Shannon P. Holmes, MSc, DVM
Magnetic resonance angiography (MRA) is an exciting area of imaging in physician medicine and has great potential in veterinary medicine. Compared to conventional angiography (CA) and computed tomography angiography (CTA), MRA is a completely non-invasive method. There is no ionizing radiation and contrast administration is not necessary for all studies. Current MRA applications in physician medicine include: diagnosis of aortic aneurysms, aortic dissection, congenital cardiovascular malformations, renal stenosis, vasculitis, atherosclerosis and thromboembolism. The number of MRA applications continues to grow with progressive development of MR technology, in the form of faster gradients, new pulse sequences and the use of contrastagents.
In routine MR images, specifically spin echo (SE) pulse sequences, blood can appear bright or dark. Its signal is determined by the velocity of the blood. Fast moving arterial blood will typically appear dark or even produce a signal void, whereas slow moving venous blood will have bright signal intensity. Movement in routine MR imaging is usually problematic and a source of numerous artifacts. With MRA, the movement of the spinning or resonating protons in blood is used to produce MR angiograms.
In the production of MR images, radiofrequency (RF) pulses are repeatedly applied to tissue to affect the resonance of its protons. The application of RF energy is said to saturate the protons so they emit signal in a form that can be acquired for image production. Black blood or blood void of signal results when the time interval between successive RF pulses (TR or repetition time) is long) allowing the blood to flow out of the image plane (i.e., slice). The effect can also be emphasized by using thin slices and a long echo time.In a fast spin-echo sequence, a long train of echoes is acquired byuse of a series of 180° RF pulses; as a result, washout effectsare even more substantial than with conventional spin-echo techniques. Flow-related enhancement is greatest when:
Blood velocity = slice thickness (mm) ÷ TR (msec).
MRA techniques most commonly use gradient-echo (GRE) pulse sequences because they are rapidly acquired. These pulse sequences produce images in which blood appears white or bright on a dark background. They are therefore referred to a bright or white blood sequences. Bright-bloodtechniques can be subcategorized into time-of-flight (TOF-MRA)and phase-contrast (PC-MRA) techniques. Different MR principles of proton movement are used with these two techniques. TOF-MRA utilizes magnitude effects, whereas phase shift effects are the foundation of PC-MRA.
Time of Flight MRA
In TOF-MRA, the inflow blood into a volume of tissue that has had its resonating protons saturated produces positive flow contrast. That is, the signal of stationary tissue is suppressed by therapid repeated application of RF pulses (Saturation Bands or SAT BAND) and will appear relatively black. The inflowing blood will appear bright, since its spins are not saturated.
The signal of TOF-MRA is best when the blood is flowing perpendicular to the orientation of the saturation band. This is desired to make the inflowing blood uniform and avoid a mixture of saturated and unsaturated blood. If a vessel is flowing parallel to the saturation band, the signal of a length of a blood vessel will be suppressed and thus blood flow will not be detected. This can be a limitation of this technique.
An advantage of the TOF-MRA is the ability to perform studies dedicated to certain parts of the vasculature. For example, it is possible to perform a TOF arteriogram or a TOF venogram. The difference in the acquisition is the position of the saturation band and the order that the volume is collected. For this reason, the saturation band is commonly referred to as a "marching" saturation band. For arterial studies, the saturation band marches toward the heart and away from the heart for venous studies. In the case of arteriograms, it may be necessary to trigger the acquisition toeliminate arterial pulsation artifacts; the time of acquisition is triggered by the R wave of the cardiac cycle. TOF venograms are superior to other MRA venograms, since this techniques best manages the slow velocity of venous blood flow.
Diagrammatic representation of TOF-MRA in a patient with a portosystemic vascular anomaly (PSVA).
Arterial blood (Aorta) is depicted in light gray and venous blood in intermediate gray that includes the caudal vena cava (CVC) and the anomalous communication between the portal vein and caudal vena cava (portosystemic vascular anomaly--PSVA). With the application of a RF pulse (SAT BAND) the downstream blood in the aorta and CVC has its signal suppressed (depicted in black).
Phase Contrast MRA
As discussed previously, the protons within the magnetic field resonate and groups of protons will be resonating in phase with each other. If a second magnetic field is applied in the form of a magnetic gradient RF pulse along a plane, the phases will differ depending the magnetic field experienced. The application to MRA is that the flow of blood along a magnetic field gradientcauses a shift in the phase of the MR signal. In PC-MRA, pairs of images are acquired that have different sensitivitiesto flow. These are then subtracted to cancel background signal,leaving only signal from flowing blood. The phase shift is proportional to the velocity of blood flow. Therefore, arteriograms and venograms can be produced through selection of specific velocity ranges in the planning of PC-MRA. This requires knowledge of expected blood velocities for different vessels, but can be challenging to plan in area of stenosis or dilation where there can be significant alteration in the velocity. Since the phase shift is equivalent to blood flow velocity, PC-MRA is capable of performing quantitative analyses of blood flow.
