternative to surgical treatment were included in these studies. Exclusion criteria included previous nonmedical or invasive treatment of BPH; prostate cancer (excluded by clinical examination, prostate-specific antigen levels, and MRI in all patients and by prostatic biopsy in suspicious cases); bladder disorders (evaluated by urodynamic studies); and other clinical data, such as neurologic disorders affecting bladder function (e.g., multiple sclerosis), known urethral stricture, large bladder diverticula or stones, chronic renal failure, or other potentially confounding bladder or urethral diseases or conditions. Patient selection was done according to institutional procedures published elsewhere [11]. Preprocedure MRI was done according to the parameters described in the MRI Protocol subsection below. The studies were approved by both the Urology and Radiology Departments and by the institutional review board, and informed consent for prostatic artery embolization as an alternative treatment was obtained from all participants. The preliminary results of prostatic artery embolization in nine of these patients (subjects 1 9) have been published elsewhere [5, 6, 8, 9]. Cumulative Prostate Volume Change After Prostatic Artery (%) Prostate Volume Before Particle Size (µm) Infarct Prostatic Artery (cm 3 ) 1 Month 3 Months For this retrospective study, focusing on the MRI findings after prostatic artery embolization, we included all patients who underwent prostatic artery embolization at our institution and who had been followed up with imaging for at least 12 months. Our exclusion criteria were surgery or repeated prostatic artery embolization before completing the 12 months of follow-up (because we would not be able to properly evaluate imaging findings in them during the whole follow-up), and any missed contrast-enhanced MRIs before or after prostatic artery embolization. From June 2008 to December 2011, a total of 29 prostatic artery embolizations were performed in 28 patients at our institution, and by December 2012, 21 patients had completed at least 12 months of imaging follow-up. We excluded four patients: one who underwent transurethral resection of the prostate, one who underwent repeated embolization (posteriorly followed by transurethral resection of the prostate) because of treatment failure (persistent need for indwelling urinary catheter 1 month after embolization), one who did not undergo contrast-enhanced MRI before and after prostatic artery embolization because of renal failure, and one who missed MRI 1 month after prostatic Months Before IPSS Before and After Prostatic Artery 1 Month After 3 Months After 1 67 Bilateral Yes NA Unilateral Yes NA Bilateral No NA Unilateral Yes NA Bilateral Yes NA Unilateral No NA Bilateral No NA Bilateral Yes NA Bilateral Yes NA Bilateral No Bilateral No NA Not 7 14 evaluated a Bilateral Yes Unilateral Yes Bilateral Yes Bilateral Yes Bilateral Yes NA Bilateral Yes Note IPSS = International Prostate Symptom Score, NA = not applicable (patient with indwelling urinary catheter). a Patient was hospitalized for aortic valve surgery Months After artery embolization. Therefore, our study population consisted of 17 subjects who underwent a total of 17 prostatic artery embolizations. For inclusion and exclusion details, see Figure S1 in the AJR electronic supplement to this article, available at The mean duration of follow-up was 13.2 months. The mean (± SD) patient age was 65.7 ± 8.2 years (range, years). The mean prostatespecific antigen level was 7.0 ± 5.7 ng/ml (range, ng/ml). The mean prostate volume was 64.9 ± 16.2 cm 3 (range, cm 3 ) (Table 1). Procedure A detailed description of the embolization technique performed by our group has been published elsewhere [5, 11]. Briefly, selective catheterization of the prostate artery arising from the inferior vesical artery was performed on each side with a microcatheter through a right femoral artery puncture approach, and tris-acryl microspheres (Embosphere Microspheres, Biosphere Medical) µm or µm in diameter were used for its embolization. Bilateral embolization was successfully accomplished in 13 patients, whereas four patients 814 AJR:203, October 2014

3 MRI After Prostatic Artery Treatment for BPH TABLE 2: Signal Intensity of Infarcts in 12 Patients After Prostatic Artery T1-Weighted Images T2-Weighted Images Patient 1 Month 3 Months Months 1 Month 3 Months Months 1 High High Intermediate Low Low Low 2 High High Intermediate Intermediate Intermediate Intermediate 4 Low Low Low High High High 5 High High Intermediate Low with hyperintense foci Low Low 8 High Intermediate Intermediate Low with hyperintense foci Low Low 9 High High Intermediate Low Low Intermediate 12 High High Intermediate Low Low Low 13 Intermediate Intermediate Intermediate Low Intermediate Intermediate 14 High Intermediate Intermediate Low Low intermediate 15 High High High Low with hyperintense foci Low with hyperintense foci Low 16 High High High Low with hyperintense foci Low Low 17 High High Intermediate Low with hyperintense foci Low Low had only unilateral embolization (three of the right prostatic artery and one of the left) because of technical difficulties, including dissection of the inferior vesical artery in one case, impossibility of identifying this artery in two cases, and failure to catheterize it in the remaining case. Two patients were embolized with µm particles and the remaining 15 with µm particles. MRI Protocol We evaluated MRI examinations of the prostate performed before prostatic artery embolization (within 3 months of the procedure) and 1, 3, and months after the procedure. MRI was performed on a 1.5-T (Signa HDxt, GE Healthcare) or a 3-T (Achieva, Philips Healthcare) superconducting unit with phased-array torso coils. The examination protocol included thin-section highspatial-resolution axial and sagittal T2-weighted fast spin-echo images and dynamic contrastenhanced (DCE) T1-weighted axial images. The contrast agent (gadoterate dimeglumine, Dotarem, Guerbet) was injected as a bolus at a dose of 0.1 mmol/kg at a rate of 2.0 ml/s. Axial and sagittal T2-weighted fast spin-echo images were obtained with the 1.5-T unit with the following parameters: TR/TE, /160.0; echo-train length, 17 20; FOV, 18.0 cm; slice thickness and spacing, 4.0 and 0.0 mm; and matrix (frequency phase), For the 3-T unit, parameters were TR/TE, /120.0; echo-train length, 20; FOV, cm; slice thickness and spacing, 3.0 and 0.0 mm; and matrix, DCE gradient-recalled 3D T1-weighted images were obtained in the axial plane with the 1.5-T A B Fig year-old man with benign prostatic hyperplasia who underwent 3-T T2-weighted MRI 12 months after prostatic artery embolization. A, On sagittal image, craniocaudal (black arrow) and anteroposterior (white arrow) diameters are measured. B, On axial image, transverse diameter (arrow) is measured. unit with the following parameters: flip angle, 15 ; TR/TE, 5.5/minimum full; FOV, 20.0 cm; slice thickness, 4.0 mm; and matrix, One volume was obtained before contrast agent injection and the 12 other volumes were sequentially acquired immediately after contrast agent administration (each volume acquisition lasted around 25 seconds, for a total duration of approximately 300 seconds). For the 3-T unit, parameters were flip angle, 12 ; TR/TE, 5.5/2.7; echo-train length, 50; FOV, 14.0 cm; slice thickness, 3.0 mm; and matrix, One volume was obtained before contrast agent injection and 14 other volumes were sequentially acquired immediately after contrast agent administration (each volume acquisition lasted around 22 seconds, for a total duration of approximately 300 seconds). The 3-T scanner was used for one examination in two patients and for two examinations in other four patients. All the remaining imaging studies were obtained with the 1.5-T scanner. This variability was related to the availability of MRI scanners in our service. Data Collection and Imaging Evaluation Clinical results were analyzed by evaluating the presence of urinary obstruction requiring indwelling urinary catheterization or by calculating the IPSS during interviews with urologists before and 1, 3, and months after prostatic artery embolization. We defined clinical success as the removal of an indwelling urinary catheter within 1 month of prostatic artery embolization or IPSS reduction to less than 8 (patient asymptomatic or mildly symptomatic) any time during followup. Because IPSS is calculated with a questionnaire that depends on urinary symptoms, it cannot be applied to patients with an indwelling cathe- AJR:203, October

4 Frenk et al. ter because of urinary obstruction; however, it is a useful score for evaluating BPH-related symptoms over time in patients who do not present with complete urinary obstruction. Regarding complications, we chose to describe and evaluate only those seen on imaging. All MRI examinations were evaluated in consensus by two abdominal radiologists with 1 and 12 years of experience with prostate MRI, respectively. Both of them were aware of the indications for the examinations. The prostatic central gland and peripheral zone (PZ) were evaluated for the following aspects: general appearance (homogeneous or heterogeneous) and signal intensity (SI) and the presence and SI characteristics of infarcted areas. Infarcted areas were defined as new regions that displayed no evident enhancement after contrast agent administration. We classified their distribution as uni- or bilateral and evaluated their SI on T1-weighted images (on the unenhanced images of the DCE series) and on T2-weighted images. We also measured the greatest transverse diameter of the largest infarcted area (when present) on each half of the gland on the axial DCE T1-weighted image and evaluated their mean size over time. Prostate dimensions were measured on the T2- weighted sequences as follows: The transverse diameter was measured in the axial plane, and the craniocaudal and anteroposterior diameters were obtained from the sagittal images. Prostate volume was calculated using the formula for volume estimation of a prolate ellipsoid (transverse diameter craniocaudal diameter anteroposterior diameter π / 6) [14] (Fig. 1). All prostates were measured by one radiologist. Statistical Analysis Statistical analysis of the results was performed using SPSS for Windows (version 18.0, IBM). ANOVA for repeated measures was used to evaluate infarct size and prostate volume over time. Student t test was used to evaluate IPSS relative to the presence of infarcts. Statistical significance was assigned for p < Results Clinical Results Initially, 11 patients had urinary obstruction and indwelling urinary catheters (and therefore could not be evaluated by the IPSS questionnaire), and the remaining six patients had a mean IPSS of 25.3 (range, 20 30). One month after prostatic artery embolization, all of the remaining subjects (except one patient who could not be evaluated because of hospitalization for aortic valve surgery) were free of indwelling catheters and had mean IPSS of 7.4 (range, 2 17). Three and months after prostatic artery embolization, no patient had indwelling catheters and the mean IPSS was 4.2 (range, 1 10) and 7.0 (range, 1 22), respectively. All patients had clinical success according to the criteria of removal of indwelling urinary catheter within 1 month of prostatic artery embolization (11 patients) or IPSS less than 8 at any time during follow-up (all patients). Patients were maintained without medication for BPH, except for three subjects (patients 10, 11, and 13) who were prescribed medical treatment at months because of symptomatic recurrence, on the basis of their overall clinical evaluation by the urologists in charge. One of the patients who had undergone unilateral left prostatic artery embolization (patient 4) presented with a small area of wall thickening and increased contrast enhancement in the left side of the bladder neck on MRI 1 month after prostatic artery embolization, which clinically correlated to a single episode of hematuria, but this imaging finding had completely resolved 2 months later (Fig. 2). This was assumed to be related to nontarget embolization to the left inferior vesical artery. No other complications were seen on MRI for any other patient. Predominant Prostatic Signal Intensity The central gland in all prostates was heterogeneous and had a macronodular appearance before and after prostatic artery embolization, with nodules of variable SI but of predominantly high SI on T2-weighted images and intermediate SI on T1-weighted images, which are the usual imaging characteristics of BPH [15]. The PZ in all prostates had predominantly high SI on T2-weighted images and homogeneous intermediate SI on T1-weighted images (Figs. 3A and 3B). Even when infarcts were seen after prostatic artery embolization (see next subsection), the remaining parenchyma characteristics did not change. Fig year-old man with benign prostatic hyperplasia who developed single episode of hematuria after unilateral embolization of left prostatic artery. Axial dynamic contrast-enhanced T1-weighted 1.5-T MRI obtained 1 month after prostatic artery embolization shows focal thickening of bladder neck on left side with slightly increased mucosal contrast enhancement (white arrow) and apparent decreased enhancement of outer layers (black arrow). These findings were attributed to nontarget embolization to left inferior vesical artery. See Figure S2 for additional data. Infarcts Infarcts were observed after prostatic artery embolization in 12 (70.6%) of the patients and occurred exclusively in the central gland. Of the 13 patients who had bilateral prostatic artery embolization, nine (69.2%) developed infarcts, bilaterally in all cases. Of the four patients who had unilateral prostatic artery embolization, three (75.0%) developed infarcts, all of them exclusively on the embolized side. Signal intensity of the infarcts is shown on Figure 4 and Table 2: on T1-weighted images, infarcts in 10 of 12 patients (83.3%) were initially hyperintense, and they tended to become isointense over time. On T2-weighted images, infarcts in 10 of 12 subjects (83.3%) were initially hypointense, with or without hyperintense foci, and they tended to lose the hyperintense foci (when present) and become isointense over time. The appearance and evolution of most infarcts are illustrated in Figures 3C 3I. It is of note that one patient (patient 4) developed an avascular area with high SI on T2-weighted images and low SI on T1-weighted images that decreased in size but did not change in SI over time, which was attributed to an infarct with cystic transformation (Fig. S2 can be seen in the AJR electronic supplement to this article, available at Infarct size (evaluated by mean size of the biggest infarct in each half of the prostate) decreased progressively over time. There was a progressive decrease in the size of the biggest infarcts from a mean of 1.7 ± 0.8 cm 1 month after prostatic artery embolization to 1.2 ± 0.7 cm 3 months after prostatic artery embolization (p < 0.001) and then to 1.0 ± 0.6 cm months after prostatic artery embolization (p = 0.006). 816 AJR:203, October 2014

5 MRI After Prostatic Artery Treatment for BPH A D G Fig year-old man with benign prostatic hyperplasia (BPH). A C, Before prostatic artery embolization, axial 1.5-T MRI was performed. T1-weighted image (A) shows homogeneous intermediate signal intensity (SI) of both central gland and peripheral zone (PZ). T2-weighted image (B) shows characteristic appearance of central gland and PZ, which are clearly individualized. Central gland (black arrow) has predominant high SI, with nodules of varied SI. PZ (white arrow) has predominant high SI, tenuous hypointense areas, and no nodules. Dynamic contrastenhanced (DCE) T1-weighted image (C) shows heterogeneous intense enhancement of central gland (attributable to different components of BPH) and homogeneous less-intense enhancement of PZ. D F, Axial images obtained 1 month after prostatic artery embolization. Arrows show infarcts as new areas in central gland with hyperintensity on T1-weighted image (D), predominant hypointensity with some hyperintense foci on T2-weighted image (E), and lack of enhancement on DCE T1-weighted image (F). G I, Axial images obtained 12 months after prostatic artery embolization. T1-weighted image (G), T2-weighted image (H), and DCE T1-weighted image (I) show evolution of infarcts (arrows), including size reduction, loss of hyperintensity on T1-weighted image, and loss of hyperintense foci amid hypointense areas on T2-weighted image. B E H C F I AJR:203, October

6 Frenk et al. Number of Patients Month 3 Months Months Time After Prostatic Artery The mean IPSS after embolization did not statistically significantly differ between patients with infarcts and patients without them. Patients with infarcts had a mean IPSS of 7.9 ± 4.9 at 1 month after prostatic artery embolization, 4.0 ± 3.1 at 3 months after prostatic artery embolization, and 5.8 ± 5.0 at months after prostatic artery embolization, whereas patients without infarcts had a mean IPSS of 6.0 ± 2.9 (p = 0.481), 4.6 ± 2.8 (p = 0.713), and 9.8 ± 8.0 (p = 0.232) at each of those respective time points. We could not statistically evaluate IPSS before prostatic artery embolization because it could be calculated in only six of the 17 subjects. The remaining 11 patients had indwelling urinary catheters because of obstruction. Low Intermediate High A Fig. 4 Signal intensity (SI) of infarcts over time. A and B, Graphs show changes in SI on T1-weighted (A) and T2-weighted (B) images. Number of Patients Prostate Volume Prostate volume over time is shown on Figure 5 and Table 1. When evaluating all patients, the mean prostate volume statistically significantly decreased from 64.9 ± 16.2 cm 3 before prostatic artery embolization to 49.9 ± 14.6 cm 3 1 month after prostatic artery embolization (p < 0.001). No statistically significant difference was seen in volumes between 1 month and 3 months after prostatic artery embolization (49.9 ± 14.6 vs 46.0 ± 15.1 cm 3 ; p = 0.131) and between 3 months and months after prostatic artery embolization (46.0 ± 15.1 vs 48.2 ± 17.4 cm 3 ; p = 0.398). Mean prostate volume months after prostatic artery embolization was statistically significantly lower than before treatment (p < 0.001). The cumulative volume reduction months after embolization was 24.6% ± 20.7%. When evaluating results according to the presence of infarcts, we found that even though patients who did not develop infarcts had decreased mean prostate volume after prostatic artery embolization, this reduction was not statistically significant. Prostate volume in these patients was 61.0 ± 21.7 cm 3 before embolization, 55.5 ± 22.9 cm 3 at 1 month after prostatic artery embolization (p = 0.169), 54.1 ± 22.5 cm 3 at 3 months after prostatic artery embolization (p = 0.090), and 57.9 ± 25.3 cm 3 at months after prostatic artery embolization (p = 0.443). By comparison, among patients who developed infarcts, the mean prostate volume statistically significantly decreased from 66.6 ± 14.1 cm 3 before prostatic artery embolization to 47.5 ± 9.8 cm 3 1 month after prostatic artery embolization (p < 0.001). Once again, no statistically significant difference was seen in volumes between 1 month and 3 months after prostatic artery embolization (66.6 ± 14.1 vs 42.7 ± 10.2 cm 3 ; p = 0.061) and between 3 months and months after prostatic artery embolization (42.7 ± 10.2 vs 44.1 ± 12.0 cm 3 ; p = 0.573). In patients who developed infarcts, the mean prostate volume months after prostatic artery embolization was statistically significantly lower than that before treatment (p < 0.001), and the cumulative volume reduction at months after embolization was 32.0% ± 19.2%. There was no statistically significant difference when we compared prostate volume before embolization between patients who did develop infarcts and those who did not (p = 0.099) Month 3 Months Months Time After Prostatic Artery Predominantly low with hyperintense foci Low Intermediate High Discussion This study focuses on the imaging findings of prostate MRI in 17 patients who underwent prostatic artery embolization for treatment of BPH. Clinical success was seen in all patients. Infarcts were observed in 70.6% of the subjects and occurred exclusively in the central gland on the embolized side; they were mostly characterized by initial hyperintensity on T1-weighted images and predominant hypointensity on T2-weighted images and tended to become smaller and isointense to the remaining central gland over time. Infarcts were not significantly associated with IPSS changes, but patients with infarcts were the only ones who developed significant prostate volume reduction. We attributed SI in most infarcts to hemorrhagic necrosis and the presence of proteinaceous content and blood breakdown products, especially methemoglobin, and their associated T1-shortening effects, analogous to what has been shown to occur with uterine fibroids after uterine artery embolization [16 18]. This is also supported by the histopathologic examination of the products from the transurethral prostatectomy performed on two patients from our institution (who were excluded from this series because of repeated prostatic artery embolization or surgery before completing 12 months of follow-up); in these cases, fragments of periurethral prostatic tissue showed foci of vascular occlusion by the embolic material, with surrounding foreign body and chronic inflammatory reactions, and areas of ischemic necrosis [19]. The characteristics of these infarcts are quite different, however, from those seen in dogs, in whom prostatic artery embolization has been shown to produce cavitary necrosis, manifesting as large cystic spaces on MRI [20 22], similar to what happened in only one of our cases. The differences found between our results and those found in dogs might be related to structural differences between BPH in dogs and in humans, which are also thought to explain the different results of prostate laser ablation therapy in them [23]. Although BPH in dogs is characterized by extensive glandular epithelial hyperplasia, in humans it predominantly affects the stromal tissue [24 27]. These differences between imaging and histopathologic effects of prostatic artery embolization in humans and dogs may limit the validity of data extrapolated from canine models of BPH. B 818 AJR:203, October 2014

