Vascular Magnetic Resonance Imaging

Vascular Magnetic Resonance Imaging

Vascular Magnetic Resonance Imaging is the generic medical name given to a category of imaging techniques based on the Magnetic Resonance Imaging (MRI) technology. Also known under the names of Magnetic Resonance Angiography (MRA) for arteries and Magnetic Resonance Venography (MRV) for veins, these procedures aim at producing accurate images of the blood vessels in order to analyze them and detect any abnormal formations – aneurysms (dilation deformations in the blood vessel walls), stenosis (narrowing of the blood vessel passage, hindering blood circulation) or occlusions, dissections (tears of the vessels), vasculitis (blood vessel inflammations), atherosclerosis (a hardening of the blood vessel walls in the extremities) or carotid artery disease (a blockage in the major blood vessels supplying the brain). These procedures can be employed to assess the condition of blood vessels in the neck, brain, thorax, abdomen, kidneys and legs.

Vascular Magnetic Resonance Imaging allows the study of many blood vessels in the human organism. However, these techniques are less successful in certain conditions (for example, for the assessment of coronary arteries) than CT angiography or the invasive procedure of catheterization. In many cases, the underlying medical condition is found to be atherosclerosis – however abnormalities of the vascular system or aneurysms may also be observed.

Vascular Magnetic Resonance Imaging has, over other imaging techniques, the advantage of being non-invasive, and that the patient is never exposed to any type of ionizing radiation. Typically, the contrast agents employed during Magnetic Resonance Angiography are less toxic than those used during other procedures as well. However, the high costs of a MRI procedure, as well as the spatial resolution of somewhat limited capability are considerable drawbacks.

Also, there are several contraindications to Vascular Magnetic Resonance Imaging procedures. Individuals who are overweight (over 300 pounds), patients with implants such as pacemakers, valves, clips or pins, patients who require continuous life support such s oxygen administration and patients suffering from agitation, confusion, anxiousness or claustrophobia, or patients who are not able to lie still for the duration of the procedure may not be able to undergo this imaging procedure. Also, pregnant females may not be able to safely undergo MRA imaging.

Vascular Magnetic Resonance Imaging Description

As we were stating at the beginning of this informational guide, there are several Vascular Magnetic Resonance Imaging procedures that may be employed in order to obtain the best visualization of the blood vessels. Some of them produce inherent pictures (based on flow effects) while others a pharmacologically generated with the aid of contrast agents. In this chapter, we will try to familiarize you with the most common Vascular Magnetic Resonance Imaging procedures.

Time-of-flight (TOF) angiography, also known under the name of Inflow angiography, is an ordinary MRI performed using certain specifications which allow the lowing blood to appear much brighter in the images than the surrounding stationary tissue.

Phase contact magnetic resonance angiography (PC-MRA) is another technique which relies on the manipulation of the MRI signal phase with special gradients, so that the signal becomes directly proportional to the blood flow velocity. This allows the recording of quantitative measurements of the blood flow, while also imaging the blood vessels.

Fresh blood imaging (FBI) is a special type of imaging procedure relying on fast (FSE) or super fast spin echo sequences (SFSE). This procedure has been designed to take advantage of the blood’s longer T2 relaxation when compared to the static surrounding tissue, and the images are recorded by means of fast spin echo sequences which are typically synchronized with the patient’s heart rate.

Contrast enhanced magnetic resonance angiography (CE-MRA) is an imaging technique assisted and enhanced through the injection of MRI contrast dyes into the patient’s bloodstream. The images are best taken during the first passing of the enhancing substance through the patient’s arteries after being injected into a vein; with appropriate timing, the image quality is very high. Alternatively, it is possible to use contrast dyes which remains in the patient’s bloodstream for longer periods of time; the extended time frame will allow the acquisition of superior quality images – however, in this situation, both veins and arteries will be simultaneously affected by the presence of the contrast dyes and as such they will be equally enhanced.

While the procedures above mostly refer to Vascular Magnetic Resonance Imaging of the arteries, in the case of vein imaging the procedures are similar.

Vascular Magnetic Resonance Imaging Related Medication

Vascular Magnetic Resonance Imaging procedures often employ the use of contrast dyes in order to enhance the resulting images. The contrast agents are special substances which, when injected into the patient’s bloodstream, improve the visibility of the blood vessels, thus allowing the examining health care professionals to better observe the structure – and, where present, the anomalies – of the blood vessels. They are able to do so through modifying the relaxation times for the tissues they permeate, causing a change in the signal. Most of the contrast agents currently employed are compounds based on Gadolinium.

The Gadolinium doses employed vary depending on the particularities of each case; in some situations, for the imaging of large vessels and ramifications, doses as low as 0.1 mmol/kg may be used, while in the case of capillaries and other small vascular structures require higher doses. The gadolinium chelate molecule size is too large to penetrate the blood-brain barrier; this makes this agent the perfect contrast option for Vascular Magnetic Resonance Imaging procedures which aim at imaging brain lesions or tumors, where the contrast agent leaks out. After finishing its circulation period, the Gadolinium compounds will distribute in the space between cells (interstitial space) or will be excreted though the urination process.

Although Gadolinium is highly toxic in its natural state, the chelated compounds are safe. The compounds may be, in turn, linear or macro-cyclic, and ionic or non-ionic. Studies have revealed that the safest compound is a cyclical ionic Gadolinium chelated compound. In very rare cases, chelated Gadolinium usage during Vascular Magnetic Resonance Imaging may lead to the onset of nephrogenic systemic fibrosis. This is a severe affection which is characterized by the onset of fibrosis in multiple organs and tissues; it is considered that patients with impaired kidney functions are at highest risk of developing this affection.