MAGNETIZATION TRANSFER IN NEUROIMAGING

Evaluation of the role of magnetization transfer imaging in prostate: a preliminary study

Results of the preliminary study on the evaluation of the role of magnetization transfer imaging (MTI) of prostate in men who had raised prostate-specific antigen (PSA) (>4 ng/ml) or abnormal digital rectal examination (DRE) are reported. MT ratio (MTR) was calculated for 20 patients from the hyper- (normal) and hypo-intense regions (area suspicious of malignancy as seen on T2-weighted MRI) of the peripheral zone (PZ) and the central gland (CG) at 1.5 T. In addition, MTR was calculated for three healthy controls. Mean MTR was also calculated for the whole of the PZ (including hyper- and hypo-intense area) in all patients. Out of 20 patients, biopsy revealed malignancy in 12 patients. Mean MTR value (8.29+/-3.49) for the whole of the PZ of patients who were positive for malignancy on biopsy was statically higher than that observed for patients who were negative for malignancy (6.18+/-3.15). The mean MTR for the whole of the PZ of controls was 6.18+/-1.63 and is similar to that of patients who were negative for malignancy. Furthermore, for patients who showed hyper- (normal portion) and hypo-intense (region suspicious of malignancy) regions of the PZ, the MTR was statistically significantly different. These preliminary results reveal the potential role of MT imaging in the evaluation of prostate cancer.

Magnetization transfer, HASTE, and FLAIR imaging

Continuous technologic developments and research have increased the clinical applications of MT, HASTE, and FLAIR imaging in neuroradiology. HASTE has become the MR imaging sequence of choice for fetal neuroimaging. Other promising uses, such as for diffusion-weighted imaging, have not been fully exploited. FLAIR has been firmly established as one of the cornerstones of brain imaging; however, post-contrast FLAIR images have not offered a clear advantage over standard T1-weighted images as suggested by early studies. FLAIR imaging with echoplanar acquisition is not considered advantageous, because the decreased imaging times are obtained at the expense of lower sensitivity. For a number of applications, diffusion-weighted imaging has surpassed FLAIR. Nevertheless, FLAIR images may be more sensitive for the detection of acute brain infarction. Recently described methods for the elimination of CSF flow artifacts may lead to improved quality and reliability of FLAIR images for subarachnoid space disease. MT preparation is now routinely incorporated in time-of-flight MR angiography and gradient-echo T2*-weighted spine imaging sequences and provides increased sensitivity for postcontrast MR imaging. These applications may not be advantageous in all clinical settings. MTR analysis offers valuable information for an increasing number of pathologic processes but has not yet gained wide clinical acceptance owing to sophisticated postprocessing and significant intercenter variations. Different modifications of these techniques are being evaluated, and further developments are expected.

Molecular aspects of magnetic resonance imaging and spectroscopy

Magnetic resonance imaging (MRI) is a well known diagnostic tool in radiology that produces unsurpassed images of the human body, in particular of soft tissue. However, the medical community is often not aware that MRI is an important yet limited segment of magnetic resonance (MR) or nuclear magnetic resonance (NMR) as this method is called in basic science. The tremendous morphological information of MR images sometimes conceal the fact that MR signals in general contain much more information, especially on processes on the molecular level. NMR is successfully used in physics, chemistry, and biology to explore and characterize chemical reactions, molecular conformations, biochemical pathways, solid state material, and many other applications that elucidate invisible characteristics of matter and tissue. In medical applications, knowledge of the molecular background of MRI and in particular MR spectroscopy (MRS) is an inevitable basis to understand molecular phenomenon leading to macroscopic effects visible in diagnostic images or spectra. This review shall provide the necessary background to comprehend molecular aspects of magnetic resonance applications in medicine. An introduction into the physical basics aims at an understanding of some of the molecular mechanisms without extended mathematical treatment. The MR typical terminology is explained such that reading of original MR publications could be facilitated for non-MR experts. Applications in MRI and MRS are intended to illustrate the consequences of molecular effects on images and spectra.

Measuring in vivo myelination of human white matter fiber tracts with magnetization transfer MR

Precise characterization of white matter pathways is important for the understanding of structural-functional relationships in the human brain. While it is known from postmortem studies that the connectivity of cortical areas is conveyed by projection, commissural, and association fibers, most clinical studies disregard useful information about specific fiber tracts. Magnetization transfer (MT) MR detects the relative proportion of free mobile protons and immobile protons bound to macromolecules. MT values correlate with histopathology and it has been proposed that in the white matter, the amount of magnetization transfer correlates with the degree of myelination. Thus, MT-MR provides measures that may reflect more accurately the physiology and natural course of diseases involving the white matter. We applied this quantitative in vivo method to five children at different ages to determine whether the maturational changes of distinct fiber tracts could be measured. All regions of interest were localized by means of Brodmann's original descriptions and the additional use of reconstructed 3D-matched data from 10 myelin-stained human brain specimens. With this atlas-guided approach we localized and measured 26 supratentorial white matter fiber tracts in each hemisphere, connecting to primary as well as association cortices. All fiber tracts showed consistent age-related MT changes and the strongest effects were found in those tracts projecting to the primary cortical areas. These results suggest that our method is suitable for in vivo MT measurements in specific fiber tracts. It can provide data that relate to myelination during ontogenesis or myelination delays in myelin disorders. In the clinical domain, the focus on specific fiber tracts appears to be advantageous over standard approaches, because such system of parcellation is based on the functional anatomy of the white matter. Consequently, it may be especially useful for topical analysis in neurology allowing the assessment of the functional consequences of white matter damage as well as the effectiveness of treatments in patients with any lesion that can be visualized by MRI.