7 MRI After Prostatic Artery Treatment for BPH We found absolutely no infarcts in the PZ. We think two reasons may contribute to this finding. First, contrast enhancement is lower in the PZ than in the central gland (as has been shown by previous studies of prostatic microvascularity [28]); this should account for a greater tolerance of the PZ to ischemia. Second, because of the nature of prostate microvascularization, which is characterized by penetrating branches of capsular arteries [29], its central part, furthest from supplying arteries, would be expected to be most at risk of hypoperfusion. This is evidenced by the greater frequency of infarcts involving the PZ in a series of necropsy studies previously published [30]. Although infarcts were related to prostate volume reduction in our patients, they were not statistically significantly associated with IPSS changes. Considering that prostate volume does play a role in the development of BPH symptoms (in fact, it is one of the criteria used to decide between open and transurethral prostatectomy [2, 3]), we think that this might be related to our small cohort size. More studies with larger sample sizes are needed to correlate the development of infarcts with IPSS and other clinically relevant efficacy parameters (such as urodynamic findings and quality of life questionnaires). Regardless of the presence of infarcts or prostatic volume reduction, all patients had clinical improvement after prostatic artery embolization. Although infarcts and volume reduction may lead to reduced mass effect over the urethra, other mechanisms must be responsible for the clinical benefit seen in patients who did not have infarcts or volume reduction. We think that decreased testosterone entry in prostate cells after embolization could lead to inhibition of prostate growth, and a possible decrease in the number of α 1 -adrenergic receptors (as has been shown to happen after transurethral fine-needle ablation and microwave thermotherapy) could lead to decreased muscle tone and decreased urine outflow obstruction [31 35]. A recent study [22] of prostatic artery embolization in dogs evaluated the effect of embolization particle size on perfusion patterns in the periurethral zone and in the rest of the prostate, and on the prostatic urethra wall size. Those authors found that embolization with small particles ( µm) was associated with less parenchymal destruction (presumed Fig. 5 Prostate volume over time. Symbols represent mean, and lines and whiskers represent SD. Volume (cm 3 ) Before Prostatic Artery from DCE findings) and less volume reduction than embolization with particles of and µm; however, particles of µm were associated with urethral wall edema (which could possibly cause outflow obstruction after embolization). Therefore, they suggested that the optimal particle size for prostatic artery embolization in dogs might be µm. Although evaluating the presence of urethral wall edema and differences in perfusion patterns between the periurethral zone and the remaining gland in our patients would have been interesting, these were not among the variables we decided to evaluate when planning our study. Another group [36] compared prostatic artery embolization with particles of 100 µm and 200 µm and found that, although the smaller particles were associated with greater prostate volume reduction, the larger ones were associated with better clinical outcome. We could not assess the effect of embolization particle size because only two patients were treated with 100- to 300-µm particles, whereas all the others were embolized with 300- to 500- µm particles. However, infarcts and volume reduction were indeed observed in patients from both groups. Other previously published studies [5 10, 12, 36, 37] have described the clinical efficacy of prostatic artery embolization in humans and prostate volume reduction after embolization, in agreement with our findings. Similar to our case of bladder ischemia, another case has been reported that required surgery for removal of necrotic tissue attached to the bladder wall but that was not associated with perforation [7, 10]. In a previous study, bilateral embolization showed superior clinical results compared with unilateral embolization [37]; we did not compare these two groups of patients because of the small sample size of Infarcts = no Infarcts = yes Whole series 1 Month After Prostatic Artery 3 Months After Prostatic Artery Months After Prostatic Artery our study, which included only four cases of unilateral embolization. Although most of