Use of 1024 MRA at 3T enhances neuro scans

During the two decades in which MR has been used clinically, progress has been more sporadic than steady. If one were to plot it out, MR advances would be represented by a series of steep climbs followed by plateaus, rather than by a slow, steady climb. The steep parts include the transition from resistive to superconducting magnets in the early 1980s, the transition from low-field superconducting magnets to 1.5T superconducting systems in the mid-1980s, the introduction of echo-planar systems in the mid-1990s, and the introduction of "cardiovascular systems" with even stronger, faster gradients in the late '90s.

It appears that we are now approaching another steep part of the curve, due to the synergy provided by the greater 3T field strength of the latest commercial MRI systems and to "parallel imaging," known more commonly as SENSE (Philips), ASSET (GE), and iPAT (Siemens). With apologies to GE and Siemens, this technique will henceforth be called SENSE, because Philips invented it. SENSE is a technique that trades signal-to-noise for speed. Just as S/N is reduced by the square root of 2 (40%) whenever the number of excitations is halved in a conventional spin-echo image, the same is true of SENSE: A SENSE factor of 2 halves the acquisition time and reduces the S/N by 40%. SENSE is therefore most useful when there is excess S/N, such as at 3T. It requires the use of phased-array coils, and the number of coil elements determines the theoretical limit on the SENSE factor. SENSE works by intentionally reducing the field-of-view and the number of phase-encoding steps, which in turn reduces the acquisition time. This would normally lead to wraparound artifact, or "aliasing," but because the local sensitivity of each coil in the phased array is known, the aliasing can be "unwrapped."

Phased-array coils also have the advantage of higher S/N than standard quadrature RF coils. The greater S/N of these coils and the higher field strength can be traded for speed or higher spatial resolution at the same acquisition time. This leads us to 1024 MR angiography.

In the circle of Willis, 1024 MRA has been a Holy Grail for neuroradiologists. Earlier MR systems either did not have the computer power to acquire a 1024 x 1024 image or didn't have the necessary S/N. Attempts to boost S/N with gadolinium were complicated by venous enhancement. As the acquisition time for the gradient-echo techniques used for 3D time-of-flight MRA is TR x Np x Ns, increasing the number of phase-encode steps (Np) to 1024 would either lead to an excessively long acquisition time or mandate a reduction in TR. At some point, a very low TR limits the time available for inflow of unsaturated blood and reduces flow-related enhancement, which is the basis for TOF MRA.

Increasing the matrix size from 512 to 1024 along one axis decreases S/N by 50% (assuming FOV is held constant). Halving pixel dimension along both the phase and frequency axes reduces S/N to 25% of what it was (holding acquisition time and slice thickness constant). Since the eye, like most organs, responds logarithmically to physiologic stimuli, a loss of 75% of the original S/N will be most noticeable at low S/N levels. The added S/N afforded by 3T magnets and the latest generation of eight-channel head coils allows this drop to go essentially unnoticed. With a rectangular FOV, a smaller number of phase steps can be used to achieve the same spatial resolution, reducing the acquisition time.

Figures 1, 2, and 3 show 1024 MRA of the circle of Willis. The images were acquired on a short-bore 3T GE TwinSpeed EXCITE with an eight-channel head coil, using a 1024 x 608 acquisition over a 16 x 12-cm FOV, yielding pixels of 160 x 200 microns. This compares with the typical 1024 digital subtraction angiogram, which has an in-plane spatial resolution of 250 microns, and a typical CT angiogram, which has spatial resolution on the order of 500 microns.

The slice thickness of the MRA is 0.8 mm (800 microns), which is zero interpolated to 400 microns. It was based on a spoiled GRASS sequence with a TR of 31 msec and TE of 6 (fractional echo). Although it took 14 minutes and 12 seconds to acquire, a SENSE factor of 2 would have brought the time down to seven minutes and six seconds.

Figure 1 is a full maximum intensity projection image of the data set in the transaxial plane. All the arteries of the circle of Willis appear larger than in lower resolution MRA. This is due to the fact that intravoxel dephasing has been minimized at the periphery of the vessel where the velocity changes (and the phase dispersion) are greatest.

This level of detail clearly will allow diagnosis of spasm from subarachnoid hemorrhage, vasculitis, and intracranial atherosclerosis, greatly extending the clinical applications of MRA in the brain. Although the spatial resolution of MRA can surpass that of DSA, detection of abnormalities is a function not only of spatial resolution but also of S/N. DSA also has the advantage of temporal information.

To use a technique like TRICKS to obtain temporal phases from an MR angiogram, contrast would have to be used and 1024 spatial resolution would probably need to be reduced to get clinically reasonable spatial resolution.

Still, the initial results are impressive. Figures 1 and 2 (MIP in the sagittal plane) show the ophthalmic and anterior choroidal arteries. Figure 3 is a MIP of this data that is set in the coronal plane. It shows the lenticulostriate arteries penetrating several centimeters into the basal ganglia. As parallel imaging and phased-array technology continue to advance, the results are only going to get better.

Dr. Bradley is a professor and chair of radiology at University of California, San Diego Healthcare in San Diego.