Johnson Ga

Magnetic resonance microscopy of the rat thorax and abdomen.

With magnetic resonance (MR) microscopy, high-resolution volumetric imaging (3DFT) of small animals is possible. Although these techniques are suitable for imaging the head and other small stationary objects, breathing and cardiac motion degrade the quality of body images. Scan synchronous ventilation and cardiac gating methods have been developed that permit acquisition of high-resolution images from anywhere in the body of small animals (150 to 400 g). Anesthetized rats were ventilated in synchrony with three-dimensional Fourier spin warp (3DFT) sequence (TR = 400 to 1000 ms, TE = 20 ms). Eight or 16 slices (1.2 or 2.5 mm thick) were acquired simultaneously. Effective pixel size was 200 X 200 mu. Imaging was performed in a 1.5 T, 1-m bore research system using a 28-cm diameter high field gradient coil and a 6-cm diameter radio frequency coil. For thoracic imaging, acquisitions were gated to the QRS of the ECG. Scan synchronous ventilation eliminated breathing motion artifacts and permitted visualization of peripheral vascular structures in the lung and liver. In images that were also cardiac gated, cardiac chambers and major thoracic vessels, including the coronary arteries, were well demonstrated. Thus, thoroughly characterized rodent models can now be studied with MR not only to explore noninvasively the intricacies of mammalian pathomorphology, but also to test the capabilities of MR and aid in interpreting MR data.

A dual micro-CT system for small animal imaging

Micro-CT is a non-invasive imaging modality usually used to assess morphology in small animals. In our previous work, we have demonstrated that functional micro-CT imaging is also possible. This paper describes a dual micro-CT system with two fixed x-ray/detectors developed to address such challenging tasks as cardiac or perfusion studies in small animals. A two-tube/detector system ensures simultaneous acquisition of two projections, thus reducing scanning time and the number of contrast injections in perfusion studies by a factor of two. The system is integrated with software developed in-house for cardio-respiratory monitoring and gating. The sampling geometry was optimized for 88 microns in such a way that the geometric blur of the focal spot matches the Nyquist sample at the detector. A geometric calibration procedure allows one to combine projection data from the two chains into a single reconstructed volume. Image quality was measured in terms of spatial resolution, uniformity, noise, and linearity. The modulation transfer function (MTF) at 10% is 3.4 lp/mm for single detector reconstructions and 2.3 lp/mm for dual tube/detector reconstructions. We attribute this loss in spatial resolution to the compounding of slight errors in the separate single chain calibrations. The dual micro-CT system is currently used in studies for morphological and functional imaging of both rats and mice.

A probe for specimen magnetic resonance microscopy.

One of the primary limits to spatial resolution in magnetic resonance (MR) microscopy is the limited signal. The purpose of this study is to build a radiofrequency (rf) probe for MR microscopy of fixed specimens at 300 MHz. The design criteria for the probe were (1) high sensitivity; (2) good rf homogeneity; (3) minimization of BO variations. All probes were Helmholtz pairs operating at 300 MHz. Coils were constructed from copper/Teflon/copper microwave substrate which eliminated susceptibility problems from solder and discrete capacitors. Signal-to-noise ratio (SNR) was compared with a conventional solenoid. Phantoms of agar gels and saline-filled tubes were used to characterize the SNR and homogeneity. SNR measurements of the coil pairs showed a marked improvement (up to 60%) over that of the reference solenoid. The region of homogeneity was defined as a 10% variation in signal intensity. This correlated with the coils inner diameter. Graphs of SNR versus diameter, separation, and copper foil width allowed for optimization of the structure. Using this coil, MR microscopy is now possible on small, fixed specimens with pixels as small as 20 x 20 x 30 microns. Work is currently under way exploiting the SNR and homogeneity provided by this probe to determine the degree to which MR microscopy might add to the pathologists diagnostic tools.

Denoising of 4D Cardiac Micro-CT Data Using Median-Centric Bilateral Filtration.

