Imaging protocols plays an important role in both patient's management and research purposes. Major imaging systems in medical practice include: radiography, magnetic resonance imaging, nuclear medicine (Scintigraphy, PET and SPECT), photo acoustic imaging, breast thermography, tomography and ultrasound. Most of those imaging systems play a significant role in providing a form of non-invasive diagnostic and treatment protocols. This article reviews a type of magnetic resonance imaging known as magnetic resonance angiography (MRA). MRI is a radiological investigative technique for visualizing detailed internal structures. It allows for the production of detailed pictures of internal organs and tissues. Exposing the patient to radio waves in a magnetic field which allows for alignment of magnetization of some atoms in the body remains the basics of the technique. Such exposure allows those body atoms to rotate in the magnetic field produce by the MRI and then become detachable by the scanner. Reconstruction of the detectable changes is then done to give a detailed image of the internal structure.
History of MRA
The quest for an imaging tool to view and evaluate changes in arteries led to the development of MRA. The early attempts to use MRA started with the studying of coronary arteries using MRI pulse sequence which was applied to thoracic region and were able to shows some parts of the coronary arteries (Duerinckx). It was cited in the article by Duerinckx that Lieberman et al (1984) where able to use this technique to explore coronary arteries of some patients. Years later, Pauline et al (1987) also reported the use of the same technique to view coronary arteries of four out of six patients making the procedure yet unreliable (Duerinckx). Later, techniques based on 3-D contrast were developed and that improved the earlier attempts to evaluate coronary arteries. First generation coronary MRA techniques are those techniques involving the use of EKG-triggered k-space segmented pulse which is a 2-D analysis. The major challenges with the process was that there is increased acquisition time which relates to time for the need to acquire many 2-D images and also lack of 3-D image collection. It also requires significant expertise and familiarity with the anatomy of the coronary vessels.
Advancement based on the initial use allows the development of the 3-D time-of-flight sequences. This allow for a good signal to noise (SNR) and also contrast to noise (CNR) ratios (Wink et al, 2002). There have been several applications of the 3-D MRA and this has serve as the basis for the advancement of the technology.
Modern use
The uses of Magnetic resonance angiography in modern imaging techniques are diverse. This broad field of application will be highlighted under sub-heading based on the regions of the body.
Head and Neck Region
Head and neck region contain several vessels and anastomoses. There several forms of injuries and diseases associated (Aetna, 2011). A need for a non-invasive imaging method makes MRA a medical imaging of choice for the purpose. The important modern uses of the imaging techniques in this region are as follows:
1. MRA serves as a follow up tool in patients with arterio-venous malformations and those who are known to have ruptured intracranial aneurysm which was found to be of size greater than 3mm.
2. In cases where there is a need for the managing health care professionals to establish the presence of stenosis or other anatomical abnormalities associated with the vertebrobasilar systems. This will be done in patients that possess symptoms that are suggestive of such anatomical abnormalities (Aetna, 2011). Such symptoms include the binocular vision loss, diplopia, dysphagia and vertigo.
3. In cases of patient with signs and symptoms that are suggestive of leaking or ruptured arterio-venous malformations or intracranial aneurysms.
4. In cases of need to evaluate the pulsatile tinnitus in patients with signs and symptoms that are suggestive of various vascular lesions (Aetna, 2011 & Edward et al, 2002).
5. It can also be applied to rule out lesions such aneurysm that could involve the arterial circle of Willis. This is done in individuals with higher risks ICA.
6. It can also be used to evaluate the following conditions affecting the carotid arteries (Aetna, 2011):
a. Aneurysm tumour
b. Injuries to the artery or branches
c. Stenosis or other associated occlusive disorders
d. In cases of cervicocranial arterial dissection
Chest region
Researches and advancement in MRA have provided individuals with any chest related arterial lesions opportunity to undergo such imaging procedure for diagnosis, treatment planning and post-operative follow ups.
1. Conditions associated with thoracic aorta. These include; aneurysm, dissection and stenosis
2. Diagnosis and treatment planning in individuals with history suggestive of congenital heart defects or developmental anomalies
a. Such anomalies include atresia, hypoplasia, coarctation, double aortic arch, interrupted inferior vena cava
3. Diagnosis of cases of suspected pulmonary embolism or substitute for ventilation/perfusion scan
Spine
MRA can also be used in cases of spinal cord arterio-venous fistula or malformations. This is yet to be fully implemented because it’s still considered to be in experimental state (Aetna, 2011).
