MRI looks set for decade of growing clinical use

May 1, 2007

MRI will continue to evolve and will remain the dominant imaging technology in medicine, according to a leading world expert on the modality.

MRI will continue to evolve and will remain the dominant imaging technology in medicine, according to a leading world expert on the modality.

"To get an idea of where MRI will be in the next 10 years, look at the last 10 years," Dr. William G. Bradley, chair of radiology at the University of California, San Diego, told delegates at the 11th Asian Oceanian Congress of Radiology in Hong Kong. "We can expect increasing applications as a result of higher fields; faster, stronger gradients; and multichannel radiofrequency subsystems to take advantage of techniques like SENSE."

PET/CT is currently the rage, but he thinks PET/MR systems may become available in the future. Research techniques such as diffusion tensor and functional MRI will become commonplace in clinical practice. To complement fMRI, MR will almost certainly be linked up in some way with magnetoencephalography to produce magnetic source images, although the linkage is more likely to be through image fusion than combined units.

Over the past three decades, MR advances have been characterized by a series of steep climbs followed by plateaus, Bradley said. Low- to midfield systems dominated the clinical arena from the late 1970s to the mid-1980s, while 1.5T with 10 mT/m gradients was dominant from the mid-1980s to the mid-1990s. From the mid-1990s to the early 2000s, 1.5T with echo-planar gradients emerged, and now interest in 3T with echo-planar gradients is growing.

A major breakthrough is parallel imaging, which is also known as SENSE (Philips Medical Systems), ASSET (GE Healthcare), or iPAT (Siemens Medical Solutions). Parallel imaging requires phased-array coils, and these coils also increase the signal-to-noise ratio, he said. A typical eight-channel phased-array coil increases SNR by 40%, independent of field strength. Parallel imaging allows users to trade off the extra SNR of 3T to go faster. Since 3T has roughly twice the SNR of 1.5T, it permits better spatial resolution and/or thinner slices. Scans can be acquired four times as fast with the same spatial resolution and slice thickness, as long as the user has at least four coil elements in the phased array.

If you cover every other line of k-space, the acquisition time is halved. But so is your field-of-view, leading to aliasing or wraparound, Bradley said. By knowing the local sensitivity of each coil in the phased array, you can "unwrap" the image. For the first time, unenhanced MR angiography of the brain using a 1024 x 1024 matrix becomes possible. By using a 1024 x 680 matrix over a 16 x 12-cm field-of-view, 160 x 200-micron pixels can be produced. This compares with 250-micron resolution for digital subtraction angiography. The 100-micron-diameter ophthalmic and lenticulostriate arteries are now commonly seen.

"Using a standard birdcage head coil, the center of the brain tends to be brighter due to dielectric effects. Phased-array coils at 1.5T tend to have greater signal immediately beneath the coil elements at the periphery of the brain. Using phased-array coils at 3T, the signal throughout the brain evens out," he said.

A beneficiary of 3T and SENSE is likely to be breast MR. At 1.5T, the typical spatial resolution for a single breast examination is 1 mm in place with a slice thickness of 2.5 to 3 mm. At 3T with SENSE, a spatial resolution of 0.5 mm can be obtained with a slice thickness of 1 mm. This added spatial resolution allows much better evaluation of lesion borders to depict spiculation. MR of the breast is already essentially 100% sensitive for invasive breast cancer, and combining the kinetic data with this improved spatial resolution will also improve the specificity of MRI for the detection of breast cancer, he said.

Bradley anticipates that over the next decade MRI will gradually replace CT as the primary imaging modality for acute stroke evaluation.

"MRI can do what CT does; i.e., detect hemorrhage," he said. "Gradient echo images are more sensitive than CT for acute hemorrhage. And the b = 0 image from the diffusion study and the baseline images from the perfusion data set are even more sensitive than traditional gradient echo for detecting acute parenchymal hemorrhage. In fact, one study demonstrated that FLAIR (fluid-attenuated inversion recovery) was not only as sensitive as CT, it was 100 times more sensitive!"

Use of intraoperative MRI as primary guidance for delivery of gene and stem cell therapy also looks set to increase. Gene therapy is currently administered for Alzheimer's and Parkinson's diseases using a stereotactic frame. With direct visualization by MRI, the tip of the needle can be seen clearly and the position confirmed before the gene therapy is administered. MRI is also critical for the resection of low-grade brain lesions. The prognosis of these lesions is inversely related to the amount of tumor left behind, because even a small amount of low-grade tumor will undergo malignant degeneration and eventually kill the patient, according to Bradley.

Contrast-enhanced MR angiography of the carotids has replaced catheter angiography in many institutions, but the coronaries may take a few more years, he said. Coronary calcium scoring is likely to be supplanted by CTA of the coronaries and, eventually, by MRA due to its superior plaque characterization.

One of the goals of molecular imaging is the development of novel contrast agents targeted to specific receptors. PET or CT is likely to be the first imaging technique to be used for such agents, but MRI will follow quickly if the agent can be tagged with enough gadolinium, he said.