Magnetic Resonance Angiography Techniques:
The principle of magnetic resonance angiography (MRA) is to acquire images where the signal returned from flowing nuclei is high, and the signal from stationary nuclei is low. In this way, contrast between vessels and background tissue is achieved. There are several techniques available to obtain this contrast. Black blood imaging combines SE or FSE sequences with spatial pre-saturation pulses to produce images in which flowing vessels appear black. High signal seen in this type of sequence may indicate stenosis or occlusion of the vessel. Bright-blood imaging combines GRE sequences with GMN to produce images where flowing vessels are bright. A signal void seen in this type of sequence may indicate either a stenosis or occlusion of the vessel.
There are additional techniques designed especially for angiography. Both allow for data acquisition in either sequential (2D) or volume (3D) mode. Each has its own advantages and disadvantages and therefore each is used for different purposes. The two types of MRA are summarized below. These are time of flight (TOF) and phase contrast (PC).
2D Time-of-Flight:
For this technique, multiple contiguous thin-section (1.5 mm) GRE images are acquired, and the images are stacked to make a volume set. The 2D GRE images can be viewed individually, or the volume set can be summed to give a "collapse" image. The collapse image is most helpful for displaying thin-volume sets through the circle of Willis and for selecting regions rest for segmented reconstructions. For the MR angiogram, the volume data set is subjected to a maximum intensity projection (MIP) ray tracing algorithm to display the vessels from multiple angled views 180 degrees around an axis perpendicular to the original acquisition plane. In general, an imaging plane is selected perpendicular to the direction of flow within the vessels to maximize the inflow and the MIP angiograms are viewed from frontal to lateral projections (usually 18 projection angiograms at 10 degree increments). Adjacent angled projections can be viewed as stereo pairs, or the images can be displayed in a cine mode to give a visual 3-dimensional appearance.
Since the slices are acquired one at a time, for arterial imaging, the TR can be quire short (25-30 msec) and the flip angle quite large (45-60 degrees) without noticeable signal loss from saturation effects, except for in-plane flow. With one signal average and a 128 x 256 matrix, the imaging times range from 5 to 7 minutes, depending on the number of slices.
When the vessels are perpendicular to the acquisition plane, the arteries or veins can be selected by prescribing presaturation (SAT) rf pulses. For example, for imaging the carotid arteries in the neck, a SAT band is placed superior to the imaging plane to saturate venous flow from above. The SAT band moves with each successive axial slice to stay a few millimeters above the slice being acquired. To image the jugular veins in the neck, the SAT band would be placed inferior to the image plane.
All MRA sequences suffer signal loss due to intravoxel spin-phase dispersion or phase incoherence. These phase shift effects occur when protons within a l accumulate varying phase from magnetic field inhomogeneity, susceptibility effects, a range of velocities within a voxel, acceleration, or turbulence. Signal loss from intravoxel phase dispersion can be minimized by using the smallest voxel size, minimum TE (8-9 mm for 2D TOF), and 1st order flow compensation.
3D Time-of-Flight:
This method is similar to a 2D TOF except that the data is collected as a 3D volume set rather than individual slies. The data is processed by the MIP technique to obtain the final angiographic imageThe 3D acquisition alllows for thinner slices (0.7 mm vs 1.5 mm), and therefore a smaller voxel size, which along with a shorter TE (<5 msec) results in less intravoxel dephasing. Compared to the 2D method, 3D TOF provides higher signal-to-noise and shorter imaging times. On the other hand, volume methods are more susceptible to saturation effects because unsaturated spins from outside the volume may have to travel a few centimeters into the imaging volume to produce enhancement. Saturation effects can be minimized in 3D TOF MRA by using the thinnest volume to encompass the vessels of interest, lower flip angles (15-20 degrees), and longer TRs (TR of 40 msec is standard). Rather than collecting the entire volume at once, a limited number of overlapping slabs can be acquired sequentially to reduce saturation effects, but this technique increases imaging time and reconstruction time. Gd-enhancement can be used to shorten the T1 relaxation time of the blood and reduce saturation through the imaging volume, but it also increases the signal from stationary tissues. Magnetization transfer is a novel method that improves vascular contrast by suppressing background tissues. 3D TOF works well fo high flow arterial systems such as the the circle of Willis Saturation effects limit its utility for imaging the venous side of the circulation.2D Phase Contrast:
Phase Contrast (PC) methods use an entirely different technique to generate vascular contrast. Following the initial 90 degrees rf pulse, bipolar phase-encoding gradients are applied separately along the three axes to impart phase shifts to moving protons. Protons in stationary tissues acquire no net phase change with the bipolar gradient pulses, but flowing protons within vessels accumulate phase as they move through the gradient fields. For a second excitation, the polarity of the bipolar gradient is inverted. A vector subtraction technique essentially elimiates background signal, yielding high contrast angiograms with PC methods.
Another important parameter in PC is the velocity encoding (VENC) factor, which can be set to select out arteries or veins. Higher VENC factors (60-80 cm/sec) will selectively image the arteries, whereas a VENC factor of 20 cm/sec will highlight the veins and sinuses.
2D PC collects and displays data as a series of thick slices or a single slab. The data si not processed by the MIP algorithm but rather viewed as a single projection in the plane of acquisition, similar to a collapse image. One major benefit of this MRA sequence is that the phase images can be displayed to show direction of flow. This information may be useful for assessing collateral flow about the circle of Willis in cases of carotid or vertebrobasilar occlusive disease or for showing direction of flow to and from AVMs. Potentially more important, PC sequences can quantify velocities within a vessel. If the cross-sectional area is measured, flow can be calculated. Flow data is valuable for assessing occlusive vascular disease and likely will have a role in measuring blood flow to AVMs before and following partial resection, embolization or radiation therapy.
Since PC methods are less susceptible to saturation effects, very short TRs can be used (<25 msec). With 2D PC, imaging time can be reduced to 3 minutes or less, or the NEX can be increased to improve signal-to-noise. PC methods are equally sensitive to intravoxel dephasing as TOF techniques. As mentioned above, signal loss can be minimized by reducing voxel size and using flow compensation and minimum TE.
