Coronary artery imaging: 3D segmented k-space data acquisition with multiple breath-holds and real-time slab following.

The purpose of this work was to develop a multiple-breath-hold (BH) imaging method for coronary arteries in which a segment of k-space is acquired in each BH. The goal was to increase the resolution, or the signal-to-noise ratio (SNR) and coverage, of three-dimensional-(3D)-BH volume-targeted scanning (VCATS). To correct for slab position differences, a real-time slab following technique using navigator echoes for motion detection was used. Sixteen normal volunteers were imaged to compare the method with a single-BH scan. Results showed that higher resolution, or larger coverage and higher SNR, were achieved by the multiple-BH method without respiratory motion artifacts. In conclusion, 3D segmented k-space data acquisition with multiple-BHs and real-time slab following is a promising approach for extending the capabilities of VCATS further.

Coronary arteries: three-dimensional MR imaging with retrospective respiratory gating.

Thin-section, three-dimensional (3D) gradient-echo magnetic resonance imaging of the coronary arteries was performed without and with retrospective respiratory gating in 12 healthy volunteers and one patient. In all examinations, results were improved with gating. In five of seven volunteer examinations, coronary artery delineation on images reconstructed by using the least-squares method for motion detection with navigator echoes was found to be equal to that obtained by using edge detection. Images in five other volunteers covered the entire heart with multiple overlapping 3D slabs. The arteries were segmented from the background and could be viewed from any orientation. The lengths of contiguously visible vessels were as follows: left main coronary artery, 11.5 mm +/- 0.4 (mean +/- standard deviation); left anterior descending branch, 115.9 mm +/- 19.7; left circumflex branch, 97.2 mm +/- 12.5; and right coronary artery, 125.9 mm +/- 18.8. This respiratory gating technique clearly improved depiction of the coronary arteries.

T2-weighted MR imaging of focal hepatic lesions: comparison of various RARE and fat-suppressed spin-echo sequences.

The authors prospectively compared four T2-weighted magnetic resonance (MR) sequences, including high-resolution 512 x 512 (matrix size) RARE (rapid acquisition with relaxation enhancement), 256 x 256 RARE, 128 x 256 breath-hold RARE, and 192 x 256 fat-suppressed spin-echo (T2FS) sequences, in the evaluation of 16 patients with focal hepatic masses. MR images were evaluated by quantitative lesion-liver signal difference-to-noise ratios (SDNRs) and subjective evaluation of image artifact and image quality. No significant differences were observed between RARE sequences in SDNR values. The T2FS sequence had a significantly higher SDNR than the 512 x 512 RARE sequence (24.6 +/- 15.0 vs 14.5 +/- 9.7) (P = .008). Image quality was rated highest for the 512 x 512 RARE and T2FS sequences (P = .006). The inherent advantage of high spatial resolution suggests that the 512 x 512 RARE sequence may be of value in detecting hepatic lesions.

Focal liver disease: comparison of dynamic contrast-enhanced CT and T2-weighted fat-suppressed, FLASH, and dynamic gadolinium-enhanced MR imaging at 1.5 T.

Dynamic contrast medium-enhanced computed tomography (CT), T2-weighted fat-suppressed spin-echo (T2FS) magnetic resonance (MR) imaging, and breath-hold T1-weighted fast low-angle shot (FLASH) MR imaging before and after dynamic gadopentetate dimeglumine injection were compared in 73 patients with clinically suspected liver disease. Observer confidence for presence of focal lesions was determined by using receiver operating characteristic analysis. For all MR images, hepatic lesion-liver signal-to-noise ratios were evaluated qualitatively. and resolution and presence of artifacts were evaluated qualitatively. Lesion detection was greatest with T2FS (n = 272) and enhanced FLASH (n = 244) and was statistically greater with both of these than with CT (n = 220) and FLASH (n = 219) (P less than .03). Correct lesion characterization was greatest with enhanced FLASH (n = 236) (P less than .01), followed by CT (n = 199), FLASH (n = 164), and T2FS (n = 144). Enhanced FLASH was particularly successful in characterization of 5-mm- to 1.5-cm-diameter lesions as cystic or solid.

Continuous distributions of NMR relaxation times applied to tumors before and after therapy with X-rays and cyclophosphamide.

In vivo measurements of T1 and T2 values in two experimental tumors growing in the legs of mice were made during tumor growth and after treatment of the tumor with either X-rays or cyclophosphamide. The T1 and T2 values were obtained by fitting the data to continuous distributions of relaxation times. This technique gives broad distributions of relaxation times which are characterized by a number of peaks with characteristic T1 and T2 values. Before treatment, the T1 and T2 values increased before a palpable tumor mass could be detected. The response to subcurative doses of either treatment method was a reduction in the T1 and T2 values and a parallel reduction in tumor weight. Although local recurrence was characterized by the same pattern of tumor growth as was observed before treatment, therapy was found to give higher relaxation time values than those measured in untreated tumors. The higher relaxation time values of tumor-bearing legs were the result of redistribution of the peaks in the distribution and not changes in the relaxation times of the individual peaks.

In vivo measurements of NMR relaxation times.

A series of solenoidal NMR probes were built to measure T1 and T2 relaxation times in vivo in the mouse, over the frequency range of 5 to 60 MHz, using inversion-recovery and spin-echo pulse sequences. KHT tumors growing in the legs of C3H mice were studied and compared with normal mouse legs. The tumor relaxation times were studied at 10 MHz during the course of tumor growth and as a function of frequency when the tumor had a mass of approximately 0.9 g. Mouse legs with tumors have higher T1 and T2 values than those without tumors over the frequency range of 5 to 60 MHz. Significant changes in both relaxation times were detected before a palpable mass could be detected. T1 contrast between normal and tumor-bearing legs decreased with increasing frequency, while T2 contrast remained nearly constant. A comparison between in vivo and in vitro measurements was done using four different types of sample preparation: live mouse, dead mouse, excised whole mouse leg, and tissue sample. These studies showed small but significant differences between the relaxation times measured in vivo and those measured in vitro.