Research article
Prostatic abscess and seminal vesicle abscess: MRI findings and quantitative
analysis of apparent diffusion coefficient values
Jing Ren
a,1, Xufang Huang
a,1, He Wang
b, Huijia Liu
a, Fang Ren
a, Zhuoli Zhang
c, Yong Yang
a,
Hong Yin
a, Yi Huan
a,*
aDepartment of Radiology, Xijing Hospital, Fourth Military Medical University, Changle West Road 127#, Xi'an City 710032, People's Republic of China bDepartment of Urology, Tangdu Hospital, Fourth Military Medical University, Xinsi Road 1#, Xi'an City 710038, People's Republic of China cDepartment of Radiology, Feinberg School of Medicine, Northwestern University, Robert H. Lurie Comprehensive Cancer Center, 737 N. Michigan Ave, 16th
Floor, Chicago, IL 60611, USA
Received 11 October 2014; accepted 14 February 2015 Available online 28 May 2015
Abstract
Purpose: To investigate the conventional magnetic resonance imaging (MRI) and diffusion-weighted imaging (DWI) findings of prostatic ab-scess (PA) and seminal vesicle abab-scess (SVA), and to evaluate the qualitative and quantitative diagnostic value of apparent diffusion coefficient (ADC) for these conditions.
Materials and methods: Ten patients with PA and 2 patients with SVA who underwent MRI and DWI examinations (b¼ 0 and 1000 s/mm2) at our institution were enrolled in this study. The diagnoses of all the patients were pathologically and clinically proved. Their conventional MRI and DWI images as well as ADC values were analyzed. The ADC values between PA and surrounding prostate tissues were compared using t test. Statistical significance was inferred at P< 0.05.
Results: Six patients were revealed with multiple foci. The other 6 showed singular lesion, including the 2 with SVA. A total of 23 focal ab-scesses were demonstrated with iso-low T1WI signal as well as hyperintense T2WI and DWI signals. Four patients with PA presented air within the lesions by both T1WI and T2WI. The mean ADCs of PA, (0.618± 0.192) 103mm2s1, were significantly lower than that of surrounding prostate tissues, (1.417± 0.147) 103mm2s1(P¼ 0.000).
Conclusion: PA and SVA can be characteristically demonstrated by MRI, and the lesions show especially restricted diffusion by DWI with low ADC values. These demonstrations facilitate their qualitative and quantitative diagnosis.
© 2015 Beijing You’an Hospital affiliated to Capital Medical University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Apparent diffusion coefficient; Diffusion weighted imaging; Magnetic resonance imaging; Prostatic abscess; Seminal vesicle abscess
1. Introduction
Both prostatic abscess (PA) and seminal vesicle abscess (SVA) are clinically rare conditions whose diagnoses are challenging [1e4]. Due to inadequate treatment of bacterial prostatitis, PA can be complicated and presents as dysuria, frequent urination, perineal pain, fever, chills and lower back
pain [5,6]. Because of their nonspecific and similar clinical symptoms, the diagnosis and differential diagnosis between PA and SVA are extremely difficult on clinical grounds. Since both the conditions require drainage in addition to antibiotics, the early recognition is vitally important.
Many radiological modalities, such as CT (computed to-mography), MRI, transrectal ultrasound (TRUS) and fluorine-18 fluoro-deoxyglucose PET, have been widely used in the diagnosis of prostate and seminal vesicle diseases[7]. MRI has been recognized as an excellent examination since it favorably distinguishes tissue of the prostate and seminal vesicle [8]. MRI diffusion-weighted imaging (DWI) is sensitive to the
* Corresponding author. Tel./fax: þ86 029 84775380. E-mail address:[email protected](Y. Huan).
1
Jing Ren and Xufang Huang contributed equally to this study.
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microscopic self-diffusion of water protons and therefore captures the water diffusion microenvironment at the cellular level. DWI measures the apparent diffusion coefficient (ADC) quantitatively, providing an absolute assessment of the average water diffusion per voxel[9]. Therefore, DWI is believed to be a quantitative biomarker to identify malignancy[10]. Previous studies have demonstrated that ADC values calculated from DWI data may have clinical significance in diagnosis of prostate cancer (PCa) and seminal vesicle invasion (SVI)[11]. Moreover, DWI can diagnose both cerebral and abdominal abscess with high accuracy. Data from recent studies indicated that DWI is more sensitive than conventional MRI imaging in distinguishing abscesses and cystic tumors[12e14].
