Ultra-fast three dimensional imaging of hyperpolarized 13C in vivo.

OBJECTIVE: PASADENA, a chemical method of enhancing nuclear spin polarization has demonstrated 13C polarizations of order unity for the nascent products of molecular addition by parahydrogen. The extreme brevity of signal enhancement obtained by hyperpolarization requires improved 13C MR in vivo imaging techniques for their optimum utility. MATERIALS AND METHODS: 13C imaging sequences, including 13C 3D FIESTA, were compiled for a GE LX 1.5 T clinical MR scanner. Two water soluble 13C imaging agents were hyperpolarized utilizing parahydrogen and an automated polarizer. 13C polarization was quantified in flow phantoms and in rats with jugular vein catheters. RESULTS: Fast 3D FIESTA 13C MR imaging technique acquired sequential 3D images (3.66 s/acquisition) with superior SNR. Hyperpolarized 13C solutions and vascular phantoms achieved a maximum signal of 26,624+/-593. In vivo 13C MR images of the cardiopulmonary circulation showed maximum 13C signal of 2,402+/-158. 13C images acquired within 3.66 s showed signal enhancement over 10,000 compared to equilibrium polarization. CONCLUSION: 3D-FIESTA was effective for sub-second in vivo imaging of hyperpolarized 13C reagents produced in a custom-built parahydrogen polarizer. Application to 13C hyperpolarized by parahydrogen is demonstrated in vitro and in vivo.

Three-dimensional gadolinium-enhanced MR angiography of vascular complications after liver transplantation.

OBJECTIVE: The goal of this study was to evaluate three-dimensional gadolinium-enhanced MR angiography as a tool for examination of liver transplant patients with potential vascular complications. MATERIALS AND METHODS: Thirty-eight consecutive three-dimensional gadolinium-enhanced MR angiograms were obtained in 34 patients. Results were retrospectively reviewed and correlated with conventional angiography in 20 of the 38 cases and sonography in 37 of the 38 cases. MR angiograms were evaluated for technical adequacy, vascular patency, and parenchymal abnormalities, and results were compared with angiography and sonography. Conventional angiography and surgery were used as gold standards when available. RESULTS: Thirty-four (90%) of 38 MR angiograms were technically adequate. Vascular abnormalities were identified in 20 patients, and 19 of these patients subsequently underwent angiography, surgery, or both. There were seven cases of hepatic artery thrombosis; all were detected with MR angiography with no false-positive or false-negative interpretations. Seven patients had moderate to severe hepatic artery stenosis (>50% narrowing as determined by conventional angiography). MR angiography revealed this stenosis in six of the seven patients, with one false-negative and three false-positive interpretations. Portal vein thrombosis was detected in three patients, and portal vein stenosis was detected in two patients. CONCLUSION: Three-dimensional gadolinium-enhanced MR angiography is useful in the examination of liver transplant patients and offers a noninvasive adjunct in patients with difficult or indeterminate sonographic examinations.

Correlation of fine needle aspiration biopsy and dynamic contrast-enhanced 3D magnetic resonance imaging of the breast.

OBJECTIVE: To correlate and assess the utility of dynamic contrast-enhanced three-dimensional gadolinium-enhanced magnetic resonance imaging (Gd-3DMRI) and fine needle aspiration biopsy (FNAB) findings in patients with suspected breast disease. STUDY DESIGN: Beginning in 1993, all patients who underwent percutaneous FNAB of the breast and had concurrent Gd-3DMRI evaluation of the breast were selected for this study. Findings for FNAB and Gd-3DMRI were stratified into two categories, positive and negative. Subsequent clinical management decisions, which included surgical intervention and/or clinical follow-up, were recorded for all patients. RESULTS: There were 69 FNABs in 59 patients with corresponding Gd-3DMRI evaluation. A positive result by both FNAB and Gd-3DMRI was found in 15 of 18 malignant cases. FNAB missed one case, and Gd-3DMRI missed two, and each of these was thought to be technical. Combining the methods yielded 100% sensitivity. False positive results on Gd-3DMRI (17 cases) were all confirmed to be benign by FNAB and subsequent tissue evaluation. All 32 cases with combined negative results by FNAB and Gd-3DMRI demonstrated a benign process, yielding a specificity of 100% (32/32). CONCLUSION: Our combined testing modalities showed a high degree of specificity and good sensitivity. FNAB used with dynamic contrast-enhanced Gd-3DMRI can contribute valuable information for physicians treating patients with suspected breast abnormalities.

