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Prostate magnetic resonance spectroscopic imaging at 1.5 tesla with endorectal coil versus 3.0 tesla without endorectal coil: comparison of spectral quality
Clinical Imaging 39 (2015) 636–641
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Prostate magnetic resonance spectroscopic imaging at 1.5 tesla with endorectal coil versus 3.0 tesla without endorectal coil: comparison of spectral quality Pieter De Visschere a,⁎, Marco Nezzo b, Eva Pattyn a, Valérie Fonteyne c, Charles Van Praet d, Geert Villeirs a a
Department of Radiology, Ghent University Hospital, De Pintelaan 185, 9000 Gent, Belgium Department of Radiology, University of Rome Tor Vergata, 00133 Rome, Italy c Department of Radiotherapy, Ghent University Hospital, Gent, Belgium d Department of Urology, Ghent University Hospital, Gent, Belgium b
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Article history: Received 31 December 2014 Received in revised form 6 February 2015 Accepted 9 February 2015 Keywords: Prostate Magnetic resonance imaging Quality assessment MR spectroscopy 3.0 Tesla
a b s t r a c t Objectives: To compare the spectral quality of prostate magnetic resonance spectroscopic imaging (MRSI) at 1.5 Tesla with endorectal coil (ER-1.5T) to MRSI at 3.0 Tesla without coil (3.0T). Methods: In 30 patients, the spectral quality of 6107 voxels at ER-1.5T and that of 5667 at 3.0T were visually evaluated by three radiologists. Results: There were 57.6% good quality voxels at ER-1.5T versus 64.3% at 3.0T (P=.121). The posterior two rows showed better quality at ER-1.5T (P=.047). Conclusion: There is no significant difference in overall spectral quality between ER-1.5T and 3.0T, although ER-1.5T shows better quality close to the endorectal coil.
© 2015 Elsevier Inc. All rights reserved.
1. Introduction Magnetic resonance imaging (MRI) is increasingly being used in the assessment of patients with suspected or known prostate cancer. Multiparametric MRI (mpMRI) is the current state-of-the art imaging technique, consisting of morphological T2-weighted images (T2-WI) supplemented with at least two functional imaging techniques such as diffusionweighted imaging, magnetic resonance spectroscopic imaging (MRSI), and dynamic contrast-enhanced imaging [1]. The optimum field strength for performing mpMRI of the prostate is still a source of debate and the same accounts for the question whether an endorectal coil is needed or not. The use of an endorectal coil considerably improves the signal-tonoise ratio (SNR) in the prostate, and at 1.5T, it is considered mandatory, especially when performing MRSI [1,2]. The endorectal coil, however, increases the examination cost and is deemed uncomfortable for the patient [2,3]. On high-field-strength MRI scanners, the SNR is higher and therefore the use of an endorectal coil may be avoided [4,5]. In this study, we evaluated the spectral quality of prostate MRSI at 1.5T with endorectal
⁎ Corresponding author. Department of Radiology, Ghent University Hospital, 0 DPP, De Pintelaan 185, 9000 Gent, Belgium. Tel.: +32-93326688. E-mail address: [email protected] (P. De Visschere). http://dx.doi.org/10.1016/j.clinimag.2015.02.008 0899-7071/© 2015 Elsevier Inc. All rights reserved.
coil (ER-1.5T) as compared to 3.0T imaging without endorectal coil (3.0T) in the same patients who underwent both examinations.
2. Materials and methods 2.1. Patients At our institution, all mpMRI exams were performed on a 1.5T scanner using an endorectal coil (ER-1.5T) before May 2011 and on a 3.0T scanner without endorectal coil (3.0T) thereafter. All patients who had both an ER-1.5T and a 3.0T exam between March 2004 and February 2012 were eligible for this study. mpMRI was performed as part of a follow-up program in patients with elevated prostate-specific antigen (PSA) but negative biopsies and/or mpMRI. Exclusion criteria were patients treated for prostate cancer before or in the interval between both scans, patients who had transurethral resection of the prostate between the scans, and patients with metallic hip prosthesis. Thirty randomly selected patients were thus included. Mean age, serum PSA, and prostate volume were 64.9 (42–77) years, 7.7 (2.8–23.0) ng/ml, and 50.3 (26.1–107.4) ml for the exams at ER-1.5T, and 67.3 (43–78) years, 10.9 (4.4–27.0) ng/ml, and 62.4 (27.6–148.3) ml at 3.0T. Mean time interval between the scans was 2.5 years (range, 0.3–7.8 years). The study was
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approved by our hospital’s Ethics Committee (EC 2011/495) and the need for informed consent was waived for this retrospective study. 2.2. MRI technique Data acquisition parameters were matched as closely as possible between the two field strengths (Table 1). Spectral maps, overlaid on the corresponding transverse T2-WI, were obtained using Syngo software (Siemens Medical Systems, Erlangen, Germany). The peak areas of choline, creatine, and citrate levels were determined in the spectrum according to their expected peak locations and widths over a frequency range 1.3–4.4 ppm. Citrate peaks were identified as quadruplets, doublets, or single peaks (according to spectral quality and field strength) around 2.6 ppm, whereas choline and creatine peaks were recognized as single peaks around 3.2 and 3.0 ppm, respectively. With lower spectral quality or in case that choline or creatine peaks were very high, both peaks were regarded as one peak [6]. 2.3. Image analysis Images were analyzed on a dedicated workstation (Leonardo, Siemens Medical Systems, Erlangen, Germany). In each patient, one radiologist (P.D.V.) selected three transverse slices (in the prostatic base, midprostate, and apex) at ER-1.5T, and chose the corresponding slices at 3.0T using anatomical landmarks on the T2-WI of both studies such as the urethra, benign hyperplastic nodules in the transition zone (TZ), and the periprostatic anatomy. In these slices, the quality of all spectroscopic voxels within the prostate contour was evaluated. In total, 6107 voxels were evaluated at ER-1.5T and 5667 were evaluated at 3.0T. The quality of each spectroscopic voxel was rated on a 4-point scale based on direct visual appreciation of the choline and citrate peak heights in relation to baseline noise. Spectroscopic voxel quality was considered to be ‘excellent’ (score 4), when all metabolic resonances were well resolved and if there were no baseline distortions due to residual water or lipid. The quality was considered ‘good’ (score 3) when the metabolic resonances could be reliably distinguished and if there were only minimal baseline distortions due to residual water or lipid. The quality was considered ‘fair’ (score 2) when the metabolite peaks were difficult to resolve from the baseline noise. The quality was considered ‘poor’ (score 1) when the voxels were of insufficient spectral quality (SNR less than 5) with substantial baseline distortions (Fig. 1). Three urogenital radiologists (M.N., P.D.V., and G.V.) with 1, 3, and 14 years of experience in prostate imaging independently evaluated all the voxels. They were not blinded to the field strength because the endorectal coil was visible on the images acquired at ER-1.5T. 2.4. Statistical analysis For analysis of the individual spectroscopic voxel quality, the results were subdivided according to the location of the voxels in the prostate.