Bilateral filtration has proven an effective tool for denoising CT data. The classic filter uses Gaussian domain and range weighting functions in 2D. More recently, other distributions have yielded more accurate results in specific applications, and the bilateral filtration framework has been extended to higher dimensions. In this study, brute-force optimization is employed to evaluate the use of several alternative distributions for both domain and range weighting: Andrews Sine Wave, El Fallah Ford, Gaussian, Flat, Lorentzian, Hubers Minimax, Tukeys Bi-weight, and Cosine. Two variations on the classic bilateral filter, which use median filtration to reduce bias in range weights, are also investigated: median-centric and hybrid bilateral filtration. Using the 4D MOBY mouse phantom reconstructed with noise (stdev. ~ 65 HU), hybrid bilateral filtration, a combination of the classic and median-centric filters, with Flat domain and range weighting is shown to provide optimal denoising results (PSNRs: 31.69, classic; 31.58 median-centric; 32.25, hybrid). To validate these phantom studies, the optimal filters are also applied to in vivo, 4D cardiac micro-CT data acquired in the mouse. In a constant region of the left ventricle, hybrid bilateral filtration with Flat domain and range weighting is shown to provide optimal smoothing (stdev: original, 72.2 HU; classic, 20.3 HU; median-centric, 24.1 HU; hybrid, 15.9 HU). While the optimal results were obtained using 4D filtration, the 3D hybrid filter is ultimately recommended for denoising 4D cardiac micro-CT data, because it is more computationally tractable and less prone to artifacts (MOBY PSNR: 32.05; left ventricle stdev: 20.5 HU).

Measuring the progression of foreign-body reaction to silicone implants using in vivo MR microscopy

We used in vivo magnetic resonance (MR) microscopy to follow the growth of fibrous capsule as a foreign body reaction to silicone implants in rats. Anesthetized rats were imaged 1, 7, 14, and 28 days after silicone-coated MR imaging coils were sutured to their neck muscles. On the twenty eighth day, rats were sacrificed and coils and adjacent tissues were removed en bloc and fixed in formalin, reimaged with MR, and sectioned for conventional histology. Three-dimensional (3-D) spin-echo [3DFT] acquisition gave in-plane resolution of 32/spl times/32 /spl mu/m in vivo and 16/spl times/16 /spl mu/m ex vivo. All MR images showed a diffuse band of elevated signal intensity between the silicone of the coil and adjacent tissue. The border of the hyperintense band was thin and not well defined at seven days post-implantation. From 7-28 days, the band showed relatively homogeneous signal intensity and its thickness increased 44% on the rectus muscle side and 78% on the subcutaneous side. The capsule thickness determined either by MR in vivo and ex vivo microscopy or conventional histology was not significantly different, and there was a significant correlation between thickness measurements among those methods. MR in vivo microscopy provides sufficient resolution and spatial information to serially evaluate the growth of the foreign body fibrous capsule over time, thus achieving greater accuracy and consistency in measurements.

A LabVIEW Platform for Preclinical Imaging Using Digital Subtraction Angiography and Micro-CT

CT and digital subtraction angiography (DSA) are ubiquitous in the clinic. Their preclinical equivalents are valuable imaging methods for studying disease models and treatment. We have developed a dual source/detector X-ray imaging system that we have used for both micro-CT and DSA studies in rodents. The control of such a complex imaging system requires substantial software development for which we use the graphical language LabVIEW (National Instruments, Austin, TX, USA). This paper focuses on a LabVIEW platform that we have developed to enable anatomical and functional imaging with micro-CT and DSA. Our LabVIEW applications integrate and control all the elements of our system including a dual source/detector X-ray system, a mechanical ventilator, a physiological monitor, and a power microinjector for the vascular delivery of X-ray contrast agents. Various applications allow cardiac- and respiratory-gated acquisitions for both DSA and micro-CT studies. Our results illustrate the application of DSA for cardiopulmonary studies and vascular imaging of the liver and coronary arteries. We also show how DSA can be used for functional imaging of the kidney. Finally, the power of 4D micro-CT imaging using both prospective and retrospective gating is shown for cardiac imaging.