To the best of our knowledge, DWI data of only 2 cases of PA was reported by Singh et al. in 2011[15]. MRI data of SVA has not been reported yet, and the diagnostic value, both qualitatively and quantitatively, of DWI and ADC for PA and SVA remains unclear. The purpose of our study was to investigate the conventional MRI and DWI data of PA and SVA, and to evaluate the qualitative and quantitative diag-nostic value of ADC for PA and SVA.
2. Materials and methods 2.1. Patients
From August 2006 to February 2014, 10 patients with PA and 2 patients with SVA aged 28e67 years (with a mean of 46.9 years) who underwent MRI examination in our institution were enrolled in this study. The diagnoses of all the patients were pathologically and clinically proved. The protocol of this study was approved by the research review board of our institution. After the risks and possible benefits of the research were fully explained, all enrolled patients gave their written informed consent.
2.2. MRI examination
MRI examination was performed using a clinical 3.0 T whole-body magnetic resonance system (Siemens Magnetom Trio, Erlangen, Germany). The system is capable of working at a maximum slew rate of 200 mT/m/ms and a maximum gradient strength of 40 mT/m. Integrated system body coils were used for radiofrequency excitation and a phased array coil was used for signal reception. The patients were scanned in the supine position following scout scans to determine optimal imaging planes. Axial and coronal T1W and T2W fast spin-echo scans were performed using the following parame-ters: T1W, repetition time/echo time: 550/15 ms, field of view (FOV): 140 mm 140 mm; slice thickness: 3 mm; matrix size: 320 256; and T2W, repetition time/echo time: 4430/ 78 ms, FOV: 140 mm 140 mm; slice thickness: 4 mm; matrix size: 320 256. Axial orientation DWI was then performed using the following parameters: single-shot SE echo planar imaging sequence, b-values were 0 and 1000 s/
mm2, repetition time/echo time: 7500/80 ms, FOV:
400 mm 400 mm, slice thickness: 5 mm, matrix size:
128 256. The slices for DWI were aligned in an identical orientation to that used for the acquisition of T2W images (essentially to permit coregistration for subsequent comparisons).
2.3. MRI image reading
For each DWI series, ADC maps were reconstructed on a voxel wise basis according to the following relation:
ADC¼ ðln½S1 ln½S0Þ=b1 b0
where S1is the signal intensity of a voxel after application of
the diffusion gradient and S0is the signal magnitude without
diffusion gradients applied (b¼ 0 s/mm2)[16]. The diffusion sensitivity is determined by the difference between b1and b0,
which, in this study, was 1000 s/mm2.
Two radiologists retrospectively reviewed all MR images independently, with no informed information about the pa-tients. They reviewed the T2W images, DW images and ADC maps for each patient and selected abscess lesions and normal surrounding prostate tissues for evaluation by using the semiautomatic software MR MedFIA 1.0 (independently designed and developed in our department) for three-dimensional registration, segmentation, and ADC value anal-ysis of multi-parametric MR imaging data.
2.4. Clinical and pathological assessments
Physical examinations, including a digital rectal examina-tion (DRE), were performed. Midstream urine and urine cul-ture were analyzed. After MRI examination, all the patients received antibiotic therapy. The patients underwent CT, ul-trasound (US) or MRI examination at least once during the 4e12 months follow-up.
2.5. Statistical analysis
All data were expressed by mean± standard deviation and tested for normal distributions. ADC values for prostatic ab-scess and the normal surrounding prostate tissues were compared using Student's t test. P < 0.05 was considered to indicate a statistically significant difference. Data analyses were performed with SPSS statistical software (version 17.0 for Windows; SPSS, Inc., IL, USA).
