Site-specific variability in trabecular bone dosimetry: considerations of energy loss to cortical bone.

With continual advances in radionuclide therapies, increasing emphasis is being placed on improving the patient specificity of dose estimates to marrow tissues. While much work has been focused on determining patient-specific assessments of radionuclide uptake in the skeleton, few studies have been initiated to explore the individual variability of absorbed fraction data for electron and beta-particle sources in various skeletal sites. The most recent values of radionuclide S values used in clinical medicine continue to utilize a formalism in which electrons are transported under a trabecular bone geometry of infinite extent. No provisions are thus made for the fraction of energy lost to the cortical bone cortex of the skeletal site and its surrounding tissues. In the present study, NMR microscopy was performed on trabecular bone samples taken from the femoral head and humeral proximal epiphysis of three subjects: a 51-year male, an 82-year female, and an 86-year female. Following image segmentation and coupling to EGS4, electrons were transported within macrostructural models of the various skeletal sites that explicitly include the spatial extent of the spongiosa, as well as the thickness of the surrounding cortical bone. These energy-dependent profiles of absorbed fractions to marrow tissues were then compared to transport simulations made within an infinite region of spongiosa. Ratios of mean absorbed fraction, as weighted by the beta energy spectra, under both transport methodologies were then assembled for the radionuclides 32P and 90Y. These ratios indicate that corrections to existing radionuclide S values for 32P can vary by as much as 5% for the male, 6% for the 82-year female, and 8% for the 86-year female. For the higher-energy beta spectrum of 90Y, these same corrections can reach 8%, 10%, and 11%, respectively.

Skeletal dosimetry via NMR microscopy: investigations of sample reproducibility and signal source.

Nuclear magnetic resonance microscopy has been used for several years as a means of quantifying the 3D microarchitecture of the cancellous regions of the skeleton. These studies were originally undertaken for the purpose of developing non-invasive techniques for the early detection of osteoporosis and other bone structural changes. Recently, nuclear magnetic resonance microscopy has also been used to acquire this same 3D data for the purpose of both (1) generating chord length data across bone trabeculae and marrow cavities and (2) generating 3D images for direct coupling to Monte Carlo radiation transport codes. In both cases, one is interested in the reproducibility of the dosimetric data obtained from nuclear magnetic resonance microscopy. In the first of two studies, a trabecular bone sample from the femoral head of a 51-y-old male cadaver was subjected to repeated image acquisition, image processing, image coupling, and radiation transport simulations. The resulting absorbed fractions at high electron energies (4 MeV) were shown to vary less than 4% among four different imaging sessions of the same sample. In a separate study, two femoral head samples were imaged under differing conditions of the NMR signal source. In the first case, the samples were imaged with intact marrow. These samples were then subjected to marrow digestion and immersed in Gd-doped water, which then filled the marrow cavities. Energy-dependent absorbed fraction profiles for both the marrow-intact and marrow-free samples showed essentially equivalent results. These studies thus provide encouragement that skeletal dosimetry models of improved patient specificity can be achieved via NMR microscopy in vivo.

Chord distributions across 3D digital images of a human thoracic vertebra.

Radiation dose estimates to the trabecular region of the skeleton are of primary importance due to recent advancements in nuclear medicine. Establishing methods for accurately calculating dose in these regions is difficult due to the complex microstructure of this anatomic site and the typical ranges of beta-particles in both bone and marrow tissues. At the present time, models of skeletal dosimetry used in clinical medicine rely upon measured distributions of straight-line path lengths (chord lengths) through bone and marrow regions. This work develops a new three-dimensional, digital method for acquiring these distributions within voxelized images. In addition, the study details the characteristics of measuring chord distributions within digital images and provides a methodology for avoiding undesirable pixel or voxel effects. The improved methodology has been applied to a digital image (acquired via NMR microscopy) of the trabecular region of a human thoracic vertebra. The resulting chord-length distributions across both bone trabeculae and bone marrow cavities were found to be in general agreement with those measured in other studies utilizing different methods. In addition, this study identified that bone and marrow space chord-length distributions are not statistically independent, a condition implicitly assumed within all current skeletal dosimetry models of electron transport. The study concludes that the use of NMR microscopy combined with the digital measurement techniques should be used to further expand the existing Reference Man database of trabecular chord distributions to permit the development of skeletal dosimetry models which are more age and gender specific.

Visualization of neural tissue water compartments using biexponential diffusion tensor MRI.

The apparent diffusion tensor (ADT) imaging method was extended to account for multiple diffusion components. A biexponential ADT imaging experiment was used to obtain separate images of rapidly and slowly diffusing water fractions in excised rat spinal cord. The fast and slow component tensors were compared and found to exhibit similar gross features, such as fractional anisotropy, in both white and gray matter. However, there were also some important differences, which are consistent with the different structures occupying intracellular and extracellular spaces. Evidence supporting the assignment of the two tensor components to extracellular and intracellular water fractions is provided by an NMR spectroscopic investigation of homogeneous samples of brain tissue. Magn Reson Med 45:580-587, 2001.

Diffusion tensor MR imaging and comparative histology of glioma engrafted in the rat spinal cord.

MR imaging using contrast material derived from the diffusion of tissue water was tested for its ability to provide a nondestructive histologic analysis of tumor morphology. An apparent diffusion tensor MR image of a glioma engrafted within a rat spinal cord was generated in which fiber orientation in three dimensions was displayed in color. This imaging method clearly separated tumor from host white and gray matter and corresponded well with conventional histologic microscopy.

