Registered (1)H and (3)He magnetic resonance microscopy of the lung.

Using in vivo magnetic resonance microscopy, registered (1)H and hyperpolarized (3)He images of the rat lung were obtained with a resolution of 0.098 x 0.098 x 0.469 mm (4.5 x 10(-3) mm(3)). The requisite stability and SNR was achieved through an integration of scan-synchronous ventilation, dual-frequency RF coils, anisotropic projection encoding, and variable RF excitation. The total acquisition time was 21 min for the (3)He images and 64 min for the (1)H image. Airways down to the 6th and 7th orders are clearly visible. Magn Reson Med 45:365-370, 2001.

Functional MR microscopy of the lung using hyperpolarized 3He.

A new strategy designed to provide functional magnetic resonance images of the lung in small animals at microscopic resolution using hyperpolarized 3He is described. The pulse sequence is based on a combination of radial acquisition (RA) and CINE techniques, referred to as RA-CINE, and is designed for use with hyperpolarized 3He to explore lung ventilation with high temporal and spatial resolution in small animal models. Ventilation of the live guinea pig is demonstrated with effective temporal resolution of 50 msec and in-plane spatial resolution of <100 microm using hyperpolarized 3He. The RA-CINE sequence allows one to follow gas inflow and outflow in the airways as well as in the distal part of the lungs. Regional analysis of signal intensity variations can be performed and can help assess functional lung parameters such as residual gas volume and lung compliance to gas inflow.

Sensitivity and resolution in 3D NMR microscopy of the lung with hyperpolarized noble gases.

Three-dimensional magnetic resonance images of the guinea pig lung were acquired in vivo using hyperpolarized (HP) noble gases and radial projection encoding (PE). Results obtained with 3He (voxel size 17 microl) demonstrated high image quality showing airway structure down to the 5th or 6th generations. Signal-to-noise ratios (SNRs) of 129Xe images (voxel size 40 microl) were lower by about 1 order of magnitude as a consequence of the smaller gyromagnetic ratio, a more rapid relaxation in the gas reservoir, and lower polarization and isotope abundance. Comparison between experimentally obtained SNRs and results from calculations based on a model that accounts for the three-dimensional PE acquisition scheme and the non-equilibrium situation in HP gas imaging yielded excellent agreement for small flip angles. A theoretical examination of the potential resolution in HP gas MR microscopy of the lungs suggests that in vivo visualization of alveolar clusters distal to respiratory bronchioles may be possible.

A high-temperature superconducting Helmholtz probe for microscopy at 9.4 T.

The design and operation of a high-temperature superconducting (HTS) probe for magnetic resonance microscopy (MRM) at 400 MHz are presented. The design of the probe includes a Helmholtz coil configuration and a stable open-cycle cooling mechanism. Characterization of coil operating parameters is presented to demonstrate the suitability of cryo-cooled coils for MRM. Specifically, the performance of the probe is evaluated by comparison of signal-to-noise (SNR) performance with that of a copper Helmholtz pair, analysis of B1 field homogeneity, and quantification of thermal stability. Images are presented to demonstrate the SNR advantage of the probe for typical MRM applications.

Three dimensional magnetic resonance microangiography of rat neurovasculature.

Techniques are described to perform three dimensional (3D) MR microangiography. We have combined the use of a blood pool agent (Gd-DTPA-complexed with bovine serum albumin), three dimensional Fourier encoding, careful animal stabilization, and volume rendering to permit imaging with voxels of 60 x 60 x 60 microns. 3DFT encoding has been performed at 7.1 T with very large arrays (256 x 512 x 512). Interactive volume rendering allows a number of unique display opportunities that effectively exploit these isotropic 3D arrays.

Reduction of ringing and blurring artifacts in fast spin-echo imaging.

A simple method was devised to reduce ringing and blurring artifacts caused by discontinuous T2 weighting of k-space data in fast spin-echo magnetic resonance (MR) imaging. The method demodulates the weighting function along the phase-encoding direction by using multiple T2 values derived from a set of non-phase-encoded echoes obtained from an extra excitation. The performance of this method was evaluated by computer simulations and experiments, which confirmed its capability of effectively reducing or, in some cases, even completely removing the ringing and blurring artifacts. The results also show that the proposed method produces better results than other artifact reduction methods. The method is particularly useful at high magnetic field strengths (7.1-9.4 T) and with strong gradients (> 20 G/cm) used in MR microscopy, in which the apparent T2 values are short for most tissues. The authors expect that the proposed method will find useful applications in various fast spin-echo pulse sequences.

High-field MR microscopy using fast spin-echoes.

Fast spin-echo imaging has been investigated with attention to the requirements and opportunities for high-field MR microscopy. Two- and three-dimensional versions were implemented at 2.0 T, 7.1 T, and 9.4 T. At these fields, at least eight echoes were collectable with a 10 ms TE from fixed tissue specimens and living animals, giving an eightfold improvement in imaging efficiency. To reduce the phase-encoding gradient amplitude and its duty cycle, a modified pulse sequence with phase accumulation was developed. Images obtained using this pulse sequence exhibited comparable signal-to-noise (SNR) to those obtained from the conventional fast spin-echo pulse sequences. Signal losses due to imperfections in RF pulses and lack of phase rewinders were offset in this sequence by reduced diffusion losses incurred with the gradients required for MR microscopy. Image SNR, contrast, edge effects and spatial resolution for three k-space sampling schemes were studied experimentally and theoretically. One method of sampling k-space, 4-GROUP FSE, was found particularly useful in producing varied T2 contrast at high field. Two-dimensional images of tissue specimens were obtained in a total acquisition time of 1 to 2 min with in-plane resolution between 30 to 70 microns, and 3D images with 256(3) arrays were acquired from fixed rat brain tissue (isotropic voxel = 70 microns) and a living rat (isotropic voxel = 117 microns) in approximately 4.5 h.

MR microscopy at 7.0 T: effects of brain iron.

The T2 of brain tissue is known to be field dependent, decreasing as B0 increases. Previous studies have attributed reduced T2 in the structures of the extrapyramidal motor system (EPMS) to high iron concentrations. The present study was designed to manipulate physiologic iron concentrations and study the effects on T2 and on the field dependence of T2 at 7.0 T in whole formalin-fixed brains. A rat model was devised in which iron concentrations in the structures of interest were altered by diet manipulation. Cerebral structures with different iron content were imaged and T2 measured with MR microscopy at both 2.0 and 7.0 T. T2 of all tissues was shorter by 40%-60% at 7.0 T. Although some dependence of T2 on iron concentration was evident, it was less than expected. The strongest correlation was in the substantia nigra. The highest-resolution studies, at 30 x 30 x 50 microns, show the myelin bundles in many of the EPMS structures but not in the substantia nigra. From these data, it appears that T2 at greater field strengths depends more on susceptibility-induced spin dephasing imposed by diffusion through the tissue microstructure than on the presence of iron.

Three-dimensional MR microscopy with large arrays.

MR microscopy of fixed specimens is described using large (256(3] arrays. Images are acquired at 7.0 T with voxels as small as 70 x 70 x 70 microns (3.4 x 10(-4) mm3), more than 25,000 times smaller than routine clinical body imaging. Separation of the acquisition, reconstruction, archival, and analysis onto networked workstations provides flexibility and efficiency in handling the large data sets. The isotropic data can be interactively displayed through any plane without loss of in-plane resolution. The potential for applications of MR microscopy in clinical pathology is addressed.

Relaxation measurements at 300 MHz using MR microscopy.

Previous data on changes in tissue T1 with field have suggested the convergence of tissue T1 values toward a common value at high (greater than 4.0 T) fields. Measures of T2 dependence have suggested reduction of T2 with field. The purpose of this study was to observe the T1 and T2 at 85.5 and 300 MHz of microstructures in excised rat kidneys by employing MR microscopy. This study represents the first attempt of MR microscopy at 7.0 T with regard to the subject of magnetic field dependence of T1 and T2. As expected, T1 did increase with increasing field strength but not as dramatically as might be expected. Subtle differences in the microstructures of the kidney and the binding of water in those structures were discernible on the basis of T1 differences at 300 MHz. T2 values decreased, raising speculation concerning the mechanism for this dependence. The improved SNR permits smaller samples to be examined at much higher resolutions (greater than 30 X 30 X 200 microns), further extending the potentials for MR microscopy.

Chemical shift imaging of atherosclerosis at 7.0 Tesla.

Chemical shift imaging (CSI) was performed on cadaveric atherosclerotic fibrous plaques, periaortic adipose tissue, and cholesterol standards using a 7.0 Tesla horizontal bore prototype imaging spectrometer. Proton spectroscopy of intact tissue and deuterated chloroform extracted samples was done at the equivalent field strength of 7.0 Tesla on a vertical bore spectrometer, including studies of temperature dependence and T2 relaxation measurements. Spectra obtained using CSI on the imaging magnet were comparable with those from the conventional vertical spectrometer. Fibrous plaques and adipose tissue had unique spectral features, differing in the ratios of their water and various fat components. Chloroform extractions revealed a typical cholesteric ester spectrum for the fibrous plaque in contrast to the triglyceride spectrum of the adipose tissue. These two tissues also had different T2 relaxation measurements of their major fat resonances, with fibrous plaques having a short T2 compared to adipose tissue (15.9 milliseconds vs. 46.2 milliseconds). Temperature dependence studies showed that spectral signal intensity of the fat resonance of the fibrous plaque increased while linewidth decreased with increasing temperature from 24 degrees C to 37 degrees C. Atherosclerotic lesions may be studied at 7.0 Tesla, and NMR parameters defined in the present study may be used for further studies at other magnetic field strengths.

Performance of high-resolution monitors for digital chest imaging.