Separate quality scores were calculated for the total of all voxels within the prostate contour; for the voxels in the prostate base, midprostate, and apex; for the voxels located in the anterior and posterior two rows of voxels in the prostate; for the intermediate voxels; for all the voxels located in the TZ; and for all the voxels located in the peripheral zone (PZ). In each of these groups, the quality evaluations of the three radiologists were integrated into a mean score expressed as a percentage of poor, fair, good, and excellent quality voxels in relation to the total number of voxels located in that group. A paired Student’s t test was used to evaluate differences between ER-1.5T and 3.0T and differences between the distinct areas within the prostate at each field strength separately. For dichotomization, the scores 3 and 4 (good and excellent quality) were grouped as one category considered as ‘quality sufficient for diagnostic use’. To compare the quality of MRSI at ER-1.5T with 3.0T in the same patient, a Pearson correlation coefficient was calculated. The level of significance was set at 0.05. For all statistical analyses, a software package (SPSS for Windows, version 22.0; SPSS, Chicago, IL) was used. 3. Results 3.1. Comparison of total spectral quality (Table 2) At ER-1.5T, 16.16% of the spectroscopic voxels were considered of poor quality (score 1); 26.26%, of fair quality (score 2); 38.64%, of good quality (score 3); and 18.94%, of excellent quality (score 4). At 3.0T, these scores were 9.52%, 26.21%, 36.97%, and 27.30%, respectively. Overall, 57.58% of spectroscopic voxels had a ‘sufficient quality for diagnostic use’ at ER-1.5T; and 64.27%, at 3.0T. This difference was not statistically significant (P=.121). Further analysis of the distribution within the prostate of the voxels with the highest quality at ER-1.5T and 3.0T revealed significant local differences that compensated for each other. 3.2. Comparison of spectral quality in relation to distance from the rectum (Table 3) More good and excellent quality voxels were found in the posterior two rows (i.e., close to the rectum and endorectal coil) at ER-1.5T than at 3.0T (72.19% versus 60.53%, P= .047), while in the anterior and intermediate rows, the quality was significantly higher at 3.0T than at ER-1.5T (45.49% versus 32.88%, P= .002 and 72.52% versus 59.33%, P=.007, respectively). At ER-1.5T, the spectral quality significantly decreased with increasing distance from the endorectal coil, from 72.19% good and excellent quality voxels in the posterior two rows of the prostate to 59.33% in the intermediate rows (Pb.001) and 32.88% in the anterior two rows (Pb.001). Conversely, at 3.0T, most (72.52%) good and excellent quality voxels were present in the intermediate rows of voxels, as compared to the anterior two rows (45.49%, Pb.001) and the posterior two rows (60.53%, P=.002) (Fig. 2).
Table 1 MRI technical parameters ER-1.5T Magnetom Symphony, Siemens, Erlangen, Germany Endorectal Coil (MRInnervu, Medrad, Pittsburg, USA) inflated with air+PPA coil T2 T2 TSE, SL 4 mm, no interslice gap TR 4750 ms, TE 139 ms, FA 180°, FOV 140×280 mm, matrix size 180×512, BW 100 Hz, acquisition time 3–4 min MRSI 3D-CSI, point-resolved spectroscopic sequence, FWHM 4 Hz TR 690 ms, TE 120 ms FOV 140×280 mm Voxel size 3 mm AP×5 mm LL×5 mm CC Nominal voxel size 6 mm3 Number of acquisitions: 6 or 7 Acquisition time 10 min
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3.0T Magnetom Trio, Siemens, Erlangen, Germany T2 TSE, SL 3 mm, no interslice gap TR 8000 ms, TE 96 ms, FA 120°, FOV 300×300 mm, matrix size 410×512, BW 305 Hz, acquisition time 6–8 min 3D-CSI, point-resolved spectroscopic sequence, FWHM 8 Hz TR 750 ms, TE 145 ms FOV 300×300 mm Voxel size 4 mm AP×5 mm LL×4 mm CC Nominal voxel size 6 mm3 Number of acquisitions: 8 Acquisition time 10 min
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Fig. 1. The quality of each spectroscopic voxel was rated on a 4-point scale based on direct visual appreciation of the choline and citrate peak heights in relation to baseline noise. Spectroscopic voxel quality was considered to be ‘excellent’ (a) when all metabolic resonances were well resolved. The quality was considered ‘good’ (b) when the metabolic resonances could be reliably distinguished and if there were only minimal baseline distortions. The quality was considered ‘fair’ (c) when the metabolite peaks were difficult to resolve from the baseline noise. The quality was considered ‘poor’ (d) when the voxels were of insufficient spectral quality with substantial baseline distortions.
3.3. Comparison of spectral quality in PZ and TZ (Table 4) There were significantly more good and excellent quality voxels in the TZ at 3.0T as compared to ER-1.5T (68.75% versus 52.25%, Pb .001).
