MRA (Magnetic resonance angiography) is a collection of MRI techniques that are used in the imaging of blood vessels. The technique is mainly used in the generation of images of arteries to evaluate them for occlusion, occlusion, stenosis (narrowing of blood vessels), aneurysms (this is the dilation of blood vessels), and other anomalies of blood vessels and the heart. The technique is often used in the evaluation of the blood vessels particularly arteries of the brain, the neck, the abdominal and thoracic aorta, the blood vessel of the legs and the renal arteries. This aids in the detection, diagnosis, and management of stroke, heart disorders and disorders of blood vessels. In this imaging technique, a dominant radio frequency and a magnetic field help in the generation of computer images of major blood vessels of the body. It is important to point out the fact that MR angiography does not use X-rays (ionizing radiation); instead, it utilizes electromagnetic energy. These are strong electromagnetic radio waves that are measured and analyzed using a computer that then forms either a two dimensional or three-dimensional image of the focus blood vessels that can then be viewed on a television monitor (Schaverien 163).  The use of these techniques has revolutionized cross-sectional imaging by aiding in the determination of inflow and outflow arteries and the evaluation of lesions along these vessels in terms of their numbers, their length, their morphology and diameter and the status of distant runoff vessels. They thus help in the stratification of patients into those who will benefit from surgical, medical or endovascular treatment.

MRA is commonly used in the examination of blood vessels in critical regions of the human body. These include the neck, the heart, the abdomen, the chest, the pelvis, arms and hand, and the legs and feet. The overall objective of this test is to identify pathology of the blood vessels such as aneurysms, arteriovenous malformations, and in the therapeutic assessment and management of these conditions. MRA attempts to answer the question, ‘Where is the site of the lesion?’ to allow doctors to make the right decision regarding the management of the patient (Gupta, 10).

MR angiography is capable of providing detailed information without any use of contrasts, however, today, particular forms of distinctions such as GD-DTPA (gadolinium diethylenetriamine Penta-acetic acid) are given to the patient to make the images even clearer (Reichenbach 985). Contrast-enhanced MR angiography is a painless procedure, and the magnetic field generated during the study does not cause tissue damage of any kind. However, sometimes, a patient may experience some mild discomfort due to the insertion if an intravenous line. Unfortunately, in patients with renal disease, the use of gadolinium-based contrast enhancing agent can be dangerous as it can result in renal failure. This to avoid these complications, the non-contrast-enhanced magnetic resonance method is highly recommended though research in the area is still in process. Notable recommendations include the 4D dynamic magnetic resonance imaging, and the gated subtraction fast spin- echo. The gated 3D-FSE MRA is particularly suitable when carrying out peripheral angiography. Is core feature is a T2-weighted 3D-fast spin–echo that is accelerated by the use of a partial Fournier acquisition (Schulz 1663). Their use in peripheral angiography is however limited in highly stenosis lesson or areas with the slow flow since the generated pulse waves are dumped. Thus the difference between diastolic and systolic signals reduces resulting in the generation of poor quality images.

Over the last decade, the use of contrasts enhancing MRA has been gaining momentum and has become the imaging modality of choice when it comes to the evaluation of the cardiovascular system of the body due to its intrinsic high spatial resolution, high signal to noise and freedom that is relative form flow-related artifacts. Usually, radiologists have only one single opportunity to get it right just like in a CT angiogram.

Principles of Non-contrast and Contrast MR Angiography

It’s important to understand the physics underlying Contrast-enhanced MR angiography.  Currently, they are classified into two broad groups, that is, flow independent methods and flow dependent methods. The principle underlying the flow-dependent method is that blood contained in blood vessels s continuously flowing while other tissues remain static; in this way, images of the blood vessels can be generated. This category of MRI can be further classified into phase –contrast MRA and time of flight MRA. The phased contrast MRA works by making use of phase differences to differentiate static tissues form blood vessels. On the other hand, TOF (time of flight MRI) also referred to as inflow angiography utilizes time flow compensation and echo-time to make blood that is flowing to appear brighter than surrounding static tissue. It thus exploits the fact that moving blood experiences limited excitation pulses which caused them to be less saturated than the surrounding tissues hence giving a much higher signal. This method category is heavily reliant on the flow of blood; the image quality is poor in areas of low tide like large aneurysms and where the flow is in the plane of the image (Wintermark 110). However, the technique is heavily used in imaging of the head and neck since it’s the method most commonly used in the angiographic evaluation of ischemic stroke patients.