High-resolution cathode-ray tubes (CRT's) are currently the most viable soft-copy display for digital radiography. We present here methods for measuring large-area contrast ratio and detail contrast ratio. A two-dimensional charge coupled device (ccd) array signal-averaged with a video frame buffer permits linear microradiometric measure of individual beam lines. Results from three different 1000-line monitors demonstrate the shift variance of resolution. The detail contrast ratio (or modulation depth) was found to vary from 100% to less than 10% across the face of one CRT. Dynamic focus in both the horizontal and vertical deflection circuitry proved effective in reducing this shift variance. Comparisons of three phosphors demonstrate the utility of long persistence phosphors (P164) for static display in producing brighter images with less flicker. Recommendations for CRT design and selection for high-resolution digital radiography are included.

Histogram-directed processing of digital chest images.

One of the potential advantages of digital chest imaging is the ability to process these images. However, such processing, when uniformly applied to the entire image, is often unsatisfactory due to the different processing requirements of lung field and mediastinum. Therefore, a method to selectively process these regions based upon the histogram of the original image has been developed. Thirteen conventional chest films were digitized with a laser film scanner. Analysis of individual lung field and mediastinum histograms showed that the chest image histogram is essentially bimodal with significant lung field-mediastinum histogram peak separation; overlap between these peaks is small (9% of the total histogram) and insensitive to minor pathologic change or radiographic technique. Using these histograms, a gray level threshold distinguishing mediastinum from lung field was selected and used to direct the regionally-selective processing of several chest images. This technique may prove especially useful for digital enhancement of the underexposed mediastinum often encountered on conventional chest radiographs.

Digital synthesis of lung nodules.

Studies evaluating observer accuracy and visual perception of pulmonary nodules usually are based upon test films obtained from clinical practice in patients with proven pulmonary nodules. Unfortunately, such nodules do not always occur in the optimal size and location to facilitate testing. Such studies would be enhanced by the ability to place nodules of desired size and location on chest radiographs. This report describes a method of placing a computer-generated (synthesized) nodule on a digitized chest radiograph. To demonstrate the similarity of these synthesized nodules to real nodules, each digitized radiograph with a computer-generated nodule was paired with a digitized chest radiograph of a patient with a clinically proven pulmonary nodule. A total of 22 pairs of chest radiographs were then shown to 13 radiologists, who were asked to distinguish the synthesized nodule from the real nodule. With this two alternative forced-choice test, the radiologists were only able to distinguish the synthesized nodule in 51% of the cases, strongly suggesting that computer generated nodules may be used to simulate real pulmonary nodules in future tests of nodule detection.

Synthesized MR images: comparison with acquired images.

Synthesized and directly acquired spin-echo images were compared in order to assess the validity of magnetic resonance (MR) image synthesis as a method enabling retrospective formation of images by interactive manipulation of scan parameters. Synthetic images subjectively compared favorably in both accuracy and precision with acquired images when formed for the same values of echo (TE) and repetition times (TR) and for interpolated and extrapolated values of both TE and TR. Plots of synthetic and acquired signals within the same pixel sectors quantitatively showed comparable values for several regions of interest in the brain. Percent error and noise-normalized differences between acquired and synthetic images were tested as a quantitative measure of accuracy. Percent error was consistently less than 5% for brain parenchyma, and synthetic signals were accurate to within four times the noise level at acquisition. The apparent signal-to-noise ratio of synthetic images was comparable, superior, or inferior to similar acquired images, depending on the values of TE and TR. Total acquisition time required for synthetic formation of images for arbitrary values of TE and TR was equivalent to that of a single direct acquisition with a TR of 2,500 msec.

Cerebral magnetic resonance image synthesis.

The authors previously described magnetic resonance (MR) image synthesis, a process that enables the investigator to manipulate imaging parameters retrospectively and generate or "synthesize" the image that corresponds to various arbitrary scanning factors. They demonstrate the validity and utility of synthetic spin-echo images in cerebral imaging. As a test of their method, spin-echo images are synthesized for echo times identical to those of the original acquired images as well as for alternate values. Subjectively, the quality of synthetic and acquired images is comparable. It is shown quantitatively for several tissue types that the reconstructed synthetic signal matches the acquired signal within the uncertainty of the acquired images. Observed and measured noise levels in the acquired and synthetic images are comparable. Because of a signal-averaging effect, the synthetic images can have a higher signal-to-noise ratio than the source images, thereby providing improved boundary definition. Applications of MR image synthesis are discussed with respect to potential reduction in scanning time. The advantages of image synthesis versus analysis of computed images are discussed.

Automated MR image synthesis: feasibility studies.

The authors describe an automated technique of magnetic resonance (MR) image synthesis. Given a specific pulse sequence, MR signals are acquired for several pulse delay and/or repetition times and used to compute images of intrinsic parameters T1, T2, and N(H). Both the computed images and operator-specified pulse delay and repetition times are then used to "synthesize" a new image based on equations descriptive of MR signal behavior and comparable to that acquired by using the operator-specified parameters in an actual MR study. Instrumentation enabling rapid operator-interactive generation of synthesized images is described and initial results presented, allowing for dependence of the signal on T2 in spin echo images. Extension to full T1, T2, and N(H) dependence for arbitrary pulse sequences is described. Major advantages of this technique include retrospective optimization of contrast between arbitrary materials, rapid and systematic image analysis, and reduced scanning time; potential limitations include accuracy, noise, motion artifacts, and multicomponent behavior.