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M+, bone marrow infiltration; TO, no malignancy e.g. scar, fibrosis, granulation tissue, hypertrophy; T+ malignancy; ++, highly accurate; +, accurate; 0, not accurate; -, not possible

M+, bone marrow infiltration; TO, no malignancy e.g. scar, fibrosis, granulation tissue, hypertrophy; T+ malignancy; ++, highly accurate; +, accurate; 0, not accurate; -, not possible

On CT, bladder neoplasm appears as sessile, peduncu-lated, soft tissue masses projecting into the bladder lumen (Fig. 10). The tumors have a density similar to that of the bladder wall on enhanced scans and occasionally the in-tra-luminal surface is encrusted with calcium. However, these tumors may show increased attenuation due to neo-vascularization. Bladder cancer involving superficial and deep muscles usually produce focal bladder wall thickening and retraction, but CT cannot reliably differentiate between the various layers of the bladder wall and cannot, therefore, distinguish lesions of the lamina propria (stage T1) from those invading the superficial (stage T2a) and deep muscle (stage T2b).

Tumors of the anterior, posterior and lateral walls are easily detected and evaluated, but difficulties arise in showing tumors at the dome and trigone because of the axial plane imaging used routinely in body CT. CT is clinically useful for detecting invasion into perivesical fat

Fig. 10. Stage T2b bladder cancer (T). On post contrast CT both bladder wall and tumor have identical signal intensity and cannot be separated

(stages T3a and T3b), as CT can distinguish tumors confined to the bladder wall from those spreading into the fat.

Macroscopic extra-vesical extension (stage T3b) is characterized by poor definition of the outer aspect of the bladder wall with an increase in density of the perivesical fat and often requires 5 mm sections. When no distinct fat planes are present between the bladder and the rectum, uterus, prostate and vagina, early tumor invasion into these neighboring structures may be difficult to exclude.

Tumor invasion of the seminal vesicle should be suspected if a soft tissue mass obliterates the seminal vesicle fat angle. This sign should be interpreted with caution because the normal seminal vesicle angle may be lost if the rectum is over-distended or if the patient is scanned in prone position.

CT can, in addition, evaluate retro-peritoneal pelvic lymph nodes. Nodes greater than 10 mm are suspected to contain metastases.

Magnetic Resonance Imaging (MRI)

MRI is superior to CT for staging urinary bladder carcinoma. The multiplanar capabilities and soft tissue characterization capabilities of MR imaging make it a valuable diagnostic tool for visualizing the urinary bladder [66, 67].

Normal MR Anatomy

On Tl-weighted images, the urine has low-signal intensity, and normal bladder wall has an intermediate signal intensity equal to skeletal muscle (Fig. 11a). On these images, the perivesical vessels and vas deferens appear as low signal intensity tubular structures surrounding the bladder base interspersing the perivesical fat. On fat saturation Tl-weighted images the bladder wall has a slightly higher signal intensity than urine or perivesical fat [66-69].

On T2-weighted images the urine has a very high, and the bladder a low signal intensity [66-69] (Fig. 11b). The thickness of the bladder wall varies with the degree of the bladder distension. The normal values range from 2.9 to 8.8 mm, with a mean of 5.4 mm. When the bladder is distended, the wall should not exceed 5 mm in thickness.

The peri-vesical fat has a high signal intensity both on T1- and on fast spin-echo T2-weighted images, and an intermediate signal intensity on spin echo T2-weighted images [66-69]. On fat saturation T2-weighted images not only the signal intensity of perivesical fat that is similar to the bladder wall. Therefore on these images the bladder wall can only be delineated if there are adjacent higher signal intensity perivesical venous plexus or seminal vesicles [66-70].