The flow independent do not rely on contrasts or the flow of blood to generate their mages. However, they instead rely on the differences of T1, T2 in addition to the chemical shifts of the voxel in the different tissues of the body. This technique has a clear advantage to the flow-dependent method since it can take images of the body with slow flow due to disease of blood vessels more easily. Besides, there is no need to administer contrasts agents which have been linked to the development of nephrogenic fibrosis in chronic renal disease patients (Saito 2).

Contrast-enhanced MR angiography

This excellent imaging technique utilizes intravenous contrasts agents to generate clear images. The contrast driver injected into the patient’s vein via an intravenous cannula at the antecubital vein or via a central catheter. Two series of images are typically taken, the first one being in the pre-contrast period, just before administration of the agent and the second series during the 1st time the agent flows through the arteries. By contrasting the two sets of acquired images in the posts- processing phase, an image of the blood vessels is obtained which in principle show only the target vessels and leaves the surrounding tissues. The image is usually of high quality.  Blood pool contrast agent such as polymeric gadolinium complexes and albumin-binding gadolinium complexes are sometimes used. These agents remain in blood circulation for over an hour, unlike traditional agents that leave within a few minutes. Their only limitation is that even veins will be enhanced when these agents are used primarily when images of high resolution are required (McDonald 773).

The current emerging technology in Contrast-enhanced MR angiography involves the use of subtraction less Contrast-enhanced MRA.  The emergence of these techniques has made it possible to generate high-quality contrast-enhanced magnetic resonance images without the need to subtract mask images taken in a non-contrast enhanced environment. The approached has revolutionized magnetic resonance imaging since it eliminates motion subtraction artifacts in addition to the images background noise thus raising the diagnostic quality of these images. However, the technology only works when body fat suppression over a large area is done using the mDIXION (Multiecho 2-point Dixon). The methods make use of body water when taking MRI scans, thus eliminating the need to suppress body fat. The use of water images removed the need to subtract mask images before generating quality MR angiograms. However, it is important to point out the fact that the gold standard for the diagnosis of CVA (cerebral vascular disease) has remained to be digital subtracted angiography.  Besides, it provided guidance inter-procedurally. However, it involves extensive use of iodinated contrast agent and x- rays which have side effects (Attali 82).

According to Caschera, (65) it is important to point out that contrast-enhanced MRA and contrast-enhanced CT angiography are almost similar except that it makes use of gadolinium-based regimen instead of Iodinated contrast compounds. In CE-MRA, gadolinium makes the images brighter by shortening the T of blood vessels similar to how Iodine attenuates x-rays of blood vessels in CT angiogram. The injection of contrast media is performed either by hand or by the use of powered injectors remotely controlled by the scanner (Shu 120). The contrast media progressively gets diluted as it floes via the lungs and the heart.

The 3-dimensional imaging

The approach currently recommended when performing magnetic resonance imaging is the 3D approach. However, the 2D approach is still used in some regions of the world.  The three D imaging allows the radiologists to acquire images from various angles. Forts, 2d images from different slices areas generated before getting combined to create the three 3 D images (Schubert 80). It is important to note that 3D data is not only used to calculate projections but also the creation of cross-sectional images. The three D imaging is particularly useful in the management of complex vessel geometries in which blood vessels flow in all directions from a central area like the heart and the circle of Willis.  In a study done by Zhang et al., 2016 showed that 3D magnetic resonance angiography has a high degree of accuracy in the visualization of vessel lesions. Additionally, it provided real-time imaging during the performance of endovascular procedures (Fushimi 230).