Half-Fourier, three-dimensional technique for dynamic contrast-enhanced MR imaging of both breasts and axillae: initial characterization of breast lesions.

PURPOSE: To evaluate the ability of asymmetric half-Fourier three-dimensional (3D) magnetic resonance (MR) imaging to characterize signal intensity changes in breasts and axillae after contrast material injection and to compare the spatial resolution and measured signal intensity change of asymmetric and symmetric (keyhole) partial Fourier techniques. MATERIALS AND METHODS: Imaging was performed in 28 adult patients by collecting a single full-Fourier 3D data set with bolus injection of contrast material during the last 10 seconds followed by collection of six half-Fourier 3D data sets without interimage delays. Postcontrast keyhole and half-Fourier images were formed from the same full-Fourier raw data set. RESULTS: The asymmetric half-Fourier 3D technique maintained the spatial resolution and lesion signal intensity of the full-Fourier image, whereas the 50% keyhole method degraded the spatial resolution by a factor of two and decreased the lesion signal intensity by 19% for a 2 x 2-pixel region of interest. Histopathologic correlation was attained in 32 lesions in 28 patients. Sensitivity was 100% (five of five) and specificity was 89% (24 of 27). CONCLUSION: The asymmetric half-Fourier 3D MR imaging technique allows imaging of both breasts and axillae without loss of lesion contrast or temporal resolution and provides the maximum spatial resolution and lesion signal intensity attainable for the views sampled.

Dynamic sequential 3D gadolinium-enhanced MRI of the whole breast.

Dynamic contrast-enhanced 2D MR imaging of the breast has shown high sensitivity and specificity for the detection and characterization of breast lesions. We investigated the ability of a dynamic fast 3D MR imaging technique that repeatedly scans the whole breast in 44-s intervals without an interscan delay time to obtain similar sensitivity and specificity as 2D imaging. Fifty-six patients scheduled for breast biopsy were entered into the study, and 83 lesions detected by 3D dynamic scanning were biopsied. Dynamic 3D contrast-enhanced breast imaging with subtraction detected and correctly classified all 23 cancers, and 44 of the 60 benign lesions yielding a sensitivity of 100%, a specificity of 73%, and a 100% predictive negative value. The enhancement profiles of metastatic lymph nodes were similar to those of primary cancer. This technique allowed detection of multifocal and multicentric lesions and did not require a prior knowledge of lesion location. These results indicate that dynamic contrast-enhanced 3D MRI of the whole breast is a useful and economically feasible method for staging breast cancer, providing a comprehensive noninvasive method for total evaluation of the breast and axilla in patients considering breast conservation surgery or lumpectomy.

Large angle spin-echo imaging.

A study was undertaken to assess the use of excitation flip angles greater than 90 degrees for T1 weighted spin-echo (SE) imaging with a single 180 degrees refocusing pulse and short TR values. Theoretical predictions of signal intensity for SE images with excitation pulse angles of 90-180 degrees were calculated based on the Bloch equations and then measured experimentally from MR images of MnCl2 phantoms of various concentrations. Liver signal-to-noise ratios (SNR) and liver-spleen contrast-to-noise ratios (CNR) were measured from breathhold MR images of the upper abdomen in 16 patients using 90 and 110 degrees excitation flip angles. The theoretical predictions showed significant improvements in SNR with excitation flip angles > 90 degrees, which were more pronounced at small TR values. The phantom studies showed reasonably good agreement with the theoretical predictions in correlating the excitation pulse angle with signal intensity. In the human imaging studies, the 110 degrees excitation pulse angle resulted in a 7.4% (p < .01) increase in liver SNR and an 8.2% (p = .2) increase in liver-spleen CNR compared to the 90 degrees pulse angle at TR = 275 ms. Increased signal intensity resulting from the use of large flip angle excitation pulses with a single echo SE pulse sequence was predicted and confirmed experimentally in phantoms and humans.

Hybrid DANTE and phase-contrast imaging technique for measurement of three-dimensional myocardial wall motion.

