|Figure 1. Three-dimensional rendering of a subject with cutaways illustrating activation in the Greene et al (2001) moral reasoning study. The red areas on the cutaway surfaces exhibited greater activation when subjects contemplated what to do in an emotional moral-personal dilemma, as opposed to either a moral-impersonal or a non-moral dilemma. These areas included (from left to right): 1) right angular gyrus; 2) posterior cingulate; and 3) medial prefrontal cortex. Courtesy of Leigh Nystrom, PhD and Brian Sommerville, Princeton University.|
RI is so much a part of everyday medical practice that it is sometimes difficult to remember that it was once an exotic technology. Upon its introduction, MRI was considered too expensive for routine use; in response to market and regulatory forces, it was deployed primarily in academic medical centers for some time. Once research had clarified the usefulness of MRI and clinical imaging had lengthened its list of applications, MRI reached broad implementation.
Since that time, advances in both the sophistication of MRI technology and the clinical and research uses to which it is put have been ongoing. Among the most important recent breakthroughs has been the introduction of 3T MR systems. The advent of higher field strength in clinical and research MRI settings is promising not only because it can improve existing imaging, but because it can permit MRI to expand into types of imaging that are partially or entirely new.
The brain’s congenital abnormalities, injuries, and neoplasms are being diagnosed through the use of MRI today, but higher field strengths permit the detection of brain tissue that does not appear to be damaged, but that does not function in the expected manner. Functional MRI (fMRI) can, in this way, be used in the diagnosis of the more subtle brain differences categorized as mental illnesses or learning disabilities.
MR angiography uses the blood’s own flow as its signal source in imaging other structures, for example. Time-of-flight MR angiography uses the blood’s own movement as its contrast agent in imaging other structures, for example. Blood flow itself can be evaluated using a contrast medium (MRI perfusion imaging), and water movement can also be seen using MRI diffusion imaging. Although these three applications were developed before the introduction of 3T MRI, they have benefited greatly from higher field strength.
In particular, a subtype of perfusion imaging called Blood-Oxygenation-Level-Dependent (BOLD) MRI has seen a great enhancement of its capabilities. Because it detects the areas of the brain that are receiving the most blood flow at a given time, it can be used to directly highlight the various parts of the brain that are affected by the external stimuli. BOLD is a type of fMRI that can provide insight not only into the disorders of individuals, but into the typical functions of the human brain under controlled conditions. BOLD research has implications, therefore, for education and psychology as well as medicine.
Although 3T MRI has not been available long enough for most of the research that employs it to reach publication, investigators are using it to obtain a better signal-to-noise ratio (SNR), to enhance spatial resolution, and to decrease image-acquisition times. These new capabilities are of particular importance to the investigators who are breaking new ground in mapping the brain.
In a study partially performed using a 3T MRI system at Princeton University-based Center for the Study of Brain, Mind, and Behavior, a research center under the direction of Jonathan D. Cohen, MD, PhD, Green et al 1 used two fMRI protocols to investigate whether moral judgment has its basis predominantly in reason or in emotion. Study subjects underwent brain scanning while they responded to 60 moral and practical dilemmas, some of which were designed to create more emotional responses than others. The resulting images showed clear differences between subjects’ responses to moral and nonmoral dilemmas (Figure 1). The authors wrote, “We argue that moral dilemmas vary systematically in the extent to which they engage emotional processing and that these variations in emotional engagement influence moral judgment. These results may shed light on some puzzling patterns in moral judgment observed by contemporary philosophers.”
Marlene Richter, PhD, is an MR research scientist involved with the ongoing work of this group at the Center for the Study of Brain, Mind, and Behavior, Princeton University, Princeton, NJ. She says of the 3T scanner, “From my viewpoint, the system is running very well and has excellent stability, and that is obviously important for fMRI studies.”
In 1999, Conturo et al2 of Washington University in St Louis developed a way of tracking neuronal fibers in living humans. This method used diffusion-tensor MRI to characterize water diffusion, permitting the researchers to reconstruct fiber trajectories within the brain by tracking the direction of the most rapid water diffusion (which indicated the fiber direction). This work revealed several details of brain structure. “Tracks covered long distances, navigated through divergences and tight curves, and manifested topological separations. Previously undescribed topologies were revealed.” They added, “In MRI, spatial resolution depends on SNR and acquisition time. By defining the limits in fiber bundle size and configuration trackable at specific SNR levels and spatial resolutions, the diffusion-tensor MRI data collection could be adjusted to the desired degree of detail.”
Fortunately, the introduction of 3T MRI is now providing investigators with the imaging power that they have wished for in the past. Using a Siemens Allegra 3T head scanner, An and Lin3 developed methods for the application of diffusion gradients that reduce the errors formerly present in MRI estimation of cerebral venous blood flow and oxygen extraction fraction.
|Figure 2. Siemens Allegra installed at the Center for the Study of Brain, Mind, and Behavior, Princeton University, NJ.|
E.M. Haake, PhD, leads investigators at Wayne State University Medical Center, Detroit, who are using 3T MRI to study brain anatomy at an unprecedented level of detail. This work may be especially valuable to medical understanding of demyelination and neurodegenerative conditions, since these involve changes in the brain that are too difficult to visualize using other methods. This group has obtained data on the entire brain at a resolution of 0.25 mm3 using a 3T magnet. In the course of this work, the researchers have developed a high-resolution, three-dimensional, gradient echo technique. Many structures never seen in the living brain have been imaged using this technique.
Mukherjee et al4 of Washington University used diffusion-tensor MRI to study the changes in water diffusion in five area of the brain in children. The retrospective study’s 153 subjects ranged in age from 1 day to 11 years. The investigators were able to define characteristic patterns of change that occur as a child ages; this work may lead to the establishment of developmental milestones that will indicate the maturity of the brain.
In work supported by the NIH and a McDonnell-Pew grant, Frank Tong, PhD, assistant professor of psychology, and colleagues at Princeton University are using fMRI to determine the areas of the brain that are active when study subjects view an ambiguous object. Some subjects attempt to control their perception of the object in various ways, while other subjects view it passively; the investigators are finding that the brain areas involved in controlling perception overlap those involved in simply monitoring perception.
The clinical and research applications of 3T MRI are likely to be diverse once the technology has been more widely disseminated. Meanwhile, it is probable that 3T scanners will be most heavily used where the enhancements that they produce can have the highest yield: in imaging the human brain. n
Kris Kyes is technical editor of Decisions in Axis Imaging News.
- Greene JD, Sommerville RB, Nystrom LE, Darley JM, Cohen JD. An fMRI investigation of emotional engagement in moral judgment. Science. 2001;293:2105-2108.
- Conturo TE, Lori NF, Cull TS, et al. Tracking neuronal fiber pathways in the living human brain. Proc Natl Acad Sci USA. 1999;96:10422-10427.
- An H, Lin W. Quantitative measurement of cerebral venous blood volume and cerebral blood oxygen saturation: asymmetric spin echo approach [abstract]. Proceedings of the International Society of Magnetic Resonance. 2002. In press.
- Mukherjee P, Miller JH, Shimony JS, et al. Normal brain maturation during childhood: developmental trends characterized with diffusion-tensor MR imaging. Radiology. 2001;221:349-358.