Abdomen
MRA play an important role in the following abdominal conditions (Aetna, 2011):
1. Renal arteries assessment for stenosis especially in patient with hypertension
2. Pelvic arteries especial the aorto-iliac arteries for stenosis in patients with suspected cases
3. Determination of extent of abdominal aortic aneurysm or occlusive condition
4. Chronic mesenteric ischemia condition
Lower limbs
1. Initial test for diagnosis and treatment planning in cases of peripheral arterial disease involving the lower limbs
Other important areas that MRA can be applied include in the process of velocity mapping for evaluation of accessory renal arteries in prospective donors and screening process for intracranial aneurysm or reno-vascular hypertension (Aetna, 2011).
Limitations
The technique do not provide an option in the case of need for immediate intervention
Lower spatial resolution as a technical limitation hence there is high chances of occurrence of motion artefacts which can lead to either false negative or false positive results.
In a situation where the patient had issues with following or maintaining breathing demands of the techniques, there is a likelihood of development of artefacts. Involuntary motion of the diaphragm and arrhythmias can also lead to development of artefacts.
Shortage of the MRA machines at the community imaging centres
False negatives within the vessels as a result of calcifications
Risks of radiation exposure however, recent improvements are making these risks to be reduced and the technique to be available for children.
Quality of images produced from some arteries are less when compared to conventional catheter bases angiography
Evaluating smaller vessels may be very difficult or separating the images of arteries from vein, may also be difficult.
Patients with difficulty in maintaining a lying state with their backs may find it difficult to undergo the tests.
Obese patients may actually not fit into the conventional MRI machine for the angiography.
Presence of implants may impaired the ability to retrieve clearer pictures
Acquisition
Image acquisition with magnetic resonance angiography has developed over time to produce 3-D images with good signal-to-noise ratios and contrast-to-noise ratios. The techniques actually do no rely on acquisition alone but also on post processing activities that will improve the quality of the images especially in the case of coronaries arteries with tortuous nature. Hence, to achieve a good image in such condition, there is a need for post processing. Image acquisition is done with 1.5 Tesla Philip Gyroscanner. This scanner use a navigator gated and corrected ECG triggered ultra-fast 3D gradient which depend on echo-echo planar imaging techniques.
Pulse sequences
Pulse sequence entails series of events which are initiated by computer and comprise of RF pulses, gradient waveforms and data acquisition. The purpose of this sequence is to effectively manage or manipulate the magnetization of the MRI to produce desired signal for the imaging technique. The application of the pulse sequence in MRA remains the core of the process and it has remain an important field for advancement and research. Pulse sequences in MRI are classified into: basic pulse sequences, angiographic pulse sequences, fast imaging pulse sequences utilizing echo trains and pulse sequences for advanced applications (Bernstein, King, & Xiao Hong). Common MRA techniques include: black blood angiography, phase contrast angiography, time-of-flight ad contrast enhanced angiography (Bernstein, King, & Xiaohong). It can also be classified as; Spin echo (SE), Fast spin echo (FSE), inversion recovery (STIR, FLAIR) and Gradient reading echo (GRE).
Spin echo
Spin echo sequence remain the basic and simplest type of sequence initiated in MRI. The major consideration in SE is the parameters which are the TR and the TE. TR represents the duration between two 90 degree excitation pulses and it is usually about 100 to 3000ms in a regular spin echo sequences. The implication of this component is that it allows for longitudinal magnetization to recover. This recovery is termed T1 relaxation. It should be known that the longer the TR, the more time it takes for completing the longitudinal magnetization recovery. Application of the TR is that if it is of short duration, not all the tissues would have undergone complete relaxation. All these considerations and changes make the process of image acquisition dependent on the process of relaxation. TE represent the duration between 90 degree excitation pulse process and echo signal acquisition. Spin echo sequence is a reference in terms of tissue contrast.
Major advantages of the SE sequences are the ability for anatomic imaging and references for tissue signal and image contrast works. The major demerits are related to the acquisition time which is very long. This type of sequence is used to explore almost all organs that MRI is used for. Spin echo images indicate blood vessels with black appearance. There can also be presence of void in cases of absence signal.
Fast spin echo
Normal regular spin echo which is characterized by many repetitions because there are lines in the k-space indicated to complete a form of slice image acquisition. In the case of fast spin echo, K-space lines are acquired for the same slices which is opposite in the case of echo sequence. FSE use single 90 degree excitation pulse with more than one 180 degrees pulse. This is done in the same repetition time with different phase encoding gradient steps in order for acquisition of multiple echoes to fill the k-space. The clinical application is that which is related to exploration of organs viewed in MRI. In terms of pros: there is reduced scan time and low sensitivity to artefacts. The related cons: is that of modification to tissue contrast.
