Conclusiones Extraídas de la simulación

T2W-imaging provides anatomical contrast because of the variety in transverse relaxation times (T2) of the different structures and tissues in the prostate. To depict prostate anatomy, high resolution T2W turbo spin-echo (TSE) MR images are generally obtained in 2 or 3 orthogonal planes.

As T2W-MRI is the clinical workhorse for depicting prostate anatomy, the most significant benefit of moving to 3 T is the possible increase in spatial resolution to improve visualization of the anatomical details (Fig. 2.1). From 1.5 to 3 T, the pixel volume can roughly be halved, if the performances of the coil setups at both fields were similar (see Prostate imaging – use of different coils).

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A state-of-the-art T2W-MRI of the prostate at 3 T in 3 directions with an in-plane resolution of 0.4 x 0.4 mm and a slice thickness of 3 mm can be performed within 13 min with a combination of a pelvic phased array and endorectal coil. The same spatial resolution with a similar coil setup at 1.5 T would require an unacceptable acquisition time of approximately 50 min. In clinical practice at 1.5 T, an in-plane resolution of 0.6 x 0.6 mm is employed at similar slice thickness and total acquisition time.

Staging accuracy of biopsy-proven local prostate cancer by T2W-MRI with an endorectal coil at 3 T has been reported up to 94% when performed by an experienced radiologist (10) compared with 83% at 1.5 T in the same institution (11). Other work from different institutions at either 1.5 or 3 T reports conflicting staging performances for the two field strengths, nicely summarized in (8). Difficulties remain in comparing these studies with only field strength in mind, as coil types, patient cohorts and staging endpoints were not the same. In the absence of large studies that directly compare staging performance in the same patient cohort with similar setups at the two field strengths, we can only assume a staging performance gain with the increase in spatial resolution at 3 T. With the spatial resolution available at 3 T, minimal capsular invasion could also be detected (10), which imposes a new challenge to the urologist: traditionally, based on DRE, the difference between organ-confined (stage T2) and locally advanced (stage T3) prostate cancer led the choice of treatment (curative intent for stage T2 or delaying and palliative intent for

Figure 2.1 - Prostate carcinoma in a 64-year-old male. A, B: T2W axial TSE image at 1.5 T (TE: 120 ms) (A) and at 3 T (TE: 119 ms) (B), both with an in-plane resolution of 0.35 x 0.35 mm. A low-signal-intensity lesion in the right peripheral zone and central gland (arrows) is visible, corresponding to positive biopsy findings. The capsule at the right side of the prostate demonstrates bulging (arrowhead) and irregular capsule suggesting invasion. C: T2W high-resolution axial image at the same slice position as A and B using TSE (TE: 162 ms) with an in-plane resolution of 0.18 x 0.18 mm at 3 T. The capsule demonstrates bulging (arrowhead) and is disrupted (open arrow) indicating extracapsular extension. D,E and F: detail (3x zoom) of A, B and C, respectively, with increased anatomical detail of F compared with A and B. Source: (15).

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stage T3). It is unknown whether for minimal capsular invasion, perhaps capturing very early onset of T3 disease, the same treatment decision is the best choice. In addition to a gain in spatial resolution, image contrast is an aspect that needs to be optimized for a specific field strength. The T2 relaxation times of tissues generally decrease slightly with increase in field strength (9). The knowledge of T2 relaxation times in the different prostate tissues should be utilized to maximize contrast between normal peripheral zone prostate tissue, normal transition zone prostate tissue and, most importantly, prostate cancer tissue. At 1.5 T, the peripheral zone T2 is longer than the transition zone and tumor tissue T2-values (122±34 ms, 88±13 ms and 82±18 ms, respectively) (12). The small difference in T2 between transition zone tissue, which often expresses BPH (T2=91±13 ms), and tumor tissue makes tumor recognition in this zone based on T2-value only more difficult than in the peripheral zone. When moving to 3 T, de Bazelaire et al. found a slight decrease in T2 of the overall gland compared with 1.5 T (88±0 and 74±9ms for 1.5 and 3 T, respectively) (13), while Gibbs et al. reported an increase in T2 in both peripheral zone (142±24 ms) and tumor tissue (109±20ms) with respect to their values measured at 1.5 T (14). The differences in T2 relaxation times in prostate between 1.5 and 3 T are thus not very pronounced, making changes in echo time (TE) for optimal contrast in T2W- MRI when moving from 1.5 to 3 T not very crucial (15–17).

Using TEs beyond the T2 of normal prostate tissue is common practice at both 1.5 and 3 T (e.g., TE of 96–132 ms versus T2 of 88 ms at 1.5 T and TE of 109-124 ms versus T2 of 74 ms at 3 T) (10,11,18–20). The difference between cancer and healthy prostate tissue becomes more profound at longer TEs, provided the tissue with the shortest T2 still has adequate SNR (20).

The amount of energy that is absorbed by tissue when it is exposed to an electromagnetic field (the specific absorption rate or SAR) is determined by the electrical component of the radiofrequency (RF) pulse. When moving from 1.5 to 3 T, the frequency of the RF oscillations doubles, which leads to an approximate quadratic increase in SAR (for an equal RF pulse). To stay within European Union imposed SAR safety limits the RF-intensive sequences, such as the TSE sequence for T2W-imaging of the prostate, need some alterations to their conventional (1.5 T) parameters. For prostate examinations at 3 T, partial refocusing pulses (120-150 degrees instead of 180 degrees) or the use of variable flip angles in the multiple echo train (21,22) are usually enough to adhere to SAR safety limits (assuming homogeneous distribution of the electrical component of the RF pulse over the whole exposed body).

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Diffusion-Weighted Imaging