these prior studies focused on the clinical efficacy of prostatic artery embolization, we preferred to concentrate especially on the imaging findings, including the incidence of infarcts, their imaging characteristics, and their possible relationship with volume reduction. Volume reduction and the development of infarcts could be identified with both the 1.5- and 3-T scanners, with no evident superiority of one over the other. We think, therefore, that both of them can be used for this kind of assessment, although we did not perform an objective comparison of examinations from the different scanners. Because all infarcts in our subjects presented with SI changes on T1-weighted images or T2-weighted images that were not observed on MRI before prostatic artery embolization, and considering the lack of contrast enhancement in the infarcted areas, we may hypothesize that contrast material administration may be dispensable for the identification of infarction after prostatic artery embolization. Further studies are necessary to confirm this finding. On the basis of our preliminary findings in this small cohort of patients, we consider that one MRI performed before embolization to confirm the diagnosis of BPH and to help exclude concomitant conditions, such as cancer, and another obtained 1 month after prostatic artery embolization to detect the development of infarcts and prostate volume reduction after embolization may be an adequate follow-up protocol. Studies with longer follow-up are needed to evaluate whether infarction seen on MRI can predict long-term effectiveness of prostatic artery embolization, as some researchers think is the case with fibroids treated with uterine artery embolization [38]. Although at this time clinical protocols of prostatic ar- AJR:203, October

8 Frenk et al. tery embolization include repeated pre- and postprocedural MRI as part of its evaluation [11], the actual role of MRI before and after prostatic artery embolization outside the research setting must be further evaluated, especially because all of our patients improved regardless of the development of infarcts or prostate volume reduction. Our study has some limitations. The sample size of patients evaluated was relatively small, and the follow-up period was also short. We recognize that the exclusion of two patients, one who needed surgery and one who needed repeated prostatic artery embolization and subsequent surgery, might have introduced bias to our study; however, we think this was necessary to obtain a homogeneous population for the complete duration of the follow-up for which we could evaluate the imaging findings that resulted from prostatic artery embolization, which was our main objective. Because of limited availability of scanners in our institution, the last evaluation timing ranged from 12 to 18 months after prostatic artery embolization. Our study population was heterogeneous when considering that most of the subjects underwent bilateral prostatic artery embolization but four underwent only unilateral prostatic artery embolization because of technical difficulties; this may be a more accurate representation of the method in future real-life situations, when bilateral prostatic artery embolization will not be feasible in all patients. In addition, most patients were embolized with particles of µm, with only two treated with particles of µm, so we could not evaluate the possible consequences of using different embolization particle sizes. Examinations were performed using either 1.5 or 3 T, but we think this did not affect the overall results, especially because we did not compare absolute SI measurements between different examinations, nor did we evaluate very small imaging findings, which could have been better depicted by the 3-T scanner because of its superior spatial resolution. Finally, because review of the MRI studies was not done in a blinded fashion, potential observer bias might be present; we attempted to minimize this by having the images evaluated by two radiologists in consensus. Prostate volume was measured by only one radiologist; therefore, interobserver variability was not evaluated. In conclusion, the most important MRI findings in the prostate after prostatic artery embolization were infarcted areas in the central gland on the embolized sides, which decreased in size over time, and prostate volume reduction, which was statistically significant only in patients who developed infarcts. 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