Table 2 Patient characteristics and spectroscopic quality of all voxels
Age (years) PSA (ng/ml) Prostate volume (ml) Total number of voxels evaluated 1 (poor quality) 2 (fair quality) 3 (good quality) 4 (excellent quality) 3 and 4 (‘sufficient for diagnostic use’)
ER-1.5T
3.0T
P Valuea
64.9 (42–77) 7.72 (2.83–23.00) 50.3 (26.1–107.4) 6107
67.3 (43–78) 10.91 (4.43–27.00) 62.4 (27.6–148.3) 5667
b.001 b.001 b.001
16.16% 26.26% 38.64% 18.94% 57.58%
9.52% 26.21% 36.97% 27.30% 64.27%
.034 .983 (NS) .416 (NS) .025 .121 (NS)
Results are percentages, calculated as the number of voxels with this score related to the total number of voxels, in 30 patients, evaluated by three readers. a Significance was calculated with paired Student’s t test. Pb.05 is considered statistically significant. (NS): not significant.
In the PZ, there were more good and excellent quality voxels at ER-1.5T as compared to 3.0T but this difference was not significant (64.09% at ER-1.5T versus 57.93% at 3.0T, P=.181). At ER-1.5T, the PZ showed significantly more good and excellent quality voxels as compared to the TZ (64.09% versus 52.25%, Pb .001), but at 3.0T, the PZ showed a little worse quality as compared to the TZ (57.93% versus 68.75%, P=.046).
Table 3 Spectroscopic quality in relation to distance to the rectum
Anterior two rows Intermediate rows Posterior two rows P ant/intermed P intermed/post P ant/post
ER-1.5T
3.0T
P Valuea
32.88% 59.33% 72.19% b.001 b.001 b.001
45.49% 72.52% 60.53% b.001 .002 .020
.002 .007 .047
Results are expressed as percentages of voxels with score 3 or 4 related to total number of voxels in the anterior two rows of voxels, posterior two rows of voxels, or the intermediate rows of voxels. a Significance was calculated with paired Student’s t test. Pb.05 is considered statistically significant. (NS): not significant.
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Fig. 2. Spectroscopic maps overlaid on corresponding transverse T2-weighted MR images at ER-1.5T (a and b) and at the same level at 3.0T (c and d). The typical difference in distribution of best spectral quality voxels is illustrated. At ER-1.5T, the best quality is observed in the posterior two rows of the prostate, gradually decreasing with increasing distance from the endorectal coil (a). At 3.0T, the best-quality voxels are found in the middle rows of the prostate, more homogeneously distributed (c). The individual spectra show a sharper and higher citrate quadruplet peak at 3.0T (d) than the doublet at ER-1.5T (b).
3.4. Comparison of spectral quality in prostate base, midprostate, and apex (Table 5) There were no significant differences between ER-1.5T and 3.0T in the percentages of good and excellent quality voxels in the apex (60.94% at ER-1.5T versus 59.27% at 3.0T, P=.780) or in the midprostate (64.22% at ER-1.5T versus 70.32% at 3.0T, P=.206). In the prostatic base, Table 4 Spectroscopic quality in PZ and TZ
TZ PZ P Value
ER-1.5T
3.0T
P Valuea
52.25% 64.09% b.001
68.75% 57.93% .046
b.001 .181 (NS)
Results are expressed as percentages of voxels with score 3 or 4 related to total number of voxels in the PZ or TZ. a Significance was calculated with paired Student’s t test. Pb.05 is considered statistically significant. (NS): not significant.
there were significantly more good and excellent quality voxels at 3.0T than at ER-1.5T (60,51% versus 49.59%, P=.025). At ER-1.5T, the voxel quality in the prostatic base was significantly lower (only 49.59% good and excellent quality voxels) as compared to
Table 5 Spectroscopic quality in apex, midprostate, and prostate base
Apex Midprostate Prostate base P apex/mid P mid/base P apex/base
ER-1.5T
3.0T
P Valuea
60.94% 64.22% 49.59% .386 (NS) b.001 .007
59.27% 70.32% 60.51% .034 .001 .852 (NS)
.780 (NS) .206 (NS) .025
Results are expressed as percentages of voxels with score 3 or 4 related to the total number of voxels in apex, midprostate, or prostate base. a Significance was calculated with paired Student’s t test. Pb.05 is considered statistically significant. (NS): not significant.
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that in the midprostate (64.22%, Pb .001) and the apex (60.94%, P= .007). At 3.0T, there were significantly more good and excellent quality voxels in the midprostate (70.32%) as compared to the apex (59.27%, P=.034) and prostate base (60.51%, P=.001). 3.5. Comparison of spectral quality at both field strengths in the same patients The Pearson correlation coefficient of MRSI quality at ER-1.5T and at 3.0T in the same patients was R=0.201 (P=.286, NS), indicating a slight positive correlation but not statistically significant. The scatter plot (Fig. 3) shows that, in the majority of patients, the total MRSI quality was slightly better in their exam at 3.0T although, in at least two patients, the exam was obviously much better at ER-1.5T than at 3.0T. 4. Discussion Obtaining good-quality magnetic resonance (MR) spectra can be challenging, especially in the learning phase of MRSI [7], resulting in unusable spectral data in a considerable number of voxels or sometimes even in a complete exam. In the current study, patients with insufficient overall MRSI quality were not excluded from the cohort and although we have more than 15 years of experience with prostate MRSI in our hospital, we still found poor individual spectral voxel quality in 16.16% of all voxels at ER-1.5T and in 9.52% at 3.0T (P=.034) and fair individual spectral quality in 26.26% of voxels at ER-1.5T and in 26.21% at 3.0T (P=.983, NS). In a 3.0T MRSI feasibility study, Scheenen et al. [4] reported an average percentage of voxels that passed the visual inspection of the automated fit procedure of 74% (median, 82%), with need to exclude four patients from the study group of 45. Other authors report exclusion of 18.46–82.35% of patients from their study cohorts because of insufficient overall spectral quality [4,8–10]. Insufficient spectral quality results in metabolic resonances that are difficult to interpret because they cannot be well resolved from baseline noise. Causes of insufficient spectral quality are inadequate shimming, phase shifts, inadequate fat suppression, baseline distortions due to residual water or lipid contamination, or voxels containing glycerophosphocholine (e.g., in the vicinity
Fig. 3. Scatter plot of good and excellent total quality (percentage scores 3 and 4) at ER1.5T as compared to 3.0T in the same patients. The Pearson correlation coefficient was 0.201 (P=.286), indicating a slight positive correlation but not statistically significant. The majority of patients have a slightly better quality at 3.0T as compared to ER-1.5T (they are below the diagonal line that indicates perfect correlation), but in two patients, the quality was obviously worse (they are located in the left upper quadrant of the graph). A poor-quality MRSI at 1.5T thus does not necessarily imply a poor spectral quality at 3.0T and vice versa.
of the seminal vesicles) [7,11]. Spectral degradation may also occur in case of inappropriate endorectal coil positioning in the rectum (too high, too low, or twisted) or due to motion artifacts. Some authors report more motion artifacts (and inevitably worse MRSI quality) using an endorectal coil because of patient discomfort, but others report less motion artifacts because the inflated balloon surrounding the endorectal coil immobilizes the prostate [5,12]. Patient factors may influence the quality of the MRSI, such as hip prosthesis causing susceptibility artifacts or patients previously treated with radiation therapy causing metabolic atrophy. In these patients, indeed poor MRSI quality will always be the case, no matter what the field strength is or if an endorectal coil is used or not. In our study, the Pearson correlation coefficient of MRSI quality at ER-1.5T compared to 3.0T in the same patients was 0.201 (P= .286, NS), indicating a slight positive correlation but not statistically significant. The majority of patients had a slightly better quality at 3.0T compared to ER-1.5T, but in two patients, the quality was obviously worse; therefore, a poorquality MRSI at ER-1.5T thus does not necessarily imply a poor spectral quality at 3.0T and vice versa. The use of an endorectal coil considerably improves the SNR in the prostatic area. At 1.5T, it is currently considered as mandatory, especially for spectroscopic imaging [1,2]. Due to the particular reception profile of an endorectal coil, the SNR decreases with increasing distance from the coil. This was clearly shown in our study by a significantly higher spectral quality close to the coil as compared to more distant voxels. As a consequence, the spectral quality in the PZ was significantly higher as compared to the TZ at ER-1.5T (64.09% versus 52.25%, Pb .001). Nevertheless, it was not significantly superior to the spectral quality as compared to the PZ at 3.0T (64.09% versus 57.93%, P= .181). This can be explained by the lower spectral quality of the voxels in the anterior horns of the PZ at ER-1.5T, reducing the overall PZ quality at ER-1.5T. Spectral quality in the PZ is an important issue because 70–80% of prostate cancers occur in the PZ, but the tumors that are located in the TZ or in the more anteriorly and lateral aspects of the PZ are the ones that may be missed at ultrasound-guided biopsy and therefore highquality MRI in these areas is at least of equal importance. We found the best-quality voxels at ER-1.5T in the posterior rows of voxels close to the endorectal coil and at 3.0T in the anterior and intermediate rows of voxels, in the TZ, and in the prostate base. Therefore, it seems reasonable that the combination of both techniques, a 3.0T exam with endorectal coil (ER-3.0T) should obtain the best overall quality. Indeed, several authors reported superior image quality and excellent anatomic details at ER-3.0T, not achievable at ER-1.5T or 3.0T without endorectal coil [13–15]. Nevertheless, because of the disadvantages of the endorectal coil such as discomfort to the patient and the additional costs, it is worthwhile to explore the possibilities of not using the endorectal coil. It would make MRSI easier, faster, less expensive, and truly noninvasive [4]. The SNR decreases just above and below the reception area of an endorectal coil. At ER-1.5T, we indeed found a significantly lower spectral quality in the prostatic base (49.59%) and the apex (60.94%) as compared to the midprostate (64.22%, Pb.001 and P=.007, respectively). At 3.0T, the best-quality voxels were found in the midprostate (70.32%) and in the middle rows (72.52%), more homogeneously distributed throughout the prostate. Although the spectral quality and SNR for a given field strength are affected by a multitude of factors (such as relaxation times, pulse sequences, voxel size, differences in resolution, and water suppression), higher field strengths generally result in a higher SNR and a larger chemical shift effect, yielding a better separation of metabolite peaks [7,12,15–17]. Citrate is a quadruplet resonating at 2.6 ppm that can be identified, as such, at 3.0T, but at 1.5T, it is usually demonstrated as a single peak or a doublet [7,18]. The choline and creatine peaks are closely adjacent (at 3.2 and 3.0 ppm, respectively) and overlap with the polyamine resonance (at 3.1 ppm) at 1.5T but can usually be separated at 3.0T [8,9,18].
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At 3.0T more than at 1.5T, there seem to be considerable differences in the spectral shapes of metabolites between spectroscopy packages and MR systems of different vendors. These differences can result in variable choline+creatine/citrate metabolite ratios in the same patient when obtained at different MR systems, which may influence the diagnostic performance [7]. Higher citrate peaks in both benign and malignant tissue have been reported at 3.0T compared to 1.5T [19,20]. This is also the case in our experience and was reflected in the quality judgment in our study where we found a higher number of score 4 (excellent quality) at 3.0T (27.30%) than at ER-1.5T (18.94%). This might be explained by the inherent higher citrate peaks at 3.0T that were better discriminated from background noise, giving a better subjective spectral quality. There are some limitations to our study. First, the evaluation of image quality was based on subjective criteria. We tried to overcome this problem by averaging the results of three independently reading radiologists with different years of experience. A second limitation is the mean time interval of 2.5 years between the two scans, ranging from 0.3 to 7.8 years. Ideally, the patients should have had both prostate MRI on the same day; however, none of the patients got surgery or radiation therapy between both scans, and morphologically, there were no significant changes on the T2-WI when selecting the corresponding slices at ER-1.5T and 3.0T. Notwithstanding the rather long mean time interval in some patients, we considered scanning the same patients at both field strengths superior to scanning two different groups of patients as was done by some other authors [3,21,22]. Finally, it was beyond the scope of this study to compare diagnostic performance of ER-1.5T or 3.0T for tumor delineation or staging. In a recent study, Bratan et al. [23] reported that prostate cancer detection rates were not influenced by field strength or coils used for imaging. The staging accuracy of T2-WI of the prostate has been reported to be comparable [3,22] or better [2,5] at ER-1.5T than at 3.0T. Future research should focus on mpMRI comparing both field strengths with and without endorectal coil, ideally performed in the same patients at the same day and with pathological reference.
5. Conclusion There is no significant difference in overall spectral quality between ER-1.5T and 3.0T, although ER-1.5T shows better spectral quality in the voxels close to the endorectal coil while 3.0T shows better quality in the TZ and in the prostate base.
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Prostate magnetic resonance spectroscopic imaging at 1.5 tesla with endorectal coil versus 3.0 tesla without endorectal coil: comparison of spectral quality Pieter De Visschere a,⁎, Marco Nezzo b, Eva Pattyn a, Valérie Fonteyne c, Charles Van Praet d, Geert Villeirs a a
Department of Radiology, Ghent University Hospital, De Pintelaan 185, 9000 Gent, Belgium Department of Radiology, University of Rome Tor Vergata, 00133 Rome, Italy c Department of Radiotherapy, Ghent University Hospital, Gent, Belgium d Department of Urology, Ghent University Hospital, Gent, Belgium b
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Article history: Received 31 December 2014 Received in revised form 6 February 2015 Accepted 9 February 2015 Keywords: Prostate Magnetic resonance imaging Quality assessment MR spectroscopy 3.0 Tesla
a b s t r a c t Objectives: To compare the spectral quality of prostate magnetic resonance spectroscopic imaging (MRSI) at 1.5 Tesla with endorectal coil (ER-1.5T) to MRSI at 3.0 Tesla without coil (3.0T). Methods: In 30 patients, the spectral quality of 6107 voxels at ER-1.5T and that of 5667 at 3.0T were visually evaluated by three radiologists. Results: There were 57.6% good quality voxels at ER-1.5T versus 64.3% at 3.0T (P=.121). The posterior two rows showed better quality at ER-1.5T (P=.047). Conclusion: There is no significant difference in overall spectral quality between ER-1.5T and 3.0T, although ER-1.5T shows better quality close to the endorectal coil.
© 2015 Elsevier Inc. All rights reserved.
1. Introduction Magnetic resonance imaging (MRI) is increasingly being used in the assessment of patients with suspected or known prostate cancer. Multiparametric MRI (mpMRI) is the current state-of-the art imaging technique, consisting of morphological T2-weighted images (T2-WI) supplemented with at least two functional imaging techniques such as diffusionweighted imaging, magnetic resonance spectroscopic imaging (MRSI), and dynamic contrast-enhanced imaging [1]. The optimum field strength for performing mpMRI of the prostate is still a source of debate and the same accounts for the question whether an endorectal coil is needed or not. The use of an endorectal coil considerably improves the signal-tonoise ratio (SNR) in the prostate, and at 1.5T, it is considered mandatory, especially when performing MRSI [1,2]. The endorectal coil, however, increases the examination cost and is deemed uncomfortable for the patient [2,3]. On high-field-strength MRI scanners, the SNR is higher and therefore the use of an endorectal coil may be avoided [4,5]. In this study, we evaluated the spectral quality of prostate MRSI at 1.5T with endorectal
⁎ Corresponding author. Department of Radiology, Ghent University Hospital, 0 DPP, De Pintelaan 185, 9000 Gent, Belgium. Tel.: +32-93326688. E-mail address: [email protected] (P. De Visschere). http://dx.doi.org/10.1016/j.clinimag.2015.02.008 0899-7071/© 2015 Elsevier Inc. All rights reserved.
coil (ER-1.5T) as compared to 3.0T imaging without endorectal coil (3.0T) in the same patients who underwent both examinations.
2. Materials and methods 2.1. Patients At our institution, all mpMRI exams were performed on a 1.5T scanner using an endorectal coil (ER-1.5T) before May 2011 and on a 3.0T scanner without endorectal coil (3.0T) thereafter. All patients who had both an ER-1.5T and a 3.0T exam between March 2004 and February 2012 were eligible for this study. mpMRI was performed as part of a follow-up program in patients with elevated prostate-specific antigen (PSA) but negative biopsies and/or mpMRI. Exclusion criteria were patients treated for prostate cancer before or in the interval between both scans, patients who had transurethral resection of the prostate between the scans, and patients with metallic hip prosthesis. Thirty randomly selected patients were thus included. Mean age, serum PSA, and prostate volume were 64.9 (42–77) years, 7.7 (2.8–23.0) ng/ml, and 50.3 (26.1–107.4) ml for the exams at ER-1.5T, and 67.3 (43–78) years, 10.9 (4.4–27.0) ng/ml, and 62.4 (27.6–148.3) ml at 3.0T. Mean time interval between the scans was 2.5 years (range, 0.3–7.8 years). The study was
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approved by our hospital’s Ethics Committee (EC 2011/495) and the need for informed consent was waived for this retrospective study. 2.2. MRI technique Data acquisition parameters were matched as closely as possible between the two field strengths (Table 1). Spectral maps, overlaid on the corresponding transverse T2-WI, were obtained using Syngo software (Siemens Medical Systems, Erlangen, Germany). The peak areas of choline, creatine, and citrate levels were determined in the spectrum according to their expected peak locations and widths over a frequency range 1.3–4.4 ppm. Citrate peaks were identified as quadruplets, doublets, or single peaks (according to spectral quality and field strength) around 2.6 ppm, whereas choline and creatine peaks were recognized as single peaks around 3.2 and 3.0 ppm, respectively. With lower spectral quality or in case that choline or creatine peaks were very high, both peaks were regarded as one peak [6]. 2.3. Image analysis Images were analyzed on a dedicated workstation (Leonardo, Siemens Medical Systems, Erlangen, Germany). In each patient, one radiologist (P.D.V.) selected three transverse slices (in the prostatic base, midprostate, and apex) at ER-1.5T, and chose the corresponding slices at 3.0T using anatomical landmarks on the T2-WI of both studies such as the urethra, benign hyperplastic nodules in the transition zone (TZ), and the periprostatic anatomy. In these slices, the quality of all spectroscopic voxels within the prostate contour was evaluated. In total, 6107 voxels were evaluated at ER-1.5T and 5667 were evaluated at 3.0T. The quality of each spectroscopic voxel was rated on a 4-point scale based on direct visual appreciation of the choline and citrate peak heights in relation to baseline noise. Spectroscopic voxel quality was considered to be ‘excellent’ (score 4), when all metabolic resonances were well resolved and if there were no baseline distortions due to residual water or lipid. The quality was considered ‘good’ (score 3) when the metabolic resonances could be reliably distinguished and if there were only minimal baseline distortions due to residual water or lipid. The quality was considered ‘fair’ (score 2) when the metabolite peaks were difficult to resolve from the baseline noise. The quality was considered ‘poor’ (score 1) when the voxels were of insufficient spectral quality (SNR less than 5) with substantial baseline distortions (Fig. 1). Three urogenital radiologists (M.N., P.D.V., and G.V.) with 1, 3, and 14 years of experience in prostate imaging independently evaluated all the voxels. They were not blinded to the field strength because the endorectal coil was visible on the images acquired at ER-1.5T. 2.4. Statistical analysis For analysis of the individual spectroscopic voxel quality, the results were subdivided according to the location of the voxels in the prostate.
Separate quality scores were calculated for the total of all voxels within the prostate contour; for the voxels in the prostate base, midprostate, and apex; for the voxels located in the anterior and posterior two rows of voxels in the prostate; for the intermediate voxels; for all the voxels located in the TZ; and for all the voxels located in the peripheral zone (PZ). In each of these groups, the quality evaluations of the three radiologists were integrated into a mean score expressed as a percentage of poor, fair, good, and excellent quality voxels in relation to the total number of voxels located in that group. A paired Student’s t test was used to evaluate differences between ER-1.5T and 3.0T and differences between the distinct areas within the prostate at each field strength separately. For dichotomization, the scores 3 and 4 (good and excellent quality) were grouped as one category considered as ‘quality sufficient for diagnostic use’. To compare the quality of MRSI at ER-1.5T with 3.0T in the same patient, a Pearson correlation coefficient was calculated. The level of significance was set at 0.05. For all statistical analyses, a software package (SPSS for Windows, version 22.0; SPSS, Chicago, IL) was used. 3. Results 3.1. Comparison of total spectral quality (Table 2) At ER-1.5T, 16.16% of the spectroscopic voxels were considered of poor quality (score 1); 26.26%, of fair quality (score 2); 38.64%, of good quality (score 3); and 18.94%, of excellent quality (score 4). At 3.0T, these scores were 9.52%, 26.21%, 36.97%, and 27.30%, respectively. Overall, 57.58% of spectroscopic voxels had a ‘sufficient quality for diagnostic use’ at ER-1.5T; and 64.27%, at 3.0T. This difference was not statistically significant (P=.121). Further analysis of the distribution within the prostate of the voxels with the highest quality at ER-1.5T and 3.0T revealed significant local differences that compensated for each other. 3.2. Comparison of spectral quality in relation to distance from the rectum (Table 3) More good and excellent quality voxels were found in the posterior two rows (i.e., close to the rectum and endorectal coil) at ER-1.5T than at 3.0T (72.19% versus 60.53%, P= .047), while in the anterior and intermediate rows, the quality was significantly higher at 3.0T than at ER-1.5T (45.49% versus 32.88%, P= .002 and 72.52% versus 59.33%, P=.007, respectively). At ER-1.5T, the spectral quality significantly decreased with increasing distance from the endorectal coil, from 72.19% good and excellent quality voxels in the posterior two rows of the prostate to 59.33% in the intermediate rows (Pb.001) and 32.88% in the anterior two rows (Pb.001). Conversely, at 3.0T, most (72.52%) good and excellent quality voxels were present in the intermediate rows of voxels, as compared to the anterior two rows (45.49%, Pb.001) and the posterior two rows (60.53%, P=.002) (Fig. 2).
Table 1 MRI technical parameters ER-1.5T Magnetom Symphony, Siemens, Erlangen, Germany Endorectal Coil (MRInnervu, Medrad, Pittsburg, USA) inflated with air+PPA coil T2 T2 TSE, SL 4 mm, no interslice gap TR 4750 ms, TE 139 ms, FA 180°, FOV 140×280 mm, matrix size 180×512, BW 100 Hz, acquisition time 3–4 min MRSI 3D-CSI, point-resolved spectroscopic sequence, FWHM 4 Hz TR 690 ms, TE 120 ms FOV 140×280 mm Voxel size 3 mm AP×5 mm LL×5 mm CC Nominal voxel size 6 mm3 Number of acquisitions: 6 or 7 Acquisition time 10 min
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3.0T Magnetom Trio, Siemens, Erlangen, Germany T2 TSE, SL 3 mm, no interslice gap TR 8000 ms, TE 96 ms, FA 120°, FOV 300×300 mm, matrix size 410×512, BW 305 Hz, acquisition time 6–8 min 3D-CSI, point-resolved spectroscopic sequence, FWHM 8 Hz TR 750 ms, TE 145 ms FOV 300×300 mm Voxel size 4 mm AP×5 mm LL×4 mm CC Nominal voxel size 6 mm3 Number of acquisitions: 8 Acquisition time 10 min
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Fig. 1. The quality of each spectroscopic voxel was rated on a 4-point scale based on direct visual appreciation of the choline and citrate peak heights in relation to baseline noise. Spectroscopic voxel quality was considered to be ‘excellent’ (a) when all metabolic resonances were well resolved. The quality was considered ‘good’ (b) when the metabolic resonances could be reliably distinguished and if there were only minimal baseline distortions. The quality was considered ‘fair’ (c) when the metabolite peaks were difficult to resolve from the baseline noise. The quality was considered ‘poor’ (d) when the voxels were of insufficient spectral quality with substantial baseline distortions.
3.3. Comparison of spectral quality in PZ and TZ (Table 4) There were significantly more good and excellent quality voxels in the TZ at 3.0T as compared to ER-1.5T (68.75% versus 52.25%, Pb .001).
Table 2 Patient characteristics and spectroscopic quality of all voxels
Age (years) PSA (ng/ml) Prostate volume (ml) Total number of voxels evaluated 1 (poor quality) 2 (fair quality) 3 (good quality) 4 (excellent quality) 3 and 4 (‘sufficient for diagnostic use’)
ER-1.5T
3.0T
P Valuea
64.9 (42–77) 7.72 (2.83–23.00) 50.3 (26.1–107.4) 6107
67.3 (43–78) 10.91 (4.43–27.00) 62.4 (27.6–148.3) 5667
b.001 b.001 b.001
16.16% 26.26% 38.64% 18.94% 57.58%
9.52% 26.21% 36.97% 27.30% 64.27%
.034 .983 (NS) .416 (NS) .025 .121 (NS)
Results are percentages, calculated as the number of voxels with this score related to the total number of voxels, in 30 patients, evaluated by three readers. a Significance was calculated with paired Student’s t test. Pb.05 is considered statistically significant. (NS): not significant.
In the PZ, there were more good and excellent quality voxels at ER-1.5T as compared to 3.0T but this difference was not significant (64.09% at ER-1.5T versus 57.93% at 3.0T, P=.181). At ER-1.5T, the PZ showed significantly more good and excellent quality voxels as compared to the TZ (64.09% versus 52.25%, Pb .001), but at 3.0T, the PZ showed a little worse quality as compared to the TZ (57.93% versus 68.75%, P=.046).
Table 3 Spectroscopic quality in relation to distance to the rectum
Anterior two rows Intermediate rows Posterior two rows P ant/intermed P intermed/post P ant/post
ER-1.5T
3.0T
P Valuea
32.88% 59.33% 72.19% b.001 b.001 b.001
45.49% 72.52% 60.53% b.001 .002 .020
.002 .007 .047
Results are expressed as percentages of voxels with score 3 or 4 related to total number of voxels in the anterior two rows of voxels, posterior two rows of voxels, or the intermediate rows of voxels. a Significance was calculated with paired Student’s t test. Pb.05 is considered statistically significant. (NS): not significant.
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Fig. 2. Spectroscopic maps overlaid on corresponding transverse T2-weighted MR images at ER-1.5T (a and b) and at the same level at 3.0T (c and d). The typical difference in distribution of best spectral quality voxels is illustrated. At ER-1.5T, the best quality is observed in the posterior two rows of the prostate, gradually decreasing with increasing distance from the endorectal coil (a). At 3.0T, the best-quality voxels are found in the middle rows of the prostate, more homogeneously distributed (c). The individual spectra show a sharper and higher citrate quadruplet peak at 3.0T (d) than the doublet at ER-1.5T (b).
3.4. Comparison of spectral quality in prostate base, midprostate, and apex (Table 5) There were no significant differences between ER-1.5T and 3.0T in the percentages of good and excellent quality voxels in the apex (60.94% at ER-1.5T versus 59.27% at 3.0T, P=.780) or in the midprostate (64.22% at ER-1.5T versus 70.32% at 3.0T, P=.206). In the prostatic base, Table 4 Spectroscopic quality in PZ and TZ
TZ PZ P Value
ER-1.5T
3.0T
P Valuea
52.25% 64.09% b.001
68.75% 57.93% .046
b.001 .181 (NS)
Results are expressed as percentages of voxels with score 3 or 4 related to total number of voxels in the PZ or TZ. a Significance was calculated with paired Student’s t test. Pb.05 is considered statistically significant. (NS): not significant.
there were significantly more good and excellent quality voxels at 3.0T than at ER-1.5T (60,51% versus 49.59%, P=.025). At ER-1.5T, the voxel quality in the prostatic base was significantly lower (only 49.59% good and excellent quality voxels) as compared to
Table 5 Spectroscopic quality in apex, midprostate, and prostate base
Apex Midprostate Prostate base P apex/mid P mid/base P apex/base
ER-1.5T
3.0T
P Valuea
60.94% 64.22% 49.59% .386 (NS) b.001 .007
59.27% 70.32% 60.51% .034 .001 .852 (NS)
.780 (NS) .206 (NS) .025
Results are expressed as percentages of voxels with score 3 or 4 related to the total number of voxels in apex, midprostate, or prostate base. a Significance was calculated with paired Student’s t test. Pb.05 is considered statistically significant. (NS): not significant.
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that in the midprostate (64.22%, Pb .001) and the apex (60.94%, P= .007). At 3.0T, there were significantly more good and excellent quality voxels in the midprostate (70.32%) as compared to the apex (59.27%, P=.034) and prostate base (60.51%, P=.001). 3.5. Comparison of spectral quality at both field strengths in the same patients The Pearson correlation coefficient of MRSI quality at ER-1.5T and at 3.0T in the same patients was R=0.201 (P=.286, NS), indicating a slight positive correlation but not statistically significant. The scatter plot (Fig. 3) shows that, in the majority of patients, the total MRSI quality was slightly better in their exam at 3.0T although, in at least two patients, the exam was obviously much better at ER-1.5T than at 3.0T. 4. Discussion Obtaining good-quality magnetic resonance (MR) spectra can be challenging, especially in the learning phase of MRSI [7], resulting in unusable spectral data in a considerable number of voxels or sometimes even in a complete exam. In the current study, patients with insufficient overall MRSI quality were not excluded from the cohort and although we have more than 15 years of experience with prostate MRSI in our hospital, we still found poor individual spectral voxel quality in 16.16% of all voxels at ER-1.5T and in 9.52% at 3.0T (P=.034) and fair individual spectral quality in 26.26% of voxels at ER-1.5T and in 26.21% at 3.0T (P=.983, NS). In a 3.0T MRSI feasibility study, Scheenen et al. [4] reported an average percentage of voxels that passed the visual inspection of the automated fit procedure of 74% (median, 82%), with need to exclude four patients from the study group of 45. Other authors report exclusion of 18.46–82.35% of patients from their study cohorts because of insufficient overall spectral quality [4,8–10]. Insufficient spectral quality results in metabolic resonances that are difficult to interpret because they cannot be well resolved from baseline noise. Causes of insufficient spectral quality are inadequate shimming, phase shifts, inadequate fat suppression, baseline distortions due to residual water or lipid contamination, or voxels containing glycerophosphocholine (e.g., in the vicinity
Fig. 3. Scatter plot of good and excellent total quality (percentage scores 3 and 4) at ER1.5T as compared to 3.0T in the same patients. The Pearson correlation coefficient was 0.201 (P=.286), indicating a slight positive correlation but not statistically significant. The majority of patients have a slightly better quality at 3.0T as compared to ER-1.5T (they are below the diagonal line that indicates perfect correlation), but in two patients, the quality was obviously worse (they are located in the left upper quadrant of the graph). A poor-quality MRSI at 1.5T thus does not necessarily imply a poor spectral quality at 3.0T and vice versa.
of the seminal vesicles) [7,11]. Spectral degradation may also occur in case of inappropriate endorectal coil positioning in the rectum (too high, too low, or twisted) or due to motion artifacts. Some authors report more motion artifacts (and inevitably worse MRSI quality) using an endorectal coil because of patient discomfort, but others report less motion artifacts because the inflated balloon surrounding the endorectal coil immobilizes the prostate [5,12]. Patient factors may influence the quality of the MRSI, such as hip prosthesis causing susceptibility artifacts or patients previously treated with radiation therapy causing metabolic atrophy. In these patients, indeed poor MRSI quality will always be the case, no matter what the field strength is or if an endorectal coil is used or not. In our study, the Pearson correlation coefficient of MRSI quality at ER-1.5T compared to 3.0T in the same patients was 0.201 (P= .286, NS), indicating a slight positive correlation but not statistically significant. The majority of patients had a slightly better quality at 3.0T compared to ER-1.5T, but in two patients, the quality was obviously worse; therefore, a poorquality MRSI at ER-1.5T thus does not necessarily imply a poor spectral quality at 3.0T and vice versa. The use of an endorectal coil considerably improves the SNR in the prostatic area. At 1.5T, it is currently considered as mandatory, especially for spectroscopic imaging [1,2]. Due to the particular reception profile of an endorectal coil, the SNR decreases with increasing distance from the coil. This was clearly shown in our study by a significantly higher spectral quality close to the coil as compared to more distant voxels. As a consequence, the spectral quality in the PZ was significantly higher as compared to the TZ at ER-1.5T (64.09% versus 52.25%, Pb .001). Nevertheless, it was not significantly superior to the spectral quality as compared to the PZ at 3.0T (64.09% versus 57.93%, P= .181). This can be explained by the lower spectral quality of the voxels in the anterior horns of the PZ at ER-1.5T, reducing the overall PZ quality at ER-1.5T. Spectral quality in the PZ is an important issue because 70–80% of prostate cancers occur in the PZ, but the tumors that are located in the TZ or in the more anteriorly and lateral aspects of the PZ are the ones that may be missed at ultrasound-guided biopsy and therefore highquality MRI in these areas is at least of equal importance. We found the best-quality voxels at ER-1.5T in the posterior rows of voxels close to the endorectal coil and at 3.0T in the anterior and intermediate rows of voxels, in the TZ, and in the prostate base. Therefore, it seems reasonable that the combination of both techniques, a 3.0T exam with endorectal coil (ER-3.0T) should obtain the best overall quality. Indeed, several authors reported superior image quality and excellent anatomic details at ER-3.0T, not achievable at ER-1.5T or 3.0T without endorectal coil [13–15]. Nevertheless, because of the disadvantages of the endorectal coil such as discomfort to the patient and the additional costs, it is worthwhile to explore the possibilities of not using the endorectal coil. It would make MRSI easier, faster, less expensive, and truly noninvasive [4]. The SNR decreases just above and below the reception area of an endorectal coil. At ER-1.5T, we indeed found a significantly lower spectral quality in the prostatic base (49.59%) and the apex (60.94%) as compared to the midprostate (64.22%, Pb.001 and P=.007, respectively). At 3.0T, the best-quality voxels were found in the midprostate (70.32%) and in the middle rows (72.52%), more homogeneously distributed throughout the prostate. Although the spectral quality and SNR for a given field strength are affected by a multitude of factors (such as relaxation times, pulse sequences, voxel size, differences in resolution, and water suppression), higher field strengths generally result in a higher SNR and a larger chemical shift effect, yielding a better separation of metabolite peaks [7,12,15–17]. Citrate is a quadruplet resonating at 2.6 ppm that can be identified, as such, at 3.0T, but at 1.5T, it is usually demonstrated as a single peak or a doublet [7,18]. The choline and creatine peaks are closely adjacent (at 3.2 and 3.0 ppm, respectively) and overlap with the polyamine resonance (at 3.1 ppm) at 1.5T but can usually be separated at 3.0T [8,9,18].
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At 3.0T more than at 1.5T, there seem to be considerable differences in the spectral shapes of metabolites between spectroscopy packages and MR systems of different vendors. These differences can result in variable choline+creatine/citrate metabolite ratios in the same patient when obtained at different MR systems, which may influence the diagnostic performance [7]. Higher citrate peaks in both benign and malignant tissue have been reported at 3.0T compared to 1.5T [19,20]. This is also the case in our experience and was reflected in the quality judgment in our study where we found a higher number of score 4 (excellent quality) at 3.0T (27.30%) than at ER-1.5T (18.94%). This might be explained by the inherent higher citrate peaks at 3.0T that were better discriminated from background noise, giving a better subjective spectral quality. There are some limitations to our study. First, the evaluation of image quality was based on subjective criteria. We tried to overcome this problem by averaging the results of three independently reading radiologists with different years of experience. A second limitation is the mean time interval of 2.5 years between the two scans, ranging from 0.3 to 7.8 years. Ideally, the patients should have had both prostate MRI on the same day; however, none of the patients got surgery or radiation therapy between both scans, and morphologically, there were no significant changes on the T2-WI when selecting the corresponding slices at ER-1.5T and 3.0T. Notwithstanding the rather long mean time interval in some patients, we considered scanning the same patients at both field strengths superior to scanning two different groups of patients as was done by some other authors [3,21,22]. Finally, it was beyond the scope of this study to compare diagnostic performance of ER-1.5T or 3.0T for tumor delineation or staging. In a recent study, Bratan et al. [23] reported that prostate cancer detection rates were not influenced by field strength or coils used for imaging. The staging accuracy of T2-WI of the prostate has been reported to be comparable [3,22] or better [2,5] at ER-1.5T than at 3.0T. Future research should focus on mpMRI comparing both field strengths with and without endorectal coil, ideally performed in the same patients at the same day and with pathological reference.
5. Conclusion There is no significant difference in overall spectral quality between ER-1.5T and 3.0T, although ER-1.5T shows better spectral quality in the voxels close to the endorectal coil while 3.0T shows better quality in the TZ and in the prostate base.
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