PC-MRA. As the magnetic gradient field strength increases, it causes a shift in the phases of the resonating protons along one plane. It is this shift that facilitates the spatial depiction of blood flow.
Contrast Enhanced MRA
Contrast enhanced MRA (CE-MRA) most commonly refers to administration of a gadolinium-based contrast agent during either a TOF-MRA or PC-MRA. Since the gadolinium contrast agents predominately remain in the blood pool, the will enhance the signal of the blood within vessels. In TOF imaging, it reduces the loss of signal in tortuous vessels that flow parallel to the saturation band and thus increases the sensitivity of this technique. A contrast enhanced TOF-MRA can be used to produce an arteriogram or venogram in the same manner as described above. With the use of contrast, the degree of suppression of blood flow is reduced because of the profound enhancement of all vessels. For example, in MR portography, which uses CE TOF-MRA, the descending aorta is often seen even though its signal should be suppressed. The trade-off is that there is superior visualization of the portal vasculature and tortuous vessels can be delineated in their entirety. The procedure for CE PC-MRA is identical to that described above and it would be performed for the same benefits as CE TOF-MRA.
The product of both MRA techniques are images of slices of body part images. These source images can be reviewed and individual vessels traced along their length. However, it is more common to reformat during postprocessing into either 2D or 3D reconstructions. Since blood is typically the brightest structure in MRA images, pixels within the image can be selected and then fused with the brightest pixels from adjacent slices. The process produces an image called a maximum-intensity projection (MIP).
2-D MIP of a CE-MRA of a canine cranial vasculature formatted in the transverse plane.
Benefits and Limitations
The major benefit is that MRA is a non-invasive technique and does not involve ionizing radiation. It can even be performed in the absence of catheter access, since intravenous contrast is not necessary as it is in CTA. The procedure is typically shorter than CA and provides high resolution 3-D reconstructions of anatomy. In human medicine, the cost of MR is less than CA.
Veterinary MR has no reported risks to the average patient. There is greater risk in veterinary MR associated with the anesthesia needed to perform the study. Like human MR, the strong magnetic field can disrupt the functioning of medical devices (such as pacemakers) and metallic implants will undergo heating. These will produce artifacts that can compromise the study (example: ameroid constrictors).
Note: Contrast administration is critical in veterinary MRA because there is less signal to acquire in dogs and cats, compared to the human body. In human medicine, there recently has been increased media coverage on a reaction to gadolinium contrast called Nephrogenic Systemic Fibrosis (NFS). The estimated incidence at this time is approximately 2% of patients administered contrast. A known risk factor is pre-existing renal disease. This syndrome has not been reported in any veterinary patients.
Whereas physician radiologists routinely interpret MRA of various parts of the body, veterinary MRA outside of MR portography is limited to descriptions of normal vascular MR anatomy and case reports of applications. In clinical veterinary practice is not routinely used. At this time, its application is limited by the availability of MR units and the knowledge required to appropriately perform these studies.
This represents the most thoroughly researched and applied area of veterinary MRA. Two prospective studies have been performed by Sequin et al. (1999) and Holmes et al. (in preparation) using MR portography in the diagnosis of portosystemic vascular anomalies (PSVA). Both studies used TOF-MRA, but the latter study implemented CE-MRA. The sensitivity and specificity approached 100% with experienced reviewers; this is similar to that reported with CTA of PSVA. Interpretation of MR portograms requires a strong knowledge of cross-sectional abdominal vascular anatomy. In addition to being superior to ultrasound and nuclear scintigraphy, the cross-sectional images provide surgeons a virtual "roadmap" of the anomalous vessel. This pre-surgical information has reduced intra-operative time in metabolically compromised patients and better pre-surgical planning. In the process of studying MR portography, numerous atypical PSVA were delineated that were not detected during abdominal ultrasound or were partially defined by ultrasound but were not surgically amenable. Concurrent MR of the brain in PSVA patients has revealed a previously unreported distribution of cerebral edema or cellular change that is presumably hepatic encephalopathy. With continued experience in MR portography, increased availability of MR units and its superior detection and delineation of PSVA, better management of PSVA patients can be achieved.
With experience in MRA, some institutes include these studies in neurologic and oncologic examinations. The application of MRA is typically performed to better define the vascular distribution in and surrounding a lesion. Examples of different clinical applications will be discussed in this session.
The utilization of MRA is expected to follow a similar path as that seen in human medicine. Knowledge of CTA and superior soft tissue contrast resolution popularized MRA and it, to a large extent, has replaced CTA. Since more CT units are available in veterinary medicine, CTA is currently a very popular diagnostic test. It is expected that there will be greater use of MRA in the future because of the desirable attributes and its ever increasing availability.
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