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Magnetic Resonance Imaging: Technique, Sequences and Indications
Indications of MRI in Urology
Compared to CT, magnetic resonance imaging (MRI) offers a higher soft tissue contrast and avoids the need for radiation exposure or iodine contrast media. Disadvantages are the lower image resolution, a higher chance of motion artifacts due to longer examination times, and the lower availability in many regions. MRI is particularly advantageous for imaging the pelvic organs, in kidney, adrenal, testicular, or penile masses, and also as a follow-up examination after unequivocal CT imaging. The contrast agent for MRI is less toxic and can be administered in patients with moderate renal insufficiency (GFR >30 ml/min) and an ionine contrast media allergy. Contrast-enhanced MRI can completely replace intravenous urography and is ideal for diagnosing the urinary tract in children and pregnant women. Further indications for MRI are the diagnosis of penile fracture and the imaging of the abdominal vascular system (MR angiography) (Bassignani et al., 2006).
Contraindications of Magnetic Resonance Imaging
Patients with iron implants or foreign bodies, intracranial clips, old pacemakers, insulin pumps, semirigid penile implants, Foley catheter with temperature probe, very ill or intensive care patients, and patients with claustrophobia.
Technique of Magnetic Resonance Imaging
MRI uses nuclear magnetic resonance (NMR): protons have a spin, which causes a magnetic moment because of the positive charge. Installing a strong magnetic field leads to an alignment of the hydrogen atoms. Short radio frequency pulses briefly deflect the hydrogen atoms and generate a measurable electric signal. The electric signal depends on the different tissue properties, such as water content and type of measurement (T1 or T2 weighted examinations). To assign the signals to the individual volume elements (voxels), spatial coding is enabled with the help of magnetic field gradients. The signal strength of the voxels is displayed in gray values.
Interpretation of MRI:
Certain tissue types have no standard values (comparable to Hounsfield units). Image contrast is determined by T1 and T2 weighting, proton density of different tissue types, and main magnetic field. Proper nomenclature: high signal or hyperintense, low signal or hypointense.
Multiparametric MRI (mpMRI) of the Prostate:
mpMRI of the prostate is a special organ-focused MRI with very high spatial resolution in the area of the prostate gland. mpMRI of the prostate uses different sequences such as T1- and T2-weighted imaging, diffusion-weighted imaging (DWI), magnetic resonance spectroscopic imaging, and dynamic contrast-enhanced imaging (DCE), please refer to table sequences of MRI. The prerequisite for a mpMRI is using modern high-field scanner of 1.5 or 3 Tesla. mpMRI proved useful in clinical practice to diagnose and stage prostate cancer. Studies are ongoing for further indications like small renal tumors.
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Bassignani 2006 BASSIGNANI, Matthew J.:
Understanding and interpreting MRI of the genitourinary tract.
In: Urol Clin North Am
33 (2006), Aug, Nr. 3, S. 301–317
Deutsche Version: Indikationen und Technik der Magnetresonanztomographie in der Urologie