Magnetic resonance angiography serves as a diagnostic procedure for graphical imaging of blood vessels. Unlike conventional examination methods, the use of X-rays is not necessary. However, there are contraindications to the use of this procedure.
What is magnetic resonance angiography?
Magnetic resonance angiography, or MRA, is an imaging procedure used for diagnostic imaging of blood vessels. Magnetic resonance angiography, also called MRA, is an imaging procedure used for diagnostic imaging of blood vessels. It is based on magnetic resonance imaging. The main objects of examination are the arteries. In rare cases, veins are also examined. In some cases, completely non-invasive techniques can be used here, without the need for surgical interventions or injections. Unlike conventional angiography, no catheter needs to be inserted. There are also methods of magnetic resonance angiography that are performed with contrast agents. However, the use of harmful X-rays is eliminated. Instead of the two-dimensional images produced by conventional angiography, magnetic resonance angiography usually acquires three-dimensional data sets. This allows the vessels to be assessed from all viewing directions. Magnetic resonance angiography is used in cases of suspected arteriosclerosis, emboli, thrombosis, aneurysms, or other vascular malformations.
Function, Effect, and Objectives
Magnetic resonance angiography, like general magnetic resonance imaging, is based on the physical principles of nuclear magnetic resonance. It is based on the fact that atomic nuclei, in this case protons (hydrogen atom nuclei), in chemical compounds have spin. The spin is defined as a torque. The torque generates a magnetic moment as a moving charge. When an external stationary magnetic field is applied, the magnetic moment of the proton aligns to this field. This produces a weak longitudinal magnetization (paramagnetism). If a strong alternating field is applied transverse to the direction of the static magnetic field, the magnetization tilts and transforms partially or completely into a transverse magnetization. This immediately starts a precession movement of the transverse magnetization around the field lines of the static magnetic field. A coil registers this precession movement by changing the electrical voltage. When the alternating field is switched off, the magnetic moments of the protons re-align to the static magnetic field. The transverse magnetization slowly decays. This decay time is called relaxation. However, the relaxation depends on the physical and chemical environment of the protons. Thus, the transverse magnetizations take different times to decay in different tissues and areas of the body. These different relaxations are expressed in the image by differences in brightness. Only in this way is the three-dimensional image created. This principle also applies to the imaging of blood vessels, in which case it is referred to as magnetic resonance angiography. There are many different techniques for magnetic resonance angiography. Three methods are used particularly frequently. These methods include time-of-flight MRA, phase-contrast MRA, and contrast-enhanced MRA. Time-of-flight MRA (TOF-MRA) is based on the difference in magnetization between fresh blood and surrounding tissue. This takes advantage of the fact that the inflowing blood is more strongly magnetized than the stationary tissue. The magnetization of the corresponding tissue has already been reduced by exposure to a high-frequency field. The different signal intensity of the blood and the tissue is displayed as an image. However, in the image representation, artifacts often occur if the blood has already flowed for a long time in the area under examination. To reduce the exposure time of the RF field to the blood, the examination field should be perpendicular to the direction of blood flow in this method. Time-of-flight MRA does not require a contrast agent because fast 2D or 3D gradient techniques can be used here. Phase-contrast MRA plays a major role as another method. Similar to time-of-flight MRA, the differences between flowing blood and surrounding tissue are signal-richly displayed.Here, however, the blood is not distinguished by the magnetization, but by the phase differences to the tissue. This method also does not require a contrast agent. The third method is called contrast-enhanced MRA. It is based on the injection of a contrast agent, which significantly shortens the relaxation. Compared with the other two methods, contrast-enhanced magnetic resonance angiography greatly shortens the image acquisition time.
Risks, side effects, and hazards
Compared with conventional angiography, magnetic resonance angiography has many advantages but also disadvantages. The use of this method does not require surgical intervention. Thus, a catheter does not need to be placed. However, the fact that examination and simultaneous treatment cannot be combined may have a disadvantage. Magnetic resonance angiography produces three-dimensional images that allow the vessels to be assessed from different viewing directions. However, there are also clear contraindications to the use of this method. These contraindications mainly relate to the effect of the magnetic field. For example, wearers of pacemakers or defibrillators must not undergo magnetic resonance angiography. The magnetic field used can damage the devices and cause health problems. Also, if there are iron fragments or other metallic objects (e.g., cavafilters) in the body, the use of this method is contraindicated. Magnetic resonance angiography should also not be used during the first 13 weeks of pregnancy. A contraindication also arises when wearing a cochlear implant (hearing prosthesis). This device contains a magnet. However, with some cochlear implants, MRA can be performed according to the exact instructions of the manufacturer. Implanted insulin pumps do not allow magnetic resonance angiography because these devices can also be damaged. In the case of tattoos with metal-containing color pigments, MRA can cause burns to the skin. Similarly, magnetic resonance angiography is also not recommended for nonremovable magnetic piercings in the area of examination.
