Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to investigate the anatomy, physiology, and pathology of the body in both health and disease. MRI scanners use strong magnetic fields and radio waves to form images of the body. The technique is widely used in hospitals for medical diagnosis, staging of disease and for follow-up without exposure to ionizing radiation.
To perform a study the patient is positioned within an MRI scanner which forms a strong magnetic field around the area to be imaged. Most medical applications rely on detecting a radio frequency signal emitted by excited hydrogen atoms in the body using energy from an oscillating magnetic field applied at the appropriate resonant frequency. The orientation of the image is controlled by varying the main magnetic field using gradient coils. As these coils are rapidly switched on and off they create the characteristic repetitive noises of an MRI scan. The contrast between different tissues is determined by the rate at which excited atoms return to the equilibrium state. Exogenous contrast agents may be given intravenously, orally or intra-articularly
MRI requires a magnetic field that is both strong and uniform. The field strength of the magnet is measured in Tesla – and while the majority of systems operate at 1.5T, commercial systems are available between 0.2T–7T. Most clinical magnets are superconducting which requires liquid helium. The lower field strengths can be achieved with permanent magnets, which are often used in "open" MRI scanners for claustrophobic patients.
The image quality of an MRI depends on signal and field strength. So a 3T machine has much more signal than a 1.5T machine. 3T images can provide extremely clear and vivid images and can often be done faster, decreasing overall scan time. Both are of great value to the patient in terms of diagnosis and comfort. 3T is ideal for imaging small bones, breast MRI, musculoskeletal MRI, neurological MRI and vascular MRI, where the minute details are especially crucial to diagnosis. With that said, a 3T machine isn’t always the best for every kind of imaging. 3T has a higher likely hood of artifacts (any object that appears in the image that is not in the original object); specifically “flow” artifacts due to blood or fluid. Additional heat and more noise compared to a 1.5T can also be a challenge with 3T.
A true open MRI means that the machine is open on all four sides. The machine looks almost like a pancake with the top of the machine close to the face/head of the patient. True open MRIs started at a very low strength of .3T, progressed to now 1.5T. Today, a 1.5T MRI is considered the standard for state-of-the-art imaging. The open options for a 1.5T are in wide-bore format. The size of the ‘hole’ (or bore) where the patient lies is 70 cm, providing more headroom than a true open MRI and is actually much more spacious – it is just not open on the sides. Many patients prefer the open feeling that a 1.5T wide-bore offers vs. the additional room for limbs. While the true open model is still advancing, many physicians and radiologists much prefer the images produced on the higher strength 1.5T MRI for more accurate diagnosis.
The MRI environment may cause harm in patients with MR-Unsafe devices such as cochlear implants and most permanent pacemakers. Several deaths have been reported in patients with pacemakers who have undergone MRI scanning without appropriate precautions. Many implants can be safely scanned if the appropriate conditions are adhered to. MR Conditional pacemakers are increasingly available for selected patients.
Ferromagnetic foreign bodies such as shell fragments, or metallic implants such as surgical prostheses and ferromagnetic aneurysm clips are also potential risks. Interaction of the magnetic and radio frequency fields with such objects can lead to heating or torque of the object during an MRI. Titanium and its alloys are safe from attraction and torque forces produced by the magnetic field, though there may be some risks associated with Lenz effect forces acting on titanium implants in sensitive areas within the subject, such as stapes implants in the inner ear.
The very high strength of the magnetic field can also cause "missile-effect" accidents, where ferromagnetic objects are attracted to the center of the magnet, and there have been incidents of injury and death. To reduce the risk of projectile accidents, ferromagnetic objects and devices are typically prohibited in the proximity of the MRI scanner and patients undergoing MRI examinations are required to remove all metallic objects, often by changing into a gown or scrubs, and ferromagnetic detection devices are used at some sites.
Pregnancy has always raised red flags in MRI. No effects of MRI on the fetus have been demonstrated. In particular, MRI avoids the use of ionizing radiation, to which the fetus is particularly sensitive. However, as a precaution, current guidelines recommend that pregnant women undergo MRI only when essential. This is particularly the case during the first trimester of pregnancy, as organogenesis takes place during this period. The concerns in pregnancy are the same as for MRI in general, but the fetus may be more sensitive to the effects—particularly to heating and to noise. The use of gadolinium-based contrast media in pregnancy is an off-label indication and may only be administered in the lowest dose required to provide essential diagnostic information.
Once all the scans are done, images are transferred via teleradiology to the radiologist’s screen for interpretation. Images can also be sent directly to the referring physician, operating room, emergency room, and ever further processed with 3D reconstruction. The beauty of teleradiology is not just to send images, the reading Radiologist has the luxury of viewing the images while the patient is still inside the scanner, incase additional scanning is need. The operating surgeon can view the MRI images while performing a procedure. The referring physician can have his input as well all while the scan is being done.
In 13 years of working in the radiology field and mainly in MRI, I can tell you that the 3T magnet is extremely superior to the others. The quality, speed, efficiency, and user friendly software takes the competition to another level. A technologist and Radiologist job become much easier and patient quality of health increases in 3T as appose to lower field magnets.