Technique and radiation

No specific preparation is required for patients undergoing MR scanning. It is however important to be aware of certain important contraindications:

• Pacemakers, cochlear implants, implanted infusion pumps, and other implanted devices can malfunction in the strong magnetic field

• Cerebral aneurysm clips, older (Bjork-Shiley) heart valves, titanium intravascular filters, metallic foreign objects (e.g., bullets, shrapnel, fragments in the eye) may undergo deflection

• Patients with claustrophobia, patients with dementia (who may be unable to remain still in the scanner), and morbidly obese patients (due to limited gantry space) are unsuitable

Most modern orthopedic implants are safe. Patients must be warned about possible noise levels, which may reach up 95 dB on occasions. In spite of the lack of evidence linking MR to

FIGURE 3.6. (a) T2-weighted MRI showing left lower ureteric TCC invading the bladder, (b) sagittal T2 MR urogram in an 18-week pregnant female showing ureter narrowing where it crosses the iliac vessels (c) and (d) axial T2 MR prostate—low signal tumor replacing the normal high signal PZ on the left, with early extracapsular disease on the left (Courtesy of Dr A Bradley, Wythenshawe, Manchester)

FIGURE 3.6. (a) T2-weighted MRI showing left lower ureteric TCC invading the bladder, (b) sagittal T2 MR urogram in an 18-week pregnant female showing ureter narrowing where it crosses the iliac vessels (c) and (d) axial T2 MR prostate—low signal tumor replacing the normal high signal PZ on the left, with early extracapsular disease on the left (Courtesy of Dr A Bradley, Wythenshawe, Manchester)

teratogenesis, MR scanning is not advised in pregnancy, especially in the first trimester. Nevertheless, in later pregnancy, MR urography may be utilised in the investigation of suspected upper tract obstruction (e.g., secondary to a ureteric calculus).

In order to enhance the inherent contrast between tissues, contrast agents, such as gadolinium chelate, may be used.

Gadolinium—

• Is a paramagnetic metal

• Shortens the T1 relaxation time

• Is freely filtered and excreted by the kidneys

• Is non-nephrotoxic

• Is rarely associated with allergic reactions

• Can be used safely in patients with compromised renal function, except in severe renal impairment (GFR <20 mL/min), in which case MR scanning must be followed up by dialysis

Some of the signal intensities of commonly encountered urologi-cal tissues are given in Table 3.6.

TABLE 3.6. Signal intensities of urological tissues

Substance

Tj Weighted

T2 Weighted

Kidneys

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