3. Results 3.1. Clinical data
Six patients showed high risk factors: 3 with diabetes, 1 with PA after a prostatic biopsy and 2 with double J ureteral stent replaced after ureteroscopy. All the patients presented with urinary urgency, frequency, dysuria, and fever. Four pa-tients showed perineal pain, and 1 with tenesmus. DRE ex-amination presented enlarged and tender prostate in all 12
patients. The urine culture grew Escherichia coli in 6 patients, Staphylococcus aureus in 3, and the other 3 sterile (Table 1). All the patients improved clinically after administration of antibiotics for 4 weeks.
3.2. MRI findings
Six patients were revealed with multiple foci, and the other 6 showed singular lesion, including 2 with SVA whose lesions involved the bilateral SV. A total of 23 focal abscesses were demonstrated whose sizes ranged from 0.5 cm to 3.3 cm (with a mean of 1.51 cm) in diameter. Most PA lesions were located in the central zone (9/10 patients) and peripheral zone (7/10 patients). Four abscesses showed irregular border, and the other 19 lesions were round or oval in shape. Four abscesses perforated the prostate capsule. The abscess wall was thin, smooth, with equal slightly high signal intensity on T1WI and low signal intensity on T2WI. Two patients with SVA showed enlarged seminal vesicles, with high signal intensity and multiseptated fluid collection on T2WI. The abscesses showed iso-low signal intensity on T1WI, hyperintensity on T2WI and DWI. Four PA lesions presented low signal areas of gas within the lesions both on T1WI and T2WI (Table 1,Figs. 1 and 2).
3.3. Quantitative ADC analysis
The mean ADCs of PA were (0.618± 0.192) 103mm2/ s, which were significantly lower than those of surrounding prostate tissues [(1.417± 0.147) 103mm2/s] (t¼ 10.574,
P ¼ 0.000, Fig. 3). The ADC values of SVA were
(0.544 ± 0.0113) 103 mm2 s1 and
(0.620± 0.0201) 103mm2s1, respectively (Fig. 2).
4. Discussion
Both PA and SVA are difficult to be diagnosed, because at the symptoms onset they resemble to several other diseases of the lower urinary tract. Although bacteremia or sepsis origi-nating anywhere in the body has been implicated, simple bacterial prostatitis is the common predisposing factor [17]. The organisms most frequently isolated from PA are E. coli and other gram-negative bacilli, and other isolates include Staphylococcus species and an expanding spectrum of bacteria and fungi [18]. The patients present with urinary urgency, frequency, and dysuria, or with retention. And the patients commonly experience other symptoms like fever, rigors, se-vere rectal or perineal pain, and sometimes tenesmus. The prostate is enlarged, usually unilaterally more serious, and is exquisitely tender. The symptoms of SVA are the same as those of PA, but the heat, swelling and tenderness, instead of being over the prostate, are higher over one or both SV. If left untreated, the abscess may burst into urethra, perirectal tissues (where it can present as an ischiorectal abscess), perineum, or rectum to form a rectourethral fistula. The first therapy is administration of antibiotics and percutaneous drainage of the abscess. Sometimes, however, surgery is necessary (trans-vesical or perineal approach). Therefore, timely and accurate diagnosis is extremely important.
Imaging is important in assessing the prostate and SV when abscess is suspected. TRUS and CT remain to be the most frequently described diagnostic examinations in the recent literature [19,20]. US is used to examine the presence of ab-scess, which can be visualized as a heterogeneous and moderately hypo-echoic round lesion, with septations and thick walls, and with increased vascularization on Doppler scanning. It is then also possible to puncture and drain the
Table 1
Character of prostatic abscess and seminal vesicle abscess. Patient Pathogen MR findings
Location Number of lesion
AOI T1WI T2WI DWI Gas
Prostatic abscess
1 S. aureus PZþ CZ 4 Levator aniþ anterior rectal wall
Iso signal, not very clear
Heterogeneous hyperintense, fluidefluid level
Strongly hyperintense þ 2 S. aureus PZþ CZ 3 Anterior rectal wall Iso signal,
not very clear
Heterogeneous hyperintense Strongly hyperintense e 3 S. aureus PZþ CZ 2 e Iso signal Heterogeneous hyperintense Strongly hyperintense e 4 E. coli PZþ CZ 3 Mesorectum Iso-low signal Heterogeneous hyperintense Strongly hyperintense þ 5 E. coli PZþ CZ 3 Mesorectum Iso-low signal Heterogeneous hyperintense,
septated
Strongly hyperintense þ 6 E. coli PZþ CZ 2 e Iso-low signal Heterogeneous hyperintense Strongly hyperintense þ
7 E. coli CZ 1 e Low signal Hyperintense Hyperintense e
8 E. coli CZ 1 e Iso signal Heterogeneous hyperintense Hyperintense e
9 e CZ 1 e Iso signal Hyperintense Hyperintense e
10 e PZ 1 e Iso signal Hyperintense Strongly hyperintense e
SVA 11 E. coli Bilateral SV 1 e Iso signal
septated
Heterogeneous hyperintense, septated
Strongly hyperintense e
12 e Bilateral SV 1 e Iso signal
septated
Heterogeneous hyperintense, septated
Strongly hyperintense e PA¼ prostatic abscess, SVA ¼ seminal vesicle abscess, CZ ¼ central zone, PZ ¼ peripheral zone, SV ¼ seminal vesicle, AOI ¼ adjacent organ invasion, E. coli¼ Escherichia coli, S. aureus ¼ Staphylococcus aureus.
abscess under transperineal or transrectal US guidance
[21,22]. CT is commonly ordered to define abscess borders, to define the infection involved perineum or pelvis, to find causative factors, and to choose the appropriate approach for
Fig. 1. A 37-year-old man with prostatic abscess at both central and peripheral zones. The abscess perforates the prostate capsule (a, b, c, d and e are the same imaging slice). (a) Transverse T1-weighted imaging shows abscess with iso-signal intensity that perforates the posterior border of the prostate. (b) The abscess is shown with hyperintensity on T2-weighted imaging. (c) The lesions show high signal intensity compared to the surrounding tissue on diffusion weighted imaging (DWI). Air in the abscess has low signal intensity (arrow) on T1, T2 and DWI (arrow). (d) Apparent diffusion coefficient (ADC) map demonstrates low signal intensity. (e) T2 and DW fusion image, (f) Coronal reconstruction of DWI shows the abscesses more comprehensively. (g) His-togram shows distribution of ADC values on a voxel-by-voxel basis for analysis by software MR MedFIA 1.0. The average ADC value is (0.590± 0.0144) 103mm2s1.
Fig. 2. A 54-year-old man with seminal vesicle abscess that involves his bilateral seminal vesicles (a and b are the same imaging slice. c, d, e and f are the same imaging slice). (a) Transverse T1-weighted imaging shows swelling, iso-low signal intensity and separated bilateral seminal vesicles. (b) The swelling and separated bilateral seminal vesicles are shown with hyper-intensity on T2-weighted imaging. (c) Sagittal post-contrast T1 TSE FS shows septal enhancement of the seminal vesicle abscess. (d) Sagittal reconstruction of diffusion-weighted imaging (DWI) shows the seminal vesicle abscess with strongly high signal intensity compared to the surrounding tissue. (e) Sagittal reconstruction of apparent diffusion coefficient (ADC) map demonstrates low signal intensity. (f) Contrast-enhanced T1 and DW fusion image shows the abscess more intuitively. (g) Histogram shows distribution of ADC values on a voxel-by-voxel basis for analysis by software MR MedFIA 1.0. The average ADC value is (0.544± 0.0113) 103mm2s1.
surgical drainage[22,23]. MRI, due to its higher spatial res-olution, better tissue contrast, and multiplanar imaging capa-bilities, is superior to CT and US in assessing infection involved prostate and SV[18]. High-resolution and functional MRI is widely used in patients with prostate carcinoma[24]. As the only noninvasive functional MR technique that ana-lyzes the diffusion of water molecules in vivo, DWI has gained recognition as an imaging biomarker for tissue characteriza-tion and funccharacteriza-tional evaluacharacteriza-tion[25]. It has been used to assess the prostate mainly in PCa. However, recently, the use of DWI has expanded to the field of diagnosis of deep pelvic abscess
[26].
