Determinación de los flujos de caja

Prostate MR at clinical field strengths (up to 3 T) is usually performed with at least a pelvic phased array coil for signal reception. The number of channels in the pelvic phased array coil is of importance for parallel imaging possibilities. The European Society of Urogenital Radiology recommends the use of a 8- to 16-channel pelvic phased array coil for all prostate MR exams (36), and current instrumental setups can contain even more coil elements.

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In addition to an external phased array coil, an endorectal coil for local signal reception or a combination of the two can be used. The combined use of an endorectal coil and pelvic phased array coil integrates the high SNR of the prostate from the endorectal coil with the wide field of view (FOV) of the pelvis from the pelvic phased array coil (Fig. 2.2). This results in higher staging accuracy and image quality at 1.5 T with the same imaging parameters (11,78,79).

The necessity of an endorectal coil for prostate MR is heavily discussed, since it causes discomfort for the patient, it requires additional MR scanner time and trained personnel for insertion and evaluation of the correct position, and it is expensive. Moreover, the SNR changes dramatically with distance from the coil conductors because of the inhomogeneous receive profile of the endorectal coil. This profile, if uncorrected for, is strongly visible in MR images (Fig. 2.2A,B,D,E), but can be reduced by applying a normalization filter, which equalizes signal intensities across the image at the cost of larger noise at larger distances from the coil (80). Bowel motion can introduce stronger artifacts when the endorectal coil is present, but can be suppressed by injections of glucagon and/or butylscopolaminebromide. If only the endorectal coil is used for MRI, the strongly localized sensitivity can be used to decrease the FOV of the images series, enabling high spatial resolution with reasonable scan matrices, as is essentially done in MRSI of the prostate. With large prostates, the anterior part of the transition zone of the prostate might suffer from lower sensitivity of an endorectal coil only when pushing towards the limits of spatial resolution in the posterior peripheral zone. If this high anatomical detail is of less importance, one could choose to use the SNR gain of moving to 3 T to eliminate the use of the endorectal coil as part of the coil setup (Fig. 2.2G-I).

Although it is difficult to compare images with and without endorectal coil at different field strengths in an objective and/or quantitative way, it has been observed that images obtained with a pelvic phased array coil alone at 3 T show comparable image quality to those obtained with both a pelvic phased array and endorectal coil at 1.5 T (19,81,82). In addition to this, the two different coil setups gave rise to equal numbers of motion and coil-related artifacts. This might imply that, if the time for inserting the endorectal coil and evaluating its position had been used as additional measurement time in a 3 T exam without an endorectal coil, the 3 T images could overall be superior to the 1.5 T images with endorectal coil, if patient and bowel motion is negligible over longer scan times. It was however also shown that MRI and MRSI with an endorectal coil outperformed MRI and MRSI with an array coil at 3 T in terms of image quality, staging performance and prostate cancer localization (20,52), and the combination of an endorectal with an array coil at 3 T increased SNR and image quality with respect to the same setup at 1.5 T (15,83). Due to the possibility of remaining coupling of the transmit body coil with the endorectal coil, the allowed

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SAR deposition when using an endorectal coil is only half that of the setup without endorectal coil. At 3 T, this occasionally prolongs scan times of T2W-image series, as repetition times need to be prolonged when the physiques of individual patients demand a lot of RF power to attain refocusing pulses.

In the above-mentioned studies, an inflatable endorectal coil was used (Medrad, Bayer HealthCare Medical Care, Indianola, PA, USA), where the inflated balloon helps to maintain appropriate positioning of the coil in the rectum. At 3 T, the inner balloon of the endorectal coil is often inflated with a substance resembling tissue susceptibility (e.g. liquid perfluorocarbon or barium sulfate), since inflation with air can cause large susceptibility artifacts, deteriorating prostate MRI and MRSI quality (84). Prostate MRI studies with a rigid type of endorectal coil have also been described (85,86). These rigid coils deform the prostate shape to a smaller extent than the inflatable coils, which can improve treatment planning (85). The rigid coils provide approximately a 2.5-fold higher SNR than inflatable coils near the peripheral zone midline, but a more dramatic lateral decrease in sensitivity is noted with the rigid coils as compared with the inflatable coils (86). The hard plastic of the rigid coil does not distort the magnetic field by itself, but bowel motion is less reduced than with an inflatable coil (86).

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