Denoising of 4D Cardiac Micro-CT Data Using Median-Centric Bilateral Filtration

Bilateral filtration has proven an effective tool for denoising CT data. The classic filter uses Gaussian domain and range weighting functions in 2D. More recently, other distributions have yielded more accurate results in specific applications, and the bilateral filtration framework has been extended to higher dimensions. In this study, brute-force optimization is employed to evaluate the use of several alternative distributions for both domain and range weighting: Andrews Sine Wave, El Fallah Ford, Gaussian, Flat, Lorentzian, Hubers Minimax, Tukeys Bi-weight, and Cosine. Two variations on the classic bilateral filter, which use median filtration to reduce bias in range weights, are also investigated: median-centric and hybrid bilateral filtration. Using the 4D MOBY mouse phantom reconstructed with noise (stdev. ~ 65 HU), hybrid bilateral filtration, a combination of the classic and median-centric filters, with Flat domain and range weighting is shown to provide optimal denoising results (PSNRs: 31.69, classic; 31.58 median-centric; 32.25, hybrid). To validate these phantom studies, the optimal filters are also applied to in vivo, 4D cardiac micro-CT data acquired in the mouse. In a constant region of the left ventricle, hybrid bilateral filtration with Flat domain and range weighting is shown to provide optimal smoothing (stdev: original, 72.2 HU; classic, 20.3 HU; median-centric, 24.1 HU; hybrid, 15.9 HU). While the optimal results were obtained using 4D filtration, the 3D hybrid filter is ultimately recommended for denoising 4D cardiac micro-CT data, because it is more computationally tractable and less prone to artifacts (MOBY PSNR: 32.05; left ventricle stdev: 20.5 HU).

Contrast enhancement of the liver. Evaluation by automated contiguous pixel search.

Eight dogs were infused with one of three different contrast agents: meglumine diatrizoate, iosulamide meglumine, or an ethiodized oil emulsion (EOE 13). Image intensity in the liver was evaluated globally and regionally by means of an automated pixel sampling method to determine differences in the rate of enhancement produced by the three agents. The results of the automated method were compared with those of the standard manual cursor method. The automated method showed that liver parenchyma was enhanced uniformly by all three contrast agents. The maximum degree of enhancement (Mean +/- SEM) for the three agents was diatrizoate, 12.0 +/- 3.5 Hounsfield units (HU); iosluamide, 21.3 +/- 3.5 HU; and EOE 13, 37.2 +/- 4.25 HU. With the manual cursor method, contrast enhancement was about 20% more than estimated by the automated method. The automated method is better for evaluating the magnitude and pattern of contrast agent enhancement of the entire liver, since the currently employed cursor technique requires multiple evaluations to evaluate the entire liver.

Gallbladder density and iodine concentration in humans during oral cholecystography. A comparison of iopanoic acid and iopronic acid.

Thompson WM, Meyers WC, Shaw M, Bates M, Johnson GA, Hedlund LW. Gallbladder density and iodine concentration in humans during oral Cholecystography: a comparison of iopanoic acid and iopronic acid. Invest Radio! 1982; 17:621–628. A comparison of two oral cholecystopaques, iopanoic acid (Telepaque®) and iopronic acid (Oravue®), was performed using normal volunteers. Using a double-blind crossover design, comparisons were made between the degree of gallbladder opacification and the amount of iodine recovered from the gallbladder. Bile was collected via a double lumen intestinal tube before, during, and after stimulating gallbladder contraction. There were no differences between the two agents in terms of opacification or iodine concentration. Only 19% of the administered dose of either agent was recovered, and the maximum iodine concentration in bile was 10 mg I/ml. The results suggest that this technique has merit for future comparative studies of agents concentrated in the gallbladder.