In this study, we believe MRI to be a very helpful tool for the diagnosis of PA and SVA. A typical MRI finding of ab-scess is the central gas sign in the lesion, which is character-ized by central low signal intensity and thus can be conveniently detected by MRI. As observed in our series, 4 (4/ 10, 40%) PA patients showed this diagnostic sign. The ab-scesses were demonstrated with iso-low signal intensity and were poorly defined by T1WI. By T2WI, however, the ab-scesses were visualized with heterogeneous hyperintense signal, and were better defined than those by T1WI. In addi-tion, strongly hyperintense signal on DWI may be another typical finding of the abscesses. In this series, 7 (7/10, 70%) PA patients and 2 (2/2, 100%) SVA patients showed strongly hyperintense signal on DWI. The other 3 PA patients showed hyperintense signal on DWI. Meanwhile, 3-dimensional reconstruction and fusion with T2WI after DWI may further facilitate spatial colocation of anatomical background and functional information obtained from DWI. Therefore, DWI may help the radiologists to diagnose abscess with higher specificity. Furthermore, some patients may not receive intravenous contrast agent due to a history of allergy or renal failure and may thus be examined with DWI as a stand-alone technique.
In 2011, Singh et al. reported DWI appearance in 2 cases of PA confirmatively diagnosed by urine test/culture[15]. The PA lesions were visualized with hyperintense T2 signal in the
prostate, while the lesions were poorly visualized on T1W images with peripheral enhancement on post gadolinium im-ages. The DWI revealed diffusion restriction in the lesions. The corresponding ADC map revealed low signal, with mean
values of (0.63 ± 0.07) 103 and
(0.61 ± 0.06) 103 mm2/s, respectively. Although the re-sults were close to ours, we described MRI and DWI features of 10 patients with PA and 2 patients with SVA. In addition, in this study, ADC values of these lesions were firstly analyzed per voxel using semiautomatic software MR MedFIA 1.0, thus avoiding errors caused by choice of region of interested (ROI).
In our series, the mean ADCs of PA was
(0.618± 0.192) 103mm2/s, significantly lower than that of surrounding prostate tissues. No overlap between the two groups was detected. Moreover, compared to our previous research data, the ADCs of PA were lower than those of PCa
[10]. The ADCs of SVA in this study were lower than those of normal SV and PCa SVI [11]. Pus in abscesses is strongly hyperintense on trace DW images and has a decreased ADC value, thus can be qualitatively and quantitatively diagnosed. Limitations of our study include the relatively small size of sampling examined by MRI and no biochemical analysis of purulent specimen from abscess to define its correlation-ship with ADC. PA, especial SVA, is an extremely uncom-mon condition and difficult to be clinically diagnosed. At present, large-scale sampling of such patients is hardly possible. Cohort with abscess-mimicking pathologies for comparison and therefore, the interpretation of our study results is limited to the characterization of abscess containing pus.
In conclusion, PA and SVA show characteristic manifesta-tions on MRI, especially restricted diffusion on DWI with low ADC values. MRI displays their size, number and invasion of adjacent organs clearly, thus can facilitate the qualitative and quantitative diagnosis of PA and SVA.
Grant support
The authors express thanks for grant support to the National Natural Science of China, Scientific and Technological Foundation of Shaanxi Province and High-tech Clinical Pro-jects of Xijing Hospital (Grants NSFC 81370039, NSFC 81220108011, 2007K14-02(5) and XJGX13LC11). The au-thors have no other relevant affiliations or financial involve-ment with any organization or entity with a financial interest in or financial conflict with the subject matter or materials dis-cussed in the manuscript apart from those disclosed. No writing assistance was utilized in the production of this manuscript.
Acknowledgments
The authors express thanks for grant support to the National Natural Science of China and High-tech Clinical Projects of
Xijing Hospital (Grants NSFC 81370039, NSFC
81220108011, and XJGX13LC11).
Fig. 3. Boxplot shows apparent diffusion coefficient (ADC) values measured between prostatic abscess and surrounding prostate tissue. The ADC values of prostatic abscess are significantly lower than those of surrounding prostate tissue (t¼ 10.574, P ¼ 0.000). Vertical bars represent standard deviations.
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