Characterization of myocardial stress and strain is necessary for a complete understanding of myocardial function. The precise quantification of regional myocardial strain is complicated by its time-varying pattern and regional variation resulting from the anisotropy of the myocardium and by complex torsional and shortening motions of the heart during the cardiac cycle. The authors have developed a technique for point-specific tracking of myocardial motion along all three axes in a constant selected section of myocardium by combining prospective section selection with in-plane DANTE (delays alternating with nutations for tailored excitation) tissue tagging and phase-contrast detection of motion perpendicular to the image plane. With this technique, it is possible to determine point-specific myocardial strain values in vivo.

Tracer-kinetic analysis for measuring regional cerebral blood flow by dynamic nuclear magnetic resonance imaging.

Measurement of regional cerebral blood flow in vivo has proved useful in the study of normal and diseased states in the brain. This circumstance has led to a variety of techniques for its quantitative determination and has continued to motivate the search for ever safer and more accurate methods of measurement. Recently, the use of nuclear magnetic resonance (NMR) in medical imaging has stimulated efforts to make it the basis for a non-invasive method of measuring flow in the brain. New advances in fast NMR imaging (MRI) provide data potentially amenable to analysis by tracer-kinetic methods. Such an analysis has not previously been available. In this paper we present theoretical results that may permit measurement of brain blood flow by NMR. The data interpreted by our model are those generated by a novel MRI protocol developed by Perman et al. (1992, Magn. Reson. Med. 28, 74-83; Radiology 185(P), Abstr. 154, 127) that is entirely compatible with existing routine MRI procedures. These data are fast dynamic NMR signals that reflect passage of an intravenously administered paramagnetic contrast agent serving as a plasma tracer. Our equations show how to use such data sequences to determine plasma mean transit time, plasma volume, and plasma and whole-blood flow in arbitrarily selected regions of interest in the brain. The theory accounts rigorously for recirculation of tracer to the imaged regions. Our analysis provides an explanation for the linear relationship observed experimentally by others between regional vascular volumes and time integrals of vascular-tracer residue curves, and shows that this relationship remains valid in the presence of tracer recirculation.

An experimental method for evaluating constitutive models of myocardium in in vivo hearts.

A new experimental method for the evaluation of myocardial constitutive models combines magnetic resonance (MR) radiofrequency (RF) tissue-tagging techniques with iterative two-dimensional (2-D) nonlinear finite element (FE) analysis. For demonstration, a nonlinear isotropic constitutive model for passive diastolic expansion in the in vivo canine heart is evaluated. A 2-D early diastolic FE mesh was constructed with loading parameters for the ventricular chambers taken from mean early diastolic-to-late diastolic pressure changes measured during MR imaging. FE solution was performed for regional, intramyocardial ventricular wall strains using small-strain, small-displacement theory. Corresponding regional ventricular wall strains were computed independently using MR images that incorporated RF tissue tagging. Two unknown parameters were determined for an exponential strain energy function that maximized agreement between observed (from MR) and predicted (from FE analysis) regional wall strains. Extension of this methodology will provide a framework in which to evaluate the quality of myocardial constitutive models of arbitrary complexity on a regional basis.

A fast 3D-imaging technique for performing dynamic Gd-enhanced MRI of breast lesions.

The characterization of breast lesions by their Gd-enhancement profiles has been proposed as a method for differentiating benign from malignant breast lesions. The limitations of dynamic contrast enhanced 2D imaging of the breast are the low number of slices that can be acquired, and the need to know the location of the lesion a priori to correctly select the noncontiguous 2D slice locations. These problems are exacerbated when multi-focal disease is present but not anticipated. Standard fast 3D gradient-echo imaging has a variable delay between successive acquisitions. We have developed a fast 3D gradient-echo imaging technique for dynamic Gd-DTPA enhanced breast imaging which obtains multiple 3D image sets of 32 contiguous images at 44 s intervals without an interscan delay time. This rapid 3D imaging technique achieves good temporal resolution and reduces patient motion between pre- and postcontrast images while covering a much larger portion of the breast and eliminating the need for a priori knowledge concerning the location of the lesion(s) when performing Gd-enhanced dynamic MR imaging.

A half-Fourier gradient echo technique for dynamic MR imaging.

Recently we developed the simultaneous dual FLASH (SDFLASH) pulse sequence that simultaneously obtains sequential images from the brain and the internal-carotid arteries in the neck with 1-sec temporal resolution using a standard MR scanner. The high temporal resolution (1 sec) of the SDFLASH technique was achieved partly by using a low number of phase-encoding views which thereby limited our in-plane spatial resolution to 6.25 x 3.12 mm pixels. To overcome this limitation we have developed a calibration technique which corrects distortions in signal intensity and object shape when using gradient echo half-Fourier spin warp imaging. Using this calibration technique, the operator can use the 41% decrease in scan time to either double the spatial or temporal resolution. We have successfully used this technique to acquire SDFLASH images of the head and neck with 1.0 sec temporal resolution and 3.12 x 1.6 mm spatial resolution.

Simultaneous MR acquisition of arterial and brain signal-time curves.

Regional cerebral blood flow (rCBF) provides important information about local neuronal functional and cerebrovascular status. Determination of rCBF requires sequential measurements of tracer concentration in arterial blood and brain tissue unless the tracer is trapped in the brain in proportion to rCBF. Since gadopentate dimeglumine is not trapped within brain tissue, we have developed the simultaneous dual FLASH pulse sequence (SDFLASH) which sequentially measures the MR signal change in both the internal carotid artery and brain parenchyma simultaneously during the passage of a bolus of paramagnetic contrast material.

Pressure-gated acquisition of cardiac MR images.

Electrocardiographically gated magnetic resonance (MR) image acquisition is not optimal for the quantification of in vivo cardiac deformation, because of the cycle-length dependence of cardiac mechanical events. The authors developed a method for acquisition of cardiac MR images gated to the first derivative of left-ventricular-developed pressure and used the method in a canine model. Application of this method may improve myocardial stress-strain analyses.

Evaluation of two new gadolinium chelates as contrast agents for MRI.

Two new gadolinium chelates were investigated for potential use as tissue-specific contrast agents for magnetic resonance imaging. In vitro measurements of stability constants, octanol/water partition coefficients and relaxation times in solutions of water and human serum albumin (HSA) were performed with each new chelate and compared with gadolinium-diethylenetriamine pentaacetic acid, Gd(DTPA). Biodistribution studies and magnetic resonance imaging in rats were used to evaluate the new chelates in vivo. The stability constants (log K) of gadolinium-N,N''-bis(3-hydroxy-6-methyl-2- pyridylmethyl)diethylenetriamine-N,N',N''-triacetic acid, Gd(DTTA-HP), and gadolinium-1,7,13-triaza-4,10,16-trioxacyclooctadecane-N,N', N''-triacetic acid, Gd(TTCT), were determined to be 23.65 and 18.07, respectively. These can be compared to a literature value of 22.46 for Gd(DTPA). Octanol/water partition coefficients for both complexes showed they were more lipophilic than Gd(DTPA). Gd(DTTA-HP) exhibited a smaller relaxivity in water but a larger relaxivity in 4% HSA than Gd(DTPA). Gd(TTCT) exhibited a lower relaxivity than Gd(DTPA) in both water and 4% HSA. Both complexes showed similar biodistributions to Gd(DTPA) no carrier-added concentrations. Gd(DTTA-HP) had a greater percent change in signal intensity than Gd(DTPA) on T1-weighted spin-echo images in the heart, liver, and kidney. Percent change in signal intensity for Gd(TTCT) was lower than Gd(DTPA) in heart, liver, and kidney.

Mathematical modeling of the heart using magnetic resonance imaging.

A hybrid three-dimensional solid mathematical model of cardiac ventricular geometry developed using magnetic resonance (MR) images of an in vivo canine heart is discussed. The modeling techniques were validated using MR images of an ex vivo heart and direct measurements of cardiac geometry and mass properties. A spin-echo MR sequence with in-plane resolution of 1.0 mm was used to image the canine heart in eleven short-axis planes at contiguous 5-mm intervals. Contour points on the epicardial, left ventricle (LV), and right ventricle (RV) boundaries were selected manually at each slice level. A boundary representation geometric model was constructed by fitting third-order nonuniform rational B-spline surfaces through each set of surface points. Compared to the anatomic specimen (AS), volume errors of the ex vivo model were 0.3, 1.5, and 5.8% for the LV cavity, RV cavity, and total enclosed volumes, respectively. Comparison of cross-sectional areas of the AS and the model at ten levels demonstrated mean model errors of 4.1, 2.5, and 2.9% for the LV, RV, and epicardial boundaries, respectively.

Regional myocardial stress distribution from magnetic resonance image-based mathematical models.

The instantaneous regional stress distribution within the myocardium, which cannot be directly measured, has been estimated using improved numerical methods and nonaxisymmetric biventricular geometry. To do this, we have employed computer-aided solid mathematical modeling to generate a three-dimensional representation for an ex vivo canine biventricular unit using magnetic resonance imaging. A two-dimensional transverse section was isolated from the solid mathematical model for regional stress analysis using p-version finite element analysis. Loading conditions and material property descriptions were taken from published reports. Analyses showed the maximum principal stresses to range from -1.76 X 10(5) to 8.52 X 10(5) dynes/cm2 during systolic loading, and from -3.85 X 10(4) to 1.13 X 10(5) dynes/cm2 during diastolic loading. This study demonstrates that magnetic resonance image-based solid mathematical biventricular models are suitable for regional stress analysis using p-version finite element analysis. p-Version finite element analysis using magnetic resonance image-based cardiac representations facilitates in vivo stress-strain analyses and may allow the clinical estimation of regional myocardial stress.

Brain surface cortical sulcal lengths: quantification with three-dimensional MR imaging.

The repeatability and accuracy of brain surface cortical sulcal length measurements obtained with three-dimensional (3D) reconstructions of volumetric, gradient-echo magnetic resonance (MR) images were tested. The brains of eight healthy adult volunteers and one cadaver were imaged in both the coronal and sagittal planes to yield a set of 128 1.5-2.0-mm-thick contiguous sections. 3D reconstructions of the brain cerebral cortical surfaces were obtained with computer software. Location and distance measurements of surface sulci were repeated on each reconstructed image. The same structures in the cadaver brain were independently measured with a 3D electromagnetic digitizer to validate the results of the 3D MR imaging method. All measurements from reconstructed images had high repeatability, and there were no statistically significant differences between measurement trials. The accuracy of measurements with 3D MR imaging was also good; the mean difference between digitizer and 3D MR measurements for sulcal lengths was 0.81 cm (average, 5.45-12.9 cm).

Liver imaging at 1.5 tesla: pulse sequence optimization based on improved measurement of tissue relaxation times.

In order to predict the most sensitive MR imaging sequence for detecting liver metastases at 1.5 T, in vivo measurements of T1 and T2 relaxation times and proton density were obtained using multipoint techniques. Based on these measurements, two-dimensional contrast contour plots were constructed demonstrating signal intensity contrast between hepatic lesions and surrounding liver parenchyma for different pulse sequences and pulse timing parameters. The data predict that inversion recovery spin echo (IRSE) imaging should yield the greatest contrast between liver metastases and liver parenchyma at 1.5 T, followed by short tau inversion recovery (STIR) and spin-echo (SE) pulse sequences. T2-weighted SE images provided greater liver/lesion contrast than T1-weighted SE pulse sequences. Calculated T1, T2, and proton density values of the spleen were similar to those of hepatic metastatic lesions, indicating that the signal intensity of the spleen may be used as an internal standard to predict the signal intensity of hepatic metastases on T1- and T2-weighted images at 1.5 T.

Rapid acquisition spin-echo (RASE) MR imaging: a new technique for reduction of artifacts and acquisition time.

The rapid acquisition spin-echo (RASE) technique combines a short repetition time, a short echo time, and a single excitation pulse sequence with half-Fourier data sampling. This allows for acquisition of 11 strongly T1-weighted sections during a single 23-second breath-holding period. Measurements obtained from volunteers and with phantoms reveal that RASE images have a lower signal-to-noise ratio and contrast-to-noise ratio than do conventional multiacquisition spin-echo (SE) images due to reduced data acquisition. However, liver-spleen contrast and spatial resolution are not affected. Moreover, contrast-to-artifact (C/A) measurements are 77% greater with RASE. When normalized for imaging time, all parameters are significantly higher with RASE, with a C/A per unit time that was 338% higher. Randomized, blinded review of RASE and SE sequences from 20 patients was conducted to evaluate qualitative performance. Excellent to good performances for phase-encoding artifact reduction, edge sharpness, and overall image quality were recorded for 89%, 88%, and 86% of RASE examinations, respectively, versus 41%, 59%, and 47% of conventional SE examinations, respectively. All results were statistically significant with P less than .001. RASE is an easily implemented imaging technique that utilizes widely available existing technology. Its major benefits relate to significant reduction in imaging time, elimination of respiratory artifacts, and the potential for performing dynamic contrast material-enhanced screening examinations.