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Table of Contents
REVIEW ARTICLE
Year : 2018  |  Volume : 29  |  Issue : 6  |  Page : 266-276

Multiparametric magnetic resonance imaging of prostate cancer


1 Department of Radiology, Taichung Veterans General Hospital; Department of Veterinary Medicine, National Chung Hsing University; School of Medical Imaging and Radiological Sciences, Chung Shan Medical University, Taichung, Taiwan
2 Department of Radiology, Taichung Veterans General Hospital, Taichung, Taiwan

Date of Web Publication22-Nov-2018

Correspondence Address:
Siu-Wan Hung
Department of Radiology, Taichung Veterans General Hospital; Department of Veterinary Medicine, National Chung Hsing University; School of Medical Imaging and Radiological Sciences, Chung Shan Medical University, Taichung
Taiwan
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/UROS.UROS_57_18

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  Abstract 

As the number of elderly population increase, prostate cancer (CaP) becomes the most common cause of urological cancer of men in Taiwan. Multiparametric MRI (mp-MRI) combines standard pulse sequences and Functional Imaging, is a promising tool for CaP detection. Its role has changed from detection to preoperative staging. A consensus scoring system, PI-RAD™, is developed for interpretation and reporting.

Keywords: Multiparametric-magnetic resonance imaging, prostate imaging reporting and data system, prostate cancer, transrectal ultrasound biopsy


How to cite this article:
Hung SW, Lin YT, Liu MC. Multiparametric magnetic resonance imaging of prostate cancer. Urol Sci 2018;29:266-76

How to cite this URL:
Hung SW, Lin YT, Liu MC. Multiparametric magnetic resonance imaging of prostate cancer. Urol Sci [serial online] 2018 [cited 2018 Dec 13];29:266-76. Available from: http://www.e-urol-sci.com/text.asp?2018/29/6/266/237604


  Introduction Top


As the number of elderly population increases, prostate cancer (CaP) becomes the most common cause of urologic cancer in men in Taiwan.[1],[2] Although the incidence of CaP in Asian-Pacific countries is lower than Western countries.[3] Its annual incidence has increased over the last three decades. From 1980 to 2013, standardized incidence and mortality rates (per million population) are increasing from 2.57–29.22 to 1.88–6.52 [Figure 1].[1] This may be due to new developing techniques and modalities for screening and diagnosis of CaP.
Figure 1: The age standardized incidence and mortality rate of prostate cancer in Taiwan from 1980 to 2013 (per 100,000 persons)

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  Diagnostic Tools of Prostate Cancer Top


Urologists use prostatic-specific antigen (PSA) for screening and monitoring CaP. However, PSA is not a cancer-specific marker. It has the lead time or overdiagnosis bias for early detection of CaP. Urologists also use digital rectum examination (DRE) for the diagnosis of CaP. Combine using PSA and DRE may improve the early diagnostic rate in men with low PSA level. With elevated PSA or/with abnormal DRE, urologists use transrectal ultrasound (TRUS) for assessment of the prostate size and guidance for biopsy. Pathologists category different histopathological findings of CaP into Gleason score. We then use risk stratification to provide care decisions and a better allocation of resources to manage localized CaP base on PSA elevation, DRE stage, and Gleason score. We divide localized CaP into three groups: (a) Low-risk: PSA <10 ng/mL, and Gleason score ≤6, and clinical stage T1-T2a; (b) intermediate-risk: PSA 10–20 ng/mL, or biopsy Gleason score 7, or clinical stage T2b or T2c; and (c) high-risk: PSA >20 ng/mL, or Gleason score 8–10, or clinical stage >T2c.[4] Computed tomography (CT) is poor in soft-tissue resolution and no good at the prostatic zonal definition. Therefore, CT is not used for initial CaP evaluation. However, CT is good for the wide range of coverage. We use CT for preoperative staging and organs metastasis. Steyn and Smith used magnetic resonance imaging (MRI) for CaP detection after 1982.[5] Since DRE and TRUS biopsy have their own limitations,[6],[7] the role of MRI is used for localization and preoperative staging of a patient with biopsy-proven CaP.


  Morphological Imaging Top


Conventional MRI pulse sequences including T1-weighted imaging (T1WI) and T2-weighted imaging (T2WI). With their high spatial resolution, they highlight the zonal anatomy of the prostate gland. After Hricak et al.,[8] we divide prostate into four regions: The central zone (CZ), transition zone (TZ), peripheral zone (PZ), and fibromuscular zone (FMZ) [Figure 2]. They are just like a baseball glove holding a baseball. We combined the CZ and TZ to call it as central gland (CG, the baseball).[9] The CZ locates at the base of prostate just superoposteriorly to the proximal urethra. It surrounds the ejaculatory ducts from the prostatic base to verumontanum where ejaculatory ducts enter the urethra [Figure 2]c. In a normal person, the PZ comprises about 70% of the glandular tissue. It is just like a baseball glove running from base to apex of the gland and surrounds the distal urethra.[10] Urologists using DRE to palpate PZ. In T1WI, both CG and PZ are isointense signal intensity (SI). In T2WI, the PZ becomes high in SI where the CG is still iso and heterogeneous SI. The FMZ comprises stromal tissue has low SI on both T1WI and T2WI. T2WI depicts prostatic zonal anatomy and surgical capsule. We consider it as morphology MRI. Untreated CaP shows a mass effect to adjacent normal prostate tissue or surgical capsule invasion. They have homogenous low SI and ill-defined margin, so-called erased charcoal sign on T2WI [Figure 3]. Intraglandular hemorrhage is common in prostate MRI (90%–95%).[11] We found over 81.5% of hemorrhage in the PZ of the prostate (range 7–58 days, not published data in our center). Prostate hemorrhage is present with high SI, without or with fat suppression T1WI, so we use T1WI to look for postbiopsy hemorrhage in the prostate. However, hemorrhage has a low SI on T2WI. Therefore, it affects the staging interpretation of CaP in T2WI [Figure 4]. We recommend the optimal timing to get a post-biopsy prostate MRI to be at least over 6-8 weeks.[1],[9]
Figure 2: Normal prostate from a 65-year-old male. (a) Axial, (b) sagittal, and (c) coronal T2-weighted imaging show normal zonal anatomy of prostate. CG: Central gland; PZ: Peripheral zone; TZ: Transitional zone; NVB: Neurovascular bundle; E: Ejaculatory ducts; U: Urethra; V: Verumontanum; F = Fibromuscular zone; SV: Seminal vesicle; R = Rectum; T: Testis and B: Bladder

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Figure 3: An 83-year-old man, prostatic-specific antigen: 65 ng/ml and enlarged prostate gland. Transrectal ultrasound biopsy confirmed prostate cancer in the left lobe. (a) T2-weighted imaging: diffuse low T2 intensity in the left central gland with mass effect to left peripheral zone (arrows) (b) apparent diffusion coefficient (b = 1000 s/mm2): Heterogeneous low value (c) diffusion-weighted imaging (b = 1000 s/mm2): high-signal intensity as diffusion restriction (d-e) Region of interest A in adjacent to prostate tissue with dynamic contrasted-enhanced curve Type II and region of interest B in prostate cancer with dynamic contrasted-enhanced curve Type III (f) The whole-mount pathology found adenocarcinoma with Gleason's score: 4 + 3 = 7 in the left lobe

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Figure 4: In (a) and (c) using the same blue line framework of a 5 × 5 matrix marking on the same level of prostate for metabolites analysis at magnetic resonance spectroscopic imaging. (b) The whole mount presents a large prostate cancer occupying nearly the left lobe of prostate (dark circle) (d) The nonprostate cancer side and (e) the prostate cancer side are the related metabolites presents on the choosing pixel (yellow square) of their corresponding area of prostate tissue.

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  Functional Magnetic Resonance Imaging Top


TRUS biopsy[7] and DRE[6] have their limitations in CaP diagnosis. As the development of functional and metabolic techniques, the role of MRI is no more only for detection and localization of CaP. In a high-risk patient with a negative biopsy, MRI acts as a guidance for repeated TRUS biopsy. MRI is also used for follow-up on postprostatectomy and active surveillance on the result of treatment.[12] We are now using multiparametric MRI (mp-MRI) for diagnosis of CaP. mp-MRI consists of Morphology MRI (T2WI) and two or more other modern functional MRI sequences. They include MR spectroscopic imaging (MRSI), dynamic contrasted-enhanced (DCE)-MRI, and diffusion-weighted imaging (DWI). MRSI measures metabolites concentrations. DCE-MRI presents angiogenesis and DWI reflects cell density within the prostate. mp-MRI using different protocols according to a patient's risk of tumor spreading, the severity of abnormal PSA, clinical stage, and Gleason score.[4]


  Magnetic Resonance Spectroscopic Imaging Top


MRSI is a promising tool and can provide biochemical information of cellular metabolites. CaP tissue reduces the area of the cell lumen and increases nuclei. Such morphology changing shows relating to primary Gleason score pattern.[13] As a result, cellular metabolites such as choline (Cho), creatine (Cr), citrate (Cit), and spermine (Spm) will change.

We measure the metabolite ratio of Cho/(Cr + Spm) or (Cho + Cr)/Cit in CaP. These ratios show positively correlating with the percentage area of nuclei lesion[14] and Gleason score[15] [Figure 3]. However, MRSI requires endorectal coil (ERC) on 1.5T MRI scanner.[16] It is easily affected by homogenous electromagnetic field,[17] requires additional software expertise, training, support, and time-consuming.[18],[19] One study on “MRSI and accuracy of 3T mp-MRI” shows that MRSI failure to provide added value for sextant localization of CaP.[20] These make both urologists and radiologists drawback to MRSI usage. Therefore, the published guidelines of ACR have omitted its routine usage.[9]


  Dynamic Contrast-Enhanced Magnetic Resonance Imaging Top


CaP pronounced angiogenesis because of vascular growth factors secretion induced by local hypoxia or lack of nutrients during tumor progression. The neogrowth vessels with defects or pores in the endothelial wall have higher permeability and causing leakage of small molecules to extracellular space. DCE-MRI needs a rapid acquisition lesser than 15s per acquisition. By injecting low molecular weight contrast media and using this rapid acquisition pulse sequence, CaP has a pattern of an earlier peak of enhancement (wash-in) and rapid washout of contrast agent than those of the normal surrounding tissue.[21] This special enhancing pattern (DCE I/O) presents three kinds of curve form, types I, II, and III [Figure 5]. These curves relate to the aggressiveness of CaP. By looking for these curves, DCE-MRI can also help to monitor treatment effects.[22] In one recent study on the current role of mp-MRI shows that DEC is less used than other functional techniques.[23] However, DCE-MRI is most helpful when the T2W-MRI and DWI are equivocal[24] [Figure 6].
Figure 5: Three types of curve in dynamic contrasted-enhanced represents their wash-in and wash-out status as for prostate imaging reporting and data system v1 scoring

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Figure 6: A 68-year-old man come to our center for physical checkup and found to have prostatic-specific antigen 1.55 and digital rectal examination firm on both lobes of prostate palpation and about 35–40 gm. He was admitted under clinical staging T2c for pre-transrectal ultrasound biopsy magnetic resonance imaging. Axial (a) T2-weighted imaging, (b) apparent diffusion coefficient (b = 1000 s/mm2), and (c) diffusion weighted imaging (b = 1000 s/mm2) revealed that two suspected lesions (M and B) (d) dynamic contrasted-enhanced shows that both lesions are markedly increased in enhancement. The curve in (e) M lesion is Type III and in (f) B lesion is Type I (g) The whole mount pathology turns out only M lesion is adenocarcinoma with Gleason's score: 3 + 3 = 6.

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  Diffusion-Weighted Imaging Top


CaP tissue increases the cell density and shows a significant positive association with the percentage area of a lumen, the lumen-to-nuclei ratio,[14] and tumor Gleason score at final pathology.[25] DWI is one of the functional MRI methods that work based on water molecules movements within a tissue. Therefore, DWI is enabling qualitative and quantitative assessment of CaP aggressiveness. DWI uses apparent diffusion coefficient (ADC) value to reflect cell density within the prostate tissue. Increases the cell density will cause water restriction at diffusion study and reduces ADC value. CaP shows a higher SI on DWI and a lower ADC value as compared with normal prostatic tissue [Figure 7].[26]
Figure 7: Postbiopsy hemorrhage in the left seminal vesicle. (a) Axial T1-weighted imaging, high-signal intensity is found in the left seminal vesicle. Axial of (b) T2-weighted imaging, (c) apparent diffusion coefficient (b = 1000 s/mm2), and (d) diffusion-weighted imaging (b = 1000 s/mm2) show abnormal signal intensity in the left seminal vesicle. This may getting wrong interpretation of the left seminal vesicle being invasion by prostate cancer

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  Staging of Prostate Cancer Top


Early detection or organ-confined CaP is curable. Local staging of CaP is important for differentiating organ-confined (stage T1 or T2) from early advanced disease with extraprostatic extension (EPE) or seminal vesicle invasion (Stage T3). Urologists also require exact localization of cancer for appropriate treatments. These choices of treatment include prostatectomy, minimally invasive therapy for organ-confined cases, and hormone ablation or radiation therapy for advanced extraprostatic-extended cases.[27] Minimally invasive treatments include cryoablation, radiofrequency ablation, brachytherapy, photodynamic therapy, or high-intensity focused ultrasonography. A 1.5T MRI image acquisition needs to use an ERC-combined pelvic phased-array coil to get high-quality images of the prostate. A 3T MRI scanner may have the benefits of better signal-to-noise ratio, a shorter time for acquisition and without the use of ERC.[28]

With advanced MRI techniques, mp-MRI is considered the best imaging tools for detection of multiple foci CaP. It is more accurate in differentiation between Stage T2 and T3 CaP than other imaging modalities. Its higher accuracy rate may let the urologists have a well preoperative planning to consider a nerve-sparing option. This may decrease the morbidities associated with radical prostatectomy (RRP). The stage distribution of CaP in Taiwan from 2004 to 2012 is: Stage 2 is 40%, Stage 3 is 11%, and Stage 4 is 29%.[1],[2] An equal portion of cases is between <= Stage 2 and >= Stage 3. This makes radiologists of Taiwan even more concern on specificity and sensitivity of mp-MRI for local staging of CaP. A meta-analysis report (2014) shows that mp-MRI has a specificity of 0.88 and a sensitivity of 0.74 for CaP detection. Its negative predictive values (NPVs) ranging from 0.65 to 0.94.[29] Another recent document reports that mp-MRI has an NPV and specificity of 93% and 90% on EPE detection. MRI can also predict index lesion (P = 0.012).[30]

There are multiple MRI findings for a diagnosis of EPE in CaP. They include broad capsular tumor contact (>10 mm) [Figure 8], contour deformity or retraction [Figure 9] and [Figure 10], focal capsular thickening or irregular and/or bulging of outer margin, direct extension of the tumor beyond the capsule [Figure 9], obliteration of the rectoprostatic angle [Figure 8], asymmetry of the neurovascular bundle [Figure 9], and change of seminal vesicles such as focal decrease in SI, T2 hypointense with enlarged seminal vesicle or ejaculatory duct or direct tumor extension from the prostatic base [Figure 8].[27]
Figure 8: A 68-year-old man with very high prostatic-specific antigen level (242 ng/ml) with transrectal ultrasound biopsy proved to have adenocarcinoma with Gleason's score: 4 + 3 = 7. (a) Axial T2-weighted imaging fat saturation: diffuse prostate cancer directly invades to right seminal vesicle. The involved right seminal vesicle is enlarged with low-signal intensity). The left signal intensity has normal signal intensity. (b) Axial T2-weighted imaging without fat saturation: prostate tumor (M) directly invades to the right anterior part of rectum (arrow). Note that the right rectoprostatic angle is obliterated as compared with the left side (dashed line)

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Figure 9: A 67-year-old man with present illness of malignant neoplasm of bladder s/p BCG × 3. Now presents with persistent hematuria, weak stream of urination, and prostatic-specific antigen rising from 3.56 ng/ml (two years ago) to 19.31 ng/ml. Axial of (a) T2-weighted imaging, (b) apparent diffusion coefficient (b = 1000 s/mm2), and (c) diffusion-weighted imaging (b = 1000 s/mm2) shows a diffuse prostate cancer in the left peripheral zone with bulging out margin and disruption of surgical capsule (D) to left lateroposterior side of the prostate gland. Note that the asymmetrical appearance of left neurovascular bundle. Direct extension of prostate cancer to left neurovascular bundle is considered. (d) Axial and (e) coronal T2-weighted imaging shows a lymph node in the left obturator chain

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Figure 10: An 83-year-old man with symptom of severe lower urinary tract symptoms, enlarged prostate gland and prostatic-specific antigen 65 ng/ml. Transrectal ultrasound biopsy revealed adenocarcinoma of Gleason's score: 4 + 3 = 7 on both lobes. (a) Axial T2-weighted imaging, a mass (M) with low-signal intensity and ill-defined (erased charcoal sign) disrupted surgical capsule is found in the left central gland. Axial of (b) apparent diffusion coefficient (b = 1000 s/mm2) and (c) diffusion-weighted imaging (b = 1000 s/mm2) show diffusion restriction. (d) Magnetic resonance spectroscopic imaging shows increased choline at corresponding pixal areas. (e) Dynamic contrasted-enhanced map shows asymmetric rapid in-flow and outflow. Studying at high average enhancement areas (red dots in region of interest [a and b]) show both Type II and Type III curves due to different intrinsic tissue. (f) The whole mount pathological slide shows a large left lobe prostate cancer

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  Differentiation and Mimicking Lesions Top


mp-MRI combines morphologic and functional techniques. This gives promising accuracy in diagnosis, localization, risk stratification, and staging of significant CaP. Typical untreated CaP of mp-MRI as mentioned before has a lenticular appearance in CG, with mass effect to adjacent normal prostate tissue and/or surgical capsule in PZ. They are predominant to have homogenous low SI ill-defined (erased charcoal sign) and without capsule in T2WI in CG. They have diffusion restriction in DWI, asymmetric rapid DCE I/O, and increase Cho peak in MRSI [Figure 3] and [Figure 7].

The intrinsic prostate tissue composition[31],[32] and aggressiveness of CaP may affect the signal characteristics among different parameters of mp-MRI[31],[32],[33] [Figure 10]. Some literature reported missing or undetected CaP in MRI studies. Tan et al. reported that 53.3% missing CaP nodules in their 3T MRI studies. There has a higher sensitivity for lesion ≥1 cm and those missing lesions are likely to be low risk (75.2%, Gleason score of 6).[34],[35] One report on using b = 2000 s/mm2 DWI at 3T may even detect high-grade CaP in size of >5 mm[36] [Figure 11]. The missing detection rate is higher in the apex lesions.[34],[37],[38],[39] Urologists usually use DCE for following up treatment of CaP after RRP or radiation. A local recurrent CaP in RRP is a focal nodule adjacent musculature or crescentic subcapsular focus after radiation with losing zonal anatomy. It shows diffuse low SI but with higher SI than radiation patient in T2WI. It also presents diffusion restriction in DWI, gives early rapid washin and washout contrast in a DCE study and increased Cho peak in MRSI[40],[41] [Figure 12] and [Figure 13].
Figure 11: A 63-year-old man with elastic prostate, about 25–30 g and mild firm over right lobe at digital rectal examination. His prostatic specific antigen level is elevated to 5.36 ng/ml. (a) Axial T2-weighted imaging with saturation shows a small intermediate-signal intensity lesion (M) in the right peripheral zone, axial of (b) apparent diffusion coefficient (b = 1000 s/mm2) and (c) diffusion-weighted imaging (b = 1000 s/mm2) show mild diffusion restriction. Besides, there are multiple foci of high signal foci in diffusion-weighted imaging images. (d) The whole mount slide turns to be benign of the above lesion (M) but with missing of two small foci (1 mm × 1 mm) of prostate cancer with Gleason's score: 3 + 3 = 6. The total tumor volume is lesser than 1% of prostatic bulk. Retrospectively, the apparent diffusion coefficient image (b) could found two corresponding low b-value targets in same area of the whole mount slide.

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Figure 12: A 75 years old man with past history of adenocarcinoma of prostate of pathological staging of pT3aN0M0 s/p RRP nine years ago. Now with PSA level 13.13 ng/ml and suspected local recurrent. (a) T2WI: a small nodule (<1cm, arrows) bulging out from right side anastomosis just in front of rectum (R). (b) ADC and (c) DWI (b =1000 s/mm2): Nodule show mild diffusion restriction. (d) without contrast and (e) after contrasted T1WI show no definite evidence of enhancement. Sometimes, may be subtraction MR image may be helpful

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Figure 13: A 67-year-old man with prostate cancer and prostatic-specific antigen level 10.52 ng/mg s/p robotic-assisted laparoscopic retropubic radical prostatectomy with adenocarcinoma in both lobes of the prostate gland. Tumor invaded to prostatic surgical capsule and involved right apex. Gleason's score: 4 + 5 = 9 and pathological tumor, node, and metastases stage: pT3bNx was resulted. The prostatic-specific antigen level decreased to 0.061 ng/ml in 6 months but gradually rise up to 0.342 ng/ml. T2-weighted imaging (a) sagittal and (b) axial views found local recurrent in the left posterior side of anastomosis (M) and a residual seminal vesicle is found on the left side

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The operative site is common to have a fibrotic scar and has a low SI in T2WI that mimics recurrence CaP. However, fibrotic scar shows no or delayed enhancement on contrast injection.[42],[43] Sometimes, subtraction MRI imaging technique may be helpful in case of equivocal findings between mp-MRI images.

In DWI, there are many benign entities that may show diffusion restriction and mimicking CaP.[44] DWI is using SE sequence for acquisition. Therefore, the SI of the study tissue depends on both the T2 signal and signal attenuation in between the two lobe gradients that are being applied. In lesions with a very long T2 relaxation time, such T2 effects may produce a high signal even using high b values in acquisitions. It will cause a wrong interpretation of diffusion restriction. We call it as T2 shine-through effect.[39],[45] If a tissue has a very short T2 relaxation time, complete loss of the signal may have indifferent to what b value is being promoted. We call this effect as T2 dark-through or blackout effect.[46] Many entities confound the detection or interpretation of mp-MRI on CaP.[38],[39] They include anterior hypertrophic fibromuscular stroma, chronic and granulomatous prostatitis, hypertrophic nodule or normal displaced of CG, focal changes after radiation, focal areas of atrophy, necrosis or calcification, hemorrhage or pseudolesion at the midline of PZ, surgical capsule, prominent periprostatic venous plexus, and ejaculatory ducts between seminal vesicles.[40],[42],[44],[47] We will discuss some of them in the following paragraph and summarize others in [Table 1].[37],[38],[39],[42],[47]
Table 1: Wide variety of entities mimicking Prostate Cancer

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Benign prostatic hyperplasia (BPH) is usually present in a round or oval shape nodule with well-defined margin in MRI. It is commonly found in the TZ. They show heterogeneous and various SI appearance in T2WI according to their compositions of glandular and stromal tissue. The stromal BPH nodules may mimic TZ cancer that may show low SI on T2WI, diffusion restriction in DWI, early enhancement on DCE images and elevated choline peaks in MRSI. However, they are rounded in appearance and have a more well-defined low SI “capsule” in T2WI and have symmetric rapid contrast wash-in and wash-out at DCE. Sometimes, a stromal nodule presents in PZ and often with normal prostate tissue between the nodule and the surgical capsule[38],[40],[47] [Figure 14] and [Figure 15].
Figure 14: Same case from Figure 14. The 63-year-old man with elastic prostate, about 25–30 gm, and mild firm over the right lobe at digital rectal examination. (a) Axial T2-weighted imaging without fat saturation shows heterogeneous low-intensity lesion (M) in the peripheral part of right central zone (central gland) with interrupted surgical capsule. Both (b) apparent diffusion coefficient (b = 1000 s/mm2) and (c) diffusion-weighted imaging (b = 1000 s/mm2) show diffusion restriction. Prostate cancer is suspected in the right central gland with direct invasion of surgical capsule. However, as mentioned above, (d) the postoperative result is not tumor over right central gland as show in whole mount slide. (e) Retrospective review the coronal view T2-weighted imaging shows that such lesion is round shape in coronal view and is lying within a well-defined low-signal surgical capsule. Prominent hyperplastic nodule displacing normal tissue of central gland is final considered

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Figure 15: A 78-year-old man under the past history of prostatic hyperplasia with medication and slightly increased prostatic specific antigen level (5.8 ng/ml). Digital rectum examination found mild firm in the right lobe. Transrectal ultrasound biopsy showed nodular hyperplasia. (a) T2-weighted imaging without fat saturation: low and slightly heterogeneous intensity (M) in the left side of enlarged central gland. (b) apparent diffusion coefficient and (c) diffusion weighted imaging (b = 1000 s/mm2): Mild diffusion restriction. (d) Dynamic contrasted-enhanced: All types of curve. The whole mount: Only nodular hyperplasia, chronic prostatitis, and some low-grade prostatic intraepithelial neoplasia (dashed line)

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Chronic prostatitis appears ill-defined margin, band-like or wedge-shaped in morphology. It is not common to have contour deformity or mass effect on the adjacent normal prostate tissue or surgical capsule as seen with CaP. They are less to have low SI at T2WI and do not have elevated choline or decreased citrate in MRSI. It has only mild diffusion restriction on the ADC map but might have symmetric rapid contrast wash-in and wash-out at DCE[40],[42],[44],[47],[48] [Figure 11], [Figure 14] and [Figure 15]. Granulomatous prostatitis is more common to have a large area of nonenhancement due to necrosis. Sometimes, a low ADC value together with a low DCE score may suggest granulomatous prostatitis rather than CaP.[49]


  Prostate Imaging Reporting and Data System Top


The accuracy of morphologic MRI for detection, localization, and characterization of CaP is high. Although the mp-MRI is a promising tool in the diagnosis of CaP, its usefulness in clinical practice is not established. There are discrepancy and inconsistency between the conduct, interpretation, and reporting of prostate mp-MRI. A group of multidisciplinary experts considers improving MRI techniques for CaP diagnosis. A formal consensus on the above discrepancy and inconsistency was developed. In response to increasing the quality and diagnostic value of prostate MRI, the ESUR published the first version of the prostate imaging reporting and data system (PI-RADS™) in early 2011.[4]

The PI-RADS™ v1 counts on a Likert-like five-grade scaling systems on each of the MRI parameters. It gives each suspected lesion an overall score including T2W, DWI, and DCE ranged between 3 and 15 to predict the chance of being a clinically significant CaP. The score >9 is rated positively for malignancy independent of the zonal location of the lesion.

In 2014, to make PI-RADS™ standardization and more globally acceptable, a joint steering committee has formed by the AdMeTech Foundation. The ACR and the ESUR released an updated version of PI-RADS™ v2 to overcome limitations of PI-RADS™ v1.[9],[50],[51] PI-RADS™ v2 improves detection, localization, characterization, and risk stratification in patients with suspected cancer in treatment naïve prostate glands. PI-RADS™ v2 uses a 5-point scale based on the likelihood (probability) for each lesion in the prostate gland. It combines mp-MRI findings on T2W, DWI, and DCE correlates with “the presence of a clinically significant cancer.”

PI-RADS™ v2 categories into PIRADS-1: Very low (clinically significant cancer is highly unlikely to be present), PIRADS-2: Low (clinically significant cancer is unlikely to be present), PIRADS-3: Intermediate (the presence of clinically significant cancer is equivocal), PIRADS-4: High (clinically significant cancer is likely to be present), and PIRADS-5: Very high clinically significant cancer is highly likely to be present). The timing of MRI following prostate biopsy is addressed to at least 6 weeks or longer for a staging patient. Recent PSA level, DRE findings, biopsy results, Gleason scores, and medications of hormones should be available to the radiologist at the time of MRI examination performance and interpretation.

As compared to PI-RADS™ v1, PI-RADS™ v2 proposes just one protocol for MRI of the prostate without separate parameters offered. PI-RADS™ v2 considering the location and the size (should be ≥0.5 cc) of a lesion at T2WI and DWI. Then, using the dichotomized score (positive or negative) for DCE findings. We use a specific sequence for point scoring depending on the zonal location of the evaluated lesion. The PZ lesion will use DWI, whereas TZ lesion will use T2WI for assessment. Index lesion should be identified and the EPE should be the first priority to be addressed. The highest RADS™ v2 Assessment Categories are the second consideration criteria. A final 5-point score for each lesion is assigned to the decision process. As CaP is usually multifocal, in PI-RADS™ v2, only the four highest PIRADS score will be reported. There is also have a strong recommendation that the imaging plane angle, location, and section thickness (3 mm) are identical for all sequences.[50],[51],[52] A sector map is recommended to use for defined localization in a report in case of extreme changing prostate anatomy in CaP [Figure 16].[50] A report on using PI-RADS™ v2 reveal very excellent interreader agreement (0.85). Using PI-RADS v2 may help preoperatively diagnose on the clinically significant CaP.[53] Another report shows that general body radiologists and prostate specialists can detect high-grade index CaP lesions with high sensitivity (79% to 90%) and agreement (88% to 95%).[54]
Figure 16: Sector map from PI-RADS™ v2 p. 33 under the statement from PI-RADS™ v2 in acceptable electronic (on computer) recording (p. 14). The prostate sector map was modified by David A. Rini, Department of Art as Applied to Medicine, Johns Hopkins University and was adapted from a European Consensus Meeting and the European Society of Urogenital Radiology prostate magnetic resonance imaging Guidelines 2012. It employs 39 sectors/regions: 36 for the prostate, 2 for the seminal vesicles, and 1 for the external urethral sphincter.

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  Future Role of mp-MRI Top


The RADS™ v2 has removed the MRSI and reduced the role of DCE in PIRADS assessment category. DCE has a limitation in specificity. For cost-down consideration, expertise pointed out that biparametric MRI (bp-MRI: T2WI + DWI) may be the next coming modified edition for PI-RADS™ v2. Our experience considers that DWI is the dominant sequence in intermediate- or high-risk CaP detection[33] both in CG and PZ.[55],[56] The recent reports show that the overall accuracy of CaP detection in bp-MRI was 79%.[57] The combined usage of bp-MRI and PSA or PSA density resulted in improved accuracy for detecting clinically significant CaP.[58]

mp-MRI is a promising noninvasive imaging tool for CaP screening. It helps detect primary cancer, recurrent cancer, localize, and staging a locally advanced disease. It has high accuracy and reliability in the diagnosis of index CaP lesion. RADS™ v2 considered that the detection of index lesion is important,[50] for it may change further CaP management pathway to focal therapy.[59]

Other research tools are also working for CaP detection. They are MRI-guided biopsy, MRSI at 7T, diffusion tensor imaging, diffusional kurtosis imaging, multiple b value assessments of fractional ADC, intravoxel incoherent motion, blood oxygenation level dependent imaging, intravenous ultra-small superparamagnetic iron oxide agents, and MR-positron emission tomography. With the advancement and adding usage of these novel tools, it may further improve the clinical practice of CaP treatment in the coming future.[50]

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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