Diffusion-weighted imaging with navigated interleaved echo-planar imaging and a conventional gradient system

PURPOSE

To demonstrate the technical feasibility and precision of a navigated diffusion-weighted (DW) MR imaging method with interleaved echo-planar imaging and test its diagnostic sensitivity for detection of ischemic stroke.

MATERIALS AND METHODS

Apparent diffusion coefficient (ADC) measurements were performed in phantoms, and six healthy adult volunteers were examined to determine intrasubject (precision) and intersubject (reference range) variations in absolute ADC and relative ADC (rADC) measurements. DW imaging maps and lesion rADC values were also obtained in 34 consecutive stroke patients to evaluate the sensitivity and reliability of DW-interleaved echo-planar imaging for detection of ischemic brain damage.

RESULTS

Phantom and volunteer ADC values were in excellent agreement with published data. The intrasubject variation of rADC was 6.2%. The ADC precision ranged from 6.5% in the subcortical white matter in the frontal lobe to 12.9% in the head of the caudate nucleus. Interleaved echo-planar imaging enabled rapid acquisition of high-quality images of the entire brain without substantial artifacts. Within the 1st week, the sensitivity of DW-interleaved echo-planar imaging for detection of acute infarction was 90% (18 of 20 true-positive studies) and independent of lesion location.

CONCLUSION

DW-interleaved echo-planar imaging with phase navigation and cardiac triggering is robust, reliable, and fast. With high sensitivity for detection of early ischemic infarction, it is useful for examining stroke patients by using MR systems with conventional gradient hardware.

5'-N-substituted carboxamidoadenosines as agonists for adenosine receptors

Novel as well as known 5'-N-substituted carboxamidoadenosines were prepared via new routes that provided shorter reaction times and good yields. Binding affinities were determined for rat A1 and A2A receptors and human A3 receptors. EC50 values were determined for cyclic AMP production in CHO cells expressing human A2B receptors. On all receptor subtypes relatively small substituents on the carboxamido moiety were optimal. Selectivity for the A3 receptor was found for several analogues (1a, 1d, 1h, and 1k). On A1 receptors a number of compounds, but not 5'-N-ethylcarboxamidoadenosine (NECA, 1b), showed small GTP shifts, which could be indicative of lower intrinsic activities at the A1 receptor. At the A2B receptor, derivatives 1i-k with modified ethyl substituents had reduced activities compared to the A2B reference agonist NECA (1b). Thiocarboxamido derivatives (8b and 8c) displayed considerable although decreased A2B receptor activity. The X-ray structure determination of compound 8b was carried out. Due to intramolecular hydrogen bonding between the carboxamido NH and the purine N3 in the crystal structure, the ribose moiety of this compound is in a syn conformation. However, theoretical calculations support that NECA (1b), and less so 8b, can readily adopt both the syn and the anti conformation, therefore not excluding the proposed anti mode of binding to the receptor.

Liver iron quantification: studies in aqueous iron solutions, iron overloaded rats, and patients with hereditary hemochromatosis

For the noninvasive liver iron quantification by MRI in human iron overload diseases, fundamental proton relaxation mechanisms were studied in aqueous solutions with ferritin and other iron compounds, in experimentally iron overloaded rats, and in patients with iron overload diseases. MR-relaxation rates as a function of iron concentrations in the range of 0-7.5 mg Fe/g aqueous iron solutions, 0-5.4 mg Fe/g rat liver in vivo, and 0.16-4.9 mg Fe/g human liver in vivo were determined from multi- and sets of single-spin echo sequences (1.5 T imager). As predicted by theory, transverse relaxation rates (1/T2) in aqueous iron solutions, in liver tissue of rats, and in human liver tissue increased linearly with the iron concentration. A preliminary calibration for the liver iron quantification by MRI was performed from in vivo measurements of liver 1/T2-relaxation rates and liver iron quantification by atomic absorption spectroscopy in biopsies from 13 patients. With the single spin-echo method, precise in vivo liver iron quantification in humans also above 2.0 mg Fe/g liver tissue (T2 < 15 ms) should be accomplished on any imager with shortest spin-echo time available, at least TE < 20 ms.

Stereochemistry of charged nitrogen-aromatic interactions and its involvement in ligand-receptor binding

Recently, new evidence was found for the involvement of charged nitrogen-aromatic interactions in ligand-receptor binding. In this study we report two favourable orientations of a phenyl ring with respect to a R-N+(CH3)3 group, based on crystal structure statistics from the Cambridge Structural Database. In the first orientation, the phenyl ring is situated in between the substituents at about 4.5 A from the nitrogen atom, and the ring is approximately oriented on the sphere around the nitrogen atom. In the second orientation, the phenyl ring is situated in the same direction as one of the N-C bonds at about 6.0 A from the nitrogen atom, and the ring is tilted with respect to the sphere around the nitrogen atom. The same two orientations were also found in the crystal structures of three ligand-receptor complexes, which implies that these orientations probably play a major role in molecular recognition mechanisms.

[Measurement of noise levels in MR tomography at 1.5 Tesla]

The noise levels at the isocentrum of an MR apparatus with a field strength of 1.5 Tesla was measured during various sequences. The rapid switching of the gradients exerts effects on the gradient coils, which result in the development of noise as from a loudspeaker. Measurements were carried out using gradients of 3 and 9.3 mT/m with head and body coils and a variety of sequences. The noise level depended largely on the type of sequence and, under extreme conditions, may attain 96 dB. Because of the short exposure times, we do not consider it necessary to use any form of protection. During the examination of patients with damaged hearing, some protection may be advisable.

[Cine-MRT for studying regional myocardial function]

Cