IMAGING

Combined MRI and MR Spectroscopy of the Prostate before Radical Prostatectomy
Wetter A, Engl TA, Nadjmabadi D, Fliessbach K, Lehnert T, Gurung J, Beecken WD,
Vogl TJ
Institute for Diagnostic and Interventional Radiology, University of Frankfurt, Frankurt, Germany
AJR Am J Roentgenol. 2006; 187: 724-30

• Objective: The purpose of this study was to evaluate a routine protocol for combined MR and spectroscopic imaging of the prostate for staging accuracy.
• Subjects and methods: Fifty patients with biopsy-proven prostate carcinoma were examined with our sequence protocol, which consisted of T2-weighted fast spin-echo sequences and a pelvic T1-weighted spin-echo sequence. For spectroscopy, we used a 3D chemical shift imaging (CSI) spin-echo sequence. Image interpretation was performed by two radiologists. The total number of tumor voxels and tumor voxels per slice were counted to estimate the tumor volume in every patient. The potential of MR spectroscopy to differentiate between T2 and T3 tumors, based on the estimated tumor volumes, was compared with the staging performance of MRI.
• Results: The MR measurement time was 19.01 minutes, and the total procedure time averaged 35 minutes. Seventy-six percent of the spectroscopic examinations were successful. Statistically significant differences in the number of tumor voxels per slice and tumor volumes were found between T2 and T3 tumors. The descriptive parameters of MRI and MR spectroscopy did not differ significantly; sensitivity and specificity were 75% and 87%, respectively, for MRI and 88% and 70%, respectively, for MR spectroscopy. The combination of both methods resulted in only a slight improvement in staging performance and was not statistically significant.
• Conclusion: Combined MRI and MR spectroscopy of the prostate has no diagnostic advantage in staging performance over MRI alone. The mean tumor volumes, estimated by MR spectroscopy, differ statistically significantly between T2 and T3 tumors.
  
  
• Editorial Comment
  Nowadays, the ideal way to adequately stage prostate cancer is by the combination of conventional MRI techniques and 3D-MR spectroscopic imaging (MRSI).In other words, 3D-MRSI of the prostate must be done together with conventional MRI.MRSI can be useful for the diagnosis and detection of extra-prostatic disease and seminal vesicle invasion based on the capability of estimation of tumor volume and tumor location. The presence of more than 4 contiguous voxels with cancer indicates higher probability of extra-prostatic extension of the disease. The authors of this manuscript concluded that the differences of the staging performance between MRI and MRSI were not statistically significant and thus they do not recommend the routine use of their combined sequence protocol for staging purposes of patients with histologically proven prostate carcinoma. By using their spectroscopic technique, they also had relatively unexpected high rates of false positive (13%) and false negative (25%).
  We must consider these data with caution since several important aspects of the technique used by the authors should be discussed since the authors used different parameters from those currently used by other investigators. First 3D-MR spectroscopic imaging is acquired by water and lipid-suppressed double-spin-echo point-resolved spectroscopy sequence, which is optimized for quantitative detection of both choline and citrate. Data sets are acquired as 16 x 8 x 8 phase-encoded spectral arrays (1024 voxels; nominal spatial resolution, 0.34 cm3; 1000/130; acquisition time, 17 minutes. The authors used a 3D-MRSI technique where by choosing k-space-weighted acquisition, the scanning time was shorter, 10 minutes 45 seconds, for a 12 x 12 x 8 scan with a TR of 1,300 milliseconds and four averages. With the application of a Hamming filter, the voxel size was increased from a nominal 6.7 x 6.7 x 10 mm to an effective size of 10 x 10 x 15 mm corresponding to an effective volume of 1.5 cm3. The nominal voxel size obtained by the authors was 0.45 cm3 significantly larger than 0.34 currently used .When we increase the nominal voxel size we might expect undesirable partial volume and loss of spatial resolution. This can be considered one important drawback of their technique and perhaps could explain their higher rates of false-negatives results. Another important point to discuss is that the authors did not mention whether they replaced or not the air within the endorectal coil by liquid perfluorocarbon. Liquid perfluorocarbon is very useful to reduce the high magnetic field susceptibility at the air-tissue interface and improve the quality of MR spectroscopic imaging data (by reducing the line width). Among 50 patients evaluated in this study, the authors had only 38 patients (76%), with MR spectroscopic imaging sufficient for analysis. One might speculate that by using perfluorocarbon within the endorectal coil instead of air their results would be significantly better.
  Another point that we must consider is that the authors uses a higher value of the ratio choline + creatine / citrate to consider tumor voxel. They consider, tumor voxel when the ratio of (choline + creatine) / citrate was equal to or higher than 1.1. Although there is no consensus about spectral interpretation, the classification system described by Kurhanewicz et al (1) has been used in the more recent studies on this subject. In that system, voxels are considered suspicious for cancer if the ratio of choline and creatine to citrate is at least 2 standard deviations (SDs) higher than the average ratio for the normal peripheral zone. Voxels are considered very suspicious for cancer if the ratio of choline and creatine to citrate is higher than 3 SDs above the average ratio (equal or higher than 0.86). By using a considerably higher ratio to consider tumor voxel one could expect larger number of false negative.
  In our opinion, the association of conventional MRI and 3D-MRSI is very important for the outcome of a patient with prostate cancer.

Reference
1. Jung JA, Coakley FV, Vigneron DB, et al. Endorectal MRSI of the prostate: investigation of a standardized evaluation system. Radiology 2004; 233:701-708.

Dr. Adilson Prando
Chief, Department of Radiology
Vera Cruz Hospital
Campinas, São Paulo, Brazil