In vivo dynamics and distribution of intracerebroventricularly administered gadodiamide, visualized by magnetic resonance imaging.

Direct injections into the cerebroventricles have been extensively utilized in neurophysiological studies. Mapping the distribution of injectate after intracerebroventricular injection has been made only by post mortem analysis, and the dynamic distribution of injectate within the brain has not been well characterized. In this report, we apply contrast-enhanced magnetic resonance imaging to study the pharmacokinetics and extent of non-ionic gadodiamide transport into brain tissue in vivo after intracerebroventricular administration. The results indicate that intracerebroventricular injectate travels quickly throughout the ventricular system from the lateral ventricular site of injection to the fourth ventricle and foramina of Luschka and Magendie within 2 min. After this, the signal intensity begins to increase in the periventricular and paraventricular brain parenchyma. Contrast enhancement is visible 2 mm into the brain tissue from the ventricles. Quantitative analysis of the data shows that the transport of gadodiamide across the ependymal layer that lines the cerebrospinal fluid space characterized a rate constant of 0.066+/-0.017 min(-1). These results provide a better understanding of chemical transport and diffusion following direct injection into the cerebroventricles. They provide information on the in vivo dynamics of injectate after intracerebroventricular administration, and show that contrast enhanced magnetic resonance imaging may be used to more precisely define the target sites of chemicals after intracerebroventricular administration into the brain.

The effect of ouabain on water diffusion in the rat hippocampal slice measured by high resolution NMR imaging.

High resolution NMR imaging of the isolated perfused rat hippocampal slice was used to quantitate ADC changes following ouabain-induced cell swelling. Hippocampal slices were studied in artificial cerebrospinal fluid and then in ouabain using a 600-MHz narrow bore spectrometer and a home-built perfusion chamber. The brain slices demonstrated biexponential diffusion behavior. After perfusion with 1 mMouabain, there was an increase in the fraction of slowly diffusing water. The ADCs of the two fractions did not change. These data support the hypothesis that the decrease in the ADC of brain water following an ischemic attack is caused by cell swelling. The relative amplitudes of the two diffusing fractions do not match the expected ratio of intracellular and extracellular fractions. This discrepancy may be principally due to the difference in T2 relaxation rates of the two compartments.

NMR microscopy of trabecular bone and its role in skeletal dosimetry.

One of the more intractable problems in internal dosimetry is the assessment of energy deposition by alpha and beta particles within trabecular, or cancellous, bone. In the past few years, new technologies have emerged that allow for the direct and nondestructive 3D imaging of trabecular bone with sufficient spatial resolution to characterize trabecular bone structure in a manner needed for radiation dosimetry models. High-field proton nuclear magnetic resonance (NMR) imaging is one such technology. NMR is an ideal modality for imaging trabecular bone due to the sharp contrast in proton density between the bone matrix and bone marrow regions. In this study, images of the trabecular regions within the bodies of a human thoracic vertebra have been obtained at a field strength of 14.1 T. These images were digitally processed to measure chord length distribution data through both the bone trabeculae and marrow cavities. These distributions, which were found to be qualitatively consistent with those measured by F. W. Spiers and colleagues at the University of Leeds using physical sectioning and automated light microscopy, yielded a mean trabecular thickness of 201 microm and a mean marrow cavity thickness of 998 microm. The NMR techniques developed here for vertebral imaging may be extended to other skeletal sites, allowing for improved site-specific skeletal dosimetry.

MR microscopy of perfused brain slices.

To study the origins of signal changes in clinical MRI we have previously studied isolated single neuronal cells by MR microscopy. To account for the extracellular environment of the cells, we have developed a prototype perfusion chamber for MR microimaging of perfused rat hippocampal brain slices. To demonstrate the utility of this model, brain slices were initially perfused in isotonic solutions and then subjected to osmotic perturbations via perfusate exchange with 20% hypertonic and 20% hypotonic solutions. In diffusion weighted images, signal intensity changes of +16(sigma(n-1) = 11)% (hypotonic) and -26(sigma(n-1) = 10)% (hypertonic) were observed. No significant variation in response was observed across the slice when several subregions were examined. These observations are consistent with the view that contrast changes are driven primarily by changes in the intra- and extracellular compartmentation of water. This is the first report of MR microimaging of the isolated brain slice. The technique will enable the correlation of MR microimaging measurements with microscopic changes using other modalities and techniques to provide a better understanding of signals in clinical MRI.

Diffusion anisotropy in excised normal rat spinal cord measured by NMR microscopy.

A conventional spin-echo NMR imaging pulse sequence was used to obtain high-resolution images of excised normal rat spinal cord at 7 and 14 T. It was observed that the large pulsed-field gradients necessary for high-resolution imaging caused a diffusion weighting that dominated the image contrast and that could be used to infer microscopic structural organization beyond that defined by the resolution of the image matrix (i.e., fiber orientation could be assigned based on diffusion anisotropy). Anisotropic diffusion coefficients were therefore measured using apparent diffusion tensor (ADT) imaging to assess more accurately fiber orientations in the spinal cord; structural anisotropy information is portrayed in the six unique images of the complete ADT. To reduce the dimensionality of the data, a trace image was generated using a separate color scale for each of the three diagonal element images of the ADT. This new image retains much of the invariance of the trace to the relative orientations of laboratory and sample axes (inherent to a greyscale trace image) but provides, by the use of color, contrast reflecting diffusion anisotropy. The colored trace image yields a pseudo-three-dimensional view of the rat spinal cord, from which it is possible to deduce fiber orientations.