Immediately after injection of Gadolinium (Gd)-contrast, the urinary bladder wall shows rapid enhancement [73]. Differential enhancement between the inner mucosa and submucosa, and the outer muscular layer has been reported on early images. The inner layer is more vascular and shows early enhancement while the less vascular muscular layer enhances later [71, 72]. About 2 minutes

Fig. 11. Stage T2b bladder cancer (T). a Tl-weighted MR-image tumor has the identical signal intensity as the wall. Urine has lower and perivesical fat has higher signal intensity. b T2-weighted MR image shows bladder wall with higher signal intensity than wall, and lower than urine. At arrows low signal bladder wall is disrupted by tumor, which argues for at least deep muscle invasion (stage T2b)

Fig. 11. Stage T2b bladder cancer (T). a Tl-weighted MR-image tumor has the identical signal intensity as the wall. Urine has lower and perivesical fat has higher signal intensity. b T2-weighted MR image shows bladder wall with higher signal intensity than wall, and lower than urine. At arrows low signal bladder wall is disrupted by tumor, which argues for at least deep muscle invasion (stage T2b)

after contrast injection, the Gd-contrast is excreted into the urine. In order to visualize enhancement within the bladder wall, these images must be acquired within 2 minutes. Within the urinary bladder urine containing concentrated Gd, contrast concentrates by its higher gravity on the dorsal side of the bladder (if the patient is supine) and is seen as a low signal intensity layer. More diluted urine containing contrast material is seen as a middle lay er of high signal intensity. Finally, another low intensity layer can be identified anteriorly, representing unen-hanced urine. Patent blood vessels with normal flow can be recognized without intravenous contrast by their typical signal void on turbo or fast spin-echo images.

On Tl-weighted images, lymph nodes have an intermediate signal intensity (Fig. 12). On T2-weighted images, the signal intensity varies (Fig. 13). It is, however, always higher than that of muscle. Tl-weighted images provide the optimal contrast between medium nodes and high signal intensity fat.

In general, axial images should be acquired using T1-and T2-weighted sequences followed by imaging in other planes. The direction of these planes depends on the site of the tumor. For example, coronal images are particularly helpful for visualizing tumors arising from the base, dome and lateral bladder wall, whereas sagittal images can be used to visualize anteriorly and cranially located tumors and tumors at the bladder sphincter. In addition, images should be acquired in a plane perpendicular to the wall at the base of the tumor. This allows better delineation of muscle involvement. Use of multiplanar

Fig. 12. Normal node. On (a) coronal T1-weighted image imaging plane of (b), parallel to psoas muscle ('obturator' plane) is indicated by white line. On (b) Tl-weighted 'obturator' plane a small 3 mm normal node (circle) can easily be separated from longitudinal running vessels

Fig. 13. Metastatic enlarged nodes (circles). On T2-weighted image, nodes have higher signal intensity than muscle, and identical signal intensity as bladder cancer

reconstruction with a 3D sequence gives the best results. An important plane direction in evaluating lymph nodes is parallel to the psoas muscle. This so-called 'obturator plane' allows visualization of nodes along their long axis, and can locate them in relation to the iliac vessels and obturator nerve (Fig. 12).

The slice thickness should be 4 mm at the most, with a maximal gap of 1 mm. A thinner slice thickness produces better anatomical details and improved partial volume; however, the signal-to-noise (SNR) then also decreases. The field of view (FOV) usually ranges from 24 to 36 cm in diameter.

Image quality in the pelvis should be improved by using surface coils. Phased array body coils are well suited for pelvic MR imaging. The high SNR obtained with these coils facilitates excellent image quality with superior spatial resolution

Pulse Sequences

Since MRI is a highly flexible and versatile system, many different imaging sequences and imaging planes are used for pelvic scanning. The choice depends largely on the type of the scanner used and its ability to obtain fast images. However, irrespective of the scanner features, T1-and T2-weighted sequences are mandatory.

T1-weighted Imaging

For T1-weighted images, turbo or fast spin echo (SE) or gradient recalled echo (GRE) sequences should be used. On these sequences, bladder carcinoma has intermediate signal intensity equal to that of the muscle and can be delineated from fat and urine. T1-weighted images are therefore used to determine tumor infiltration into the perivesical fat. As mentioned before, these sequences are also most suitable for imaging lymph nodes (Fig. 12). In addi

Fig. 12. Normal node. On (a) coronal T1-weighted image imaging plane of (b), parallel to psoas muscle ('obturator' plane) is indicated by white line. On (b) Tl-weighted 'obturator' plane a small 3 mm normal node (circle) can easily be separated from longitudinal running vessels

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