Technical aspects and applications of diagnostic radiology

Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) produces images by first magnetizing a patient in the bore of a powerful magnet and then broadcasting short pulses of radiofrequency (RF) energy at 46.3 MHz that resonate mobile protons (hydrogen nuclei) in the fat, protein and water of the patient’s soft tissues and bone marrow. The protons produce RF echoes when their resonant energy is released; their density and location can be exactly correlated into an image matrix by complex mathematical algorithms.

The spinning proton of the hydrogen nucleus acts like a tiny bar magnet, aligning either with or against the magnetic field, and producing a small net magnetic vector. RF energy from various types of coil, either built into the scanner or attached to specific body parts, generates a second magnetic field perpendicular to the static magnetic field that rotates or ‘flips’ the protons away from the static magnetic field. When the RF pulse is switched off, the protons flip back (relax) to their original position of equilibrium, emitting the RF energy they had acquired into the antenna around the patient. This information is then amplified, digitized and spatially encoded by the array processor.

MRI systems are graded according to the strength of the magnetic field they produce. Routine high-field systems are those capable of producing a magnetic field strength of 3–7 T (Tesla), using a superconducting electromagnet immersed in liquid helium. Open magnets for claustrophobic patients and limb scanners use permanent magnets of 0.2–0.75 T. For comparison, Earth’s magnetic field varies from 30 to 60 μT. MRI does not present any recognized biological hazard. Patients who have any ferromagnetic intracranial aneurysm clips, certain types of cardiac valve replacement or intraocular metallic foreign bodies must never be examined because there is a high risk of death or blindness. Patients with implanted electronic devices such as a cardiac pacemaker or spinal cord stimulator must be screened carefully if MRI is being considered. Many new devices are now MRI-compatible, but the precise product details must be made available to the MRI team to ensure the patient’s safety (e.g. the need to induce a ‘scanner mode’ in the device, or establishing definitive MRI-incompatibility). Many extracranial vascular clips and orthopaedic prostheses are now ‘MRI-friendly’ but may cause local artefacts; newer sequences exist to reduce artefact. Loose metal items, ‘MR-unfriendly’ anaesthetic equipment and credit cards must be excluded from the examination room. Pillows containing metallic coiled springs have been known nearly to suffocate patients, and heavy floor-buffing equipment has been found wedged in the magnet bore because domestic staff had been suboptimally informed.

New methods of analysing normal and pathological brain anatomy are now at the forefront of research. These are MR spectroscopy (MRS); functional MRI (fMRI); diffusion tensor imaging (DTI); high angular resolution diffusion imaging (HARDI) for MR tractography (MRT, see below); and molecular MRI (mMRI), which has taken on a new direction since the description of the human genome.

MRS assesses function within the living brain. It capitalizes on the fact that protons residing in differing chemical environments possess slightly different resonant properties (chemical shift). For a given volume of brain, the distribution of these proton resonances can be displayed as a spectrum. Discernible peaks can be seen for certain neurotransmitters: N -acetylaspartate varies in multiple sclerosis, stroke and schizophrenia while choline and lactate levels have been used to evaluate certain brain tumours.

fMRI depends on the fact that haemoglobin is diamagnetic when oxygenated but paramagnetic when deoxygenated. These different signals can be weighted to the smaller vessels, and hence closer to the active neurones, by using larger magnetic fields. mMRI uses biomarkers that interact chemically with their surroundings and alter the image according to molecular changes occurring within the area of interest, potentially enabling early detection and treatment of disease and basic pharmaceutical development and quantitative testing.

High-field-strength magnets give significant improvement in spatial resolution and contrast. MR images of the microvasculature of the live human brain that allow close comparison with the detail seen in histological slides have been acquired at 8 T. This has significant implications for the treatment of reperfusion injury and research into the physiology of solid tumours and angiogenesis. There is every reason to believe that continued efforts to push the envelope of high-field-strength applications will open new vistas in what appears to be a never-ending array of potential clinical applications.

T1-weighted images best accentuate fat and other soft tissues, whereas fluid is low-signal; these images are nicknamed the ‘anatomy weighting’ amongst radiologists who publish or teach anatomy. T2-weighted images reveal fluid as high signal as well as fat. Fat suppression sequences using T2 ‘fat sat’ (T2FS) or short tau inversion recovery (STIR) are very sensitive in highlighting the soft tissue or bone marrow oedema that almost invariably accompanies pathological states such as inflammation or tumours. Contrast-enhanced images with gadolinium, when used with T1 fat saturation (T1FS) sequences, also exquisitely highlight hypervascularity, particularly that associated with tumours and inflammation, especially in pathologies where the blood–brain barrier is compromised. Metallic artefact reduction sequences (MARS) are superior in imaging periprosthetic soft tissues after joint replacement or other orthopaedic metalwork implantation.