This study demonstrated that each type of endorectal probe exhibited both advantages and disadvantages, as summarized in . The SNR was increased with the smaller, rigid probes, but this increase decreased laterally and toward the apex of the prostate. The biopsy probe demonstrated a decrease in sensitivity toward the apex. This is due to the shape of the probe bending away from the prostate to allow a biopsy needle to pass through the probe into the prostate. This probe is intended to be repositioned along the S/I direction during imaging to acquire biopsy samples from different locations and so such repositioning could compensate for this falloff. The inflatable probe took less time to position and stayed in place better than the rigid probes. This may have been due to the inflatable coil positioning itself in the S/I direction. Also, the nurses who inserted the probes were more experienced using the inflatable probes than the rigid probes, so some of the longer positioning time might decrease with experience. Another difference between the probes is that the inflatable probe produced less motion artifacts than the rigid probes. The balloon in the air- or PFC-inflated probes may have absorbed some of the motion. Motion in the anatomic images primarily caused right to left repeated ghosts of the bright rectal wall. These can overlap the borders of the peripheral zone, making it more difficult to detect extracapsular extension near the peripheral zone and rectum. Ability to diagnose extracapsular extension was not evaluated in this study, however. Newly-designed rigid probe holders may address some of these issues with placement time and placement position and motion.
Comparison of the Different Endorectal-Coil Probe Setups
The design of the air-inflated probe compresses the rectal wall and introduces a pocket of air between the coil and the peripheral zone, increasing magnetic susceptibility artifacts, making shimming more difficult relative to the other probes, and increasing the line-width of the spectral peaks. This problem was addressed in the PFC-inflated probe setup. While the rigid probes used in this study are not currently commercially available, they have been used and may become commercially available. Also, similar designs have been developed for particular applications, such as dual tuned, 13
H endorectal receive only coils, recently designed for hyperpolarized 13
C prostate studies (17
). One limitation of this study was that the biopsy probe was only evaluated in five subjects, which limits interpretation of the results for this probe.
In this study, PFC was used to reduce magnetic susceptibility between the rectum and the prostate. Rosen et al (8
) compared a barium sulfate solution, PFC, and air for inflation of an endorectal coil and found that both the barium sulfate solution and PFC provided significantly higher magnetic field homogeneity across the prostate vs. air, but were not significantly different from each other. Inflation with both the barium sulfate solution and PFC improved spectra quality (8
). A manganese chloride solution has also been used to inflate endorectal probes to reduce magnetic susceptibility (18
), which should also lead to higher quality spectra. The rigid probes used in the current study have a plastic shell. Hard plastics have been shown to have a magnetic susceptibility closer to tissue than air (19
). The rigid probes in this study provided smaller water line-widths than the air-inflated probes, likely due to this and to the fact that the rectal wall was not as compressed, leading to the peripheral zone of the prostate neighboring tissue rather than neighboring the endorectal probe. Different inflation substances were not compared in the current study; however, all likely improve spectral quality over air-inflation.
In conclusion, for general use, the PFC-inflated probe is recommended, due to its larger coverage volume, shorter placement time, low motion artifacts, and high-quality spectra, but it does cause greater tissue distortions than the rigid probes. Rigid probes may be required for MR-guided biopsies and may be preferred for specific applications requiring higher SNR or less tissue distortion (such as for radiation treatment planning) or for cases in which the prostate volume is known to be small. In consideration of these factors, new coil and probe designs should incorporate advantageous characteristics of both the inflatable and rigid probes described here, taking into consideration the requirements of the desired application.