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tion, there is a good contrast between the metastasis and surrounding fatty bone marrow. Gradient echo images are susceptible to motion and therefore may cause motion artifacts. They can be helpful for demonstrating pelvic side-wall infiltration, vessels, and for distinguishing these from enlarged lymph nodes. In addition, as these sequences have a short imaging time they can be used for dynamic contrast-enhanced imaging, or for quickly acquiring 3D data. The combined time-saving of turbo or fast SE sequences and excellent image quality has led to the widespread use of this sequence type. With an attempt to tradeoff the time-saving for an improvement in image quality at many institutions, fast spin echo (FSE) has replaced conventional SE for T1- and T2-weighted imaging scans.

T2-weighted Imaging

For T2-weighting, turbo or fast SE sequences are considered state-of-the-art. On T2-weighted images, urine has high signal intensity and tumor has an intermediate intensity, higher than bladder wall or fibrosis and lower than the urine (Fig. 11b). The zonal anatomy of the prostate or uterus and vagina can also be easily recognized on these images. The T2-weighting should, however, not be too strong, as then bladder cancer has a low signal intensity that is too low, and results in decreased discrimination from the wall. To obtain this goal, a TE of no longer than 90 ms should be used. These sequences allow high resolution images in a relatively short time, and are used for determining depth of tumor infiltration within the bladder wall, for differentiating tumor from fibrosis, for assessment of invasion into the prostate, uterus or vagina, and for confirming bone marrow metastasis seen on Tl-weighted images (Fig. 14a). For last purpose a Short Tou Inversion Recovery (STIR) sequence can also be used can be used (Fig. 15). Metastases have a high, and bone marrow has a low signal intensity on this sequence type (Fig. 14b) [73].

Fig. 15. Bone marrow metastasis in pubic bone. On STIR image bone marrow metastasis has high signal intensity (arrow)

Dynamic Contrast Enhanced Sequences

MRI using extra-cellular Gd-based contrast medium can be used to improve visualization of bladder cancer (Fig. 16). The success of dynamic contrast-enhanced MRI depends on its ability to demonstrate intrinsic differences between a variety of tissues that affect contrast medium behavior [64-47]. Gd-contrast is given intravenously as a bolus injection with a power injector in the anticubital vein in a dose of 0.1 mmol/kg. This extracellular contrast agent readily passes from the vasculature into the ex-travascular, extracellular space and gives rise to enhancement. The contrast medium causes shortening of the T1-relaxation time, and thus causes increased signal intensity on T1-weighted images. The early phase of contrast-enhancement - often referred to as the first pass - includes the arrival of contrast medium. In this phase, the increased signal seen on T1-weighted images arises from both the

Fig. 14. Bone marrow metastases (arrows). a T1-weighted MR image shows metastases as low signal intensity round lesions. b On the post-Gd fat saturated T1-weighted image, due to enhancement, bone marrow metastases can be recognized as high signal intensity lesions

Fig. 16. Stage T4b urinary bladder cancer infiltrating prostate and pelvic sidewall muscle (arrows). a T2-weighted tSE image shows bladder cancer (T) infiltrating in prostate and periprostatic fat (arrows). b On the post-Gd fat saturated T1-weighted image, enhancing tumor infiltration in muscle of pelvic sidewall (arrows) is also visible

Fig. 16. Stage T4b urinary bladder cancer infiltrating prostate and pelvic sidewall muscle (arrows). a T2-weighted tSE image shows bladder cancer (T) infiltrating in prostate and periprostatic fat (arrows). b On the post-Gd fat saturated T1-weighted image, enhancing tumor infiltration in muscle of pelvic sidewall (arrows) is also visible vascular and interstitial component. Also, when a bolus of contrast medium passes through the capillary bed, it produces magnetic field inhomogeneities that result in a decrease in the signal intensity of the surrounding tissues.

Urinary bladder carcinoma develops neovasculariza-tion [56, 59, 77-84], therefore after intravenous administration of gadolinium, bladder cancer shows earlier and more enhancement than normal bladder wall or other non-malignant tissues. Urinary bladder cancer starts to enhance 7 seconds after the beginning of arterial enhancement, which is at least 4 seconds earlier than most other structures [56].

Contrast enhancement has many advantages, including improved detection of small bladders tumors, especially those measuring less than 1 cm that may be missed on T2-weighted images because the high signal tumor can be obscured by urine; improved differentiation of bladder tumors from blood clots and other debris; improved detection of muscular and perivesical fat invasion and improved conspicuity of (small) vessels. For contrast-enhanced imaging of bladder cancer, T1-weighted sequences with a time resolution of less than 20 seconds should be used. In the early phase (first pass phase), discrimination between cancer and wall or fibrosis is best visible. Also, this speed avoids the filling of the bladder lumen due to renal excretion. To enhance tumor visualization in the perivesical fat, either subtraction or fat-saturation must be applied. Usually GRE sequences are used for dynamic MRI. The imaging plane can be preselected based on the previously performed T1- and or T2-weighted sequences

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