Inversion recovery
This is another sequence protocols that allows for contrast manipulations. It provide for application of 180 degree inversion pulse at the onset of the sequence. This pulse then act to change the direction of the longitudinal magnetization. T1 relaxation is the time T1 for a regular spin echo is performed. T1 indicates that the longitudinal magnetization of a chose tissue is null. This sequence provides imaging which relates to building of tissues with magnetization and those with none. Those tissues with magnetization in this sequence tend to appear in the dark while those without it appear in grey or bright colour. Problem is increased scan time while merit is fat signal suppression. FLAIR is used in neuroradiology while STIR is for musculoskeletal system. Artefacts issues have led to the development of techniques that will create minimal artefacts. This is a form of inversion technique that was implemented to reduce residual artefacts associated with respiratory motion. Advancement in the pulse sequences and the hardware components of the techniques has allowed for the acquisition of 3-D images in a single breath holding phase. "This type of image acquisition gives room for extended coverage of anatomical structures and improved SNR and sophisticated k-space sampling schemes and isotropic spatial resolution" (Dekker, 2003).
Gradient echo
This is different from the sequence technique as a result of the use of excitation pulse with a flip angle which is lower than 90 degrees while there also no 180 degrees rephrasing pulse. The importance of those lowered flip angle is that it decreases the amount of magnetization that are tipped into transverse plane. The merit of this is that there is an increased and faster recovery of longitudinal magnetization hence shorter scan time. It allows for better or new contrast between tissues. It allows for creation of bright blood and there is usually no washout effect. Demerits are that it's more sensitive to magnetic susceptibility artefacts. Clinical application is associated with haemorrhage and calcification conditions.
Contrast enhanced MRA
This is a form of MRA techniques that make use of Gadolinium chelate agents. Their use has been associated with their pragmatic nature. Those agents cause shortening of the T1 relaxation of blood when compared with the underlying tissue. The differences between this mode of MRA to other modes such as the time-of-flight and phase contrast are that the signal of the blood in the technique is based on the intrinsic T1 signal of blood and not on the flow effects. This makes the technique less flow sensitive or dependent. Breath holding can be performed for the use in the chest and abdomen while post processing of images with maximum and appealing output is also achievable with the technique.
Time-of-flight
This technique make use of flow unlike the contrast enhanced MRA. Protons movement are detected within the imaging plane. That detection or image acquisition is achievable by involving saturation of the signal in the slice with rapid RF pulses (Bart et al, 2007). The idea is that the backgroud tissues will be suppressed by the RF pulse while the fresh moving blood will be retained when it enters the slice. The technique is used for both arterial and venous imaging.
Phase Contrast MRA
Phase contrast MRA make use of blood flow however, it’s not limited by the in-plane flow which is commonly seen in Time-of-flight cases. It makes use of bipolar flow encoding gradient which is a magnetic field related gradient and characterised by reversal of directions at the midpoint. The gradient usually induce velocity dependent phase shift in moving situations. The important aspect of the technique is that two images are usually acquired and those images are subtracted from one another to amplify the signal of vascular tissue flow. The time of flow and the magnitude determine the maximum velocity. This technique is indicated in the cardiac and thoracic imaging especially when there is a need to identify the flow and velocity information (Bart et al, 2007).
Fresh Blood Imaging
Fresh blood imaging is a form of MRA technique that does not require the use of those exogenous contrast media that are used in most techniques. The principle of use is based on FASE 3D ECG gated sequence. The technique usually produces two different sets of 3-D data. The sets of data are produced separately, one in systole and other in diastole. FBI also suppresses signals from other anatomical region to produce bright blood angiograms. The benefits of the technique are that it tends to allow for large anatomical coverage because data can be collected in both planes (sagittal and coronal).
4D- Dynamic MRA
It is a recent development in the MRA imaging technology. This was achieved by combining ASL with segmented multiphase TrueFISP readout. The benefit of this 4-D is that it will provide great resolution and it can be used to delineate dynamic blood flow in arteriovenous malformations. The application is yet to be fully adapted to MRA.
Benefits
No exposure to ionizing radiation, non-invasive mode of imaging, shorter period when compared to traditional catheter angiogram, less costly compare to the use of catheter angiography, and provision of high quality images of many vessels without using any contrast materials
Risks
It usually poses no risks to patients once the appropriate guidelines are followed. Sedation that is used before investigation may pose a risk when is too much, strong magnetic field may affect patients with any implants, and allergic reactions can be experienced when contrast materials are used.
An important future use is in the areas of stroke patient screening. This is because Boggs (2010) recently reported the use of MRA in a stroke patient that does not respond to Altepase. He added that the imaging technique could actually reveal all those areas with recanalization and their rates of doing so (Boggs). Further investigation is on-going to appreciate the use in stroke patient management.
MRA is an imaging system that produces remarkably good images of internal vessels. The imaging procedure can be used to evaluate diseased vessel. Although the use in angiography is yet to fully take control but in the nearest future MRA use will undoubtedly increase and take over the process.
References
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