Modalities of brain imaging

The brain is a vital organ in the human body. Alteration in blood flow to these organ has detrimental effects on the overall health of the affected patients. Thus, it usually is critical to ensure that imaging will provide quality images that will aid in the management of affected patients. In the past century, there has been a technological breakthrough in barn imaging which has allowed scientist to study the brain in detail and understand the relationship between its different areas (Sarraf 10). Past civilization lacked an effective means to obtain knowledge about the nervous system though past about the brain was discovered in Edwin Smith papyrus in ancient Egypt. William Harvey later proved the theory of blood circulation in the de Motu Cordis in the 1600s. In Thomas Willis in 1664 published text on Cerebri anatomy which proved to be groundbreaking research in neuroscience. This remained influential for brain studies and anatomy for the next two centuries (Pillai 8).  In 1882, the first neuroimaging technique was invented by Angelo Masso. It was a noninvasive measure and was included in the human circulated balance. The distribution of blood in the human ran during intellectual and emotional activity. The discovery of X-rays in 1895 by Roentgen brought about the origin of structural brain imaging. A few years later, lindenetahal and Haschek were able to produce radiographs of blood vessels by injecting radio-opaque solution in cadavers. However, the fast cerebral angiography was performed in 1927 by Egas Moniz. Since then technological advancement in clinical imaging, computing and, mathematics  have resulted in the development of the following techniques; positron emission tomography (PET scan), ,computed tomography  by Carmack & Hounsesfeld (awarded  1979 Nobel prize), the magnetic resonance imaging for which Mansfeld and lauteurbur received 2003 Nobel prize and the magnetic resonance imaging. These techniques in addition to functional magnetic resonance imaging, EEG, photo-acoustic imaging, transcranial Doppler ultrasound, and digital subtraction angiography have e contributed immensely in the promotion & improvement of our understanding and knowledge of the complexity of the CNS (Laviña 70).

The narrowing of the blood vessels of the brain is of critical importance due to the high risk it places on an individual for developing cerebral vascular accident.  The narrowing of these blood vessels can eventually result in a transient ischemic attack or an overt stroke. It is thus critical today to have an imaging modality that will aid is an accurate diagnosis of all the affected vessel especially in critical regions of the brain like the circle of Willis.  Currently, the most commonly used modalities include CT angiograms, Trans-cranial ultrasounds and magnetic resonance imaging (Laviña 70).

CT of the brain

Computed tomography has been used in the past to study the brain extensively. It is a technology that came much earlier than MRI and just like MRI, CT is noninvasive and painless. Later, the technology was boosted with the emergence of CT angiograms which growth broadened the diagnostic significance of CT technology in studying the brain vasculature.  The use of color-coded CT angiography provided additional information of the brain hemodynamics, particularly the retrograde and anterograde flows. Newer technology in this area includes Nano CT and micro CT. However, CT angiogram still has a lower resolution when it comes to the detection of a vaso-occlusive condition in the brain. The technology is however for more common in developing nations due to its relatively lower cost (Laviña 70).

Trans-cranial Doppler ultrasound

This is a technology that was developed by Rune Asalid in 1982. It aids in the detection of blood flow in the brain’s basal intra-cerebral arteries. The technique is advantageous in the sense that it is noninvasive while enabling users to get images of the primary vessel of the brain via the skull while at the same time monitoring the velocity of cerebral blood flow and the pulsatility of the vessels under consideration. The technology, however, has limited used I the detection and diagnosis of stenotic occlusive pathology of the cerebral vasculature (Errico 752).

Digital subtraction angiography

This is a technology that permits the visualization of blood vessels in both skeletal and soft tissues.  The technology was introduced in 1980 and has slowly progressed from 2D imaging, 3D imaging and finally 4D imaging. The technology allows visualization of the of intracranial cerebral vascular structures and vascular abnormalities like aneurysms, arteriovenous malformations, carotid stenosis, collateral circulation in occlusion of middle cerebral occlusion as well as Moyamoya disease grading. However, the images are of poor quality and may lead to diagnostic errors as a result of significant lag time brought about by subtraction (Demené 472).

Intracranial vessel wall imaging

This is a new technology that allows direct inspection of the vessel wall. It provides a useful diagnostic tool that may be used in the improvement of patients with vessel wall pathology. However, its use is limited by the tortuosity and small diameter of the cerebral vasculature. According to Alexander (589), this limits its use in the assessment of stenotic occlusive lesions, atherosclerotic lesions, CNS vasculitis, and arteriovenous malformations.

Magnetic resonance imaging

The advantages that MRI has over CT scan has always been the fact that it based on magnetic forces rather than ionizing radiation that is potentially harmful to generate its images. Its images are also of higher quality. The images are of two major types, that is, the T1 weighted images and T2 weighted images. Advanced in this technology have provided useful biomarkers that are used as either predictive tools, prognostic tools or diagnostic tools.  MRI is employed in the hemodynamic studies of suffering from cerebral vascular disease. Advances in the technology such as 4D flow MRI, MBTI and 2D phase contrast MRI have the potential of increasing its diagnostic significance in cerebral vascular pathology. However, due to its inherent limitations, this is still not suitable for the diagnosis and management of cerebral stenotic occlusive lesions (Haider, 749).

MRA of the brain

Magnetic resonance imaging is currently the gold standard when it comes to imaging of brain vascular pathology. This is used particularly in the diagnosis and management of stenotic occlusive brain lesions.  As discussed in the introductory paragraphs, MRA can be either contrast-enhanced or non-contrast enhanced. The contrast-enhanced technique was 1st introduced in the market in 19994 and has gradually gained widespread acceptance. MRA is thus the most accurate diagnostic method currently in existence for the visualization of cerebrovascular territories and to evaluate aneurysms, occlusion, stenosis and other cerebral malformations like arteriovenous malformations (Campeau, 233).  The hardware and software that are used in MRA are undergone significant improvement n the past decade especially with the introduction of 3 T. Other improvements that have been made in this technology included those of the incorporation of parallel imaging techniques, the K-space sampling schemes, parameter optimization, advances in radio frequently transmission and new gadolinium-based agents (Maksimovich 12). These new technologies facilitate the generation of high-quality images using MRA 3 T.

MRA angiography is better suited at replacing another vascular imaging modalities for the assessment of stenotic occlusive lesions in the human brain.  A study done in 2007 by Chol, p439, shows that 3D TOF-MRA had a high sensitivity, specificity, and positive predictive value for the diagnosis of vaso-occlusive brain lesions. The study proved that MRA is a reliable diagnostic tool for the detection of clinically significant major intracranial arteries vaso-occlusive disease. For optimal therapeutic injuries, accurate assessment of these patients is critical. However, the efficient of 3D TOF-MRA was found to be restricted by factors such as phase dispersion or loss of flow signal as a result of saturation, limitation of scan range or spatial resolution, and high susceptibility to artifacts (Scarabino 47).

The following table illustrates 3D TOF-MRA imaging

Conclusion

Standardized contrast-enhanced MRA should be taken as the gold standard for the diagnosis and management of vaso-occlusive lesions of the brain. Vaso-occlusive conditions of the brain particularly those secondary to emboli and atherosclerotic plaque lesions have increasingly become common in the population. This illustrated by the rising number of stroke patients in the society for instance in Algeria, the president suffered a stroke which caused him to be bedridden (Hara 622). Contrasts enhanced angiography should thus be adopted as a standardized screening methodology for vaso-occlusive lesions of the brain territory particularly for at-risk patients based on the assessment of the physician. This will aid in the early detection of potentially lethal lesions to facilitate early endovascular therapy depending on the need of the affected patients. Moreover, all patients with transient ischemic attack must undergo the test.  Contract enhanced angiography is the most recommended method out of all brain imaging modality due to its high sensitivity and specificity as a result of its higher quality images when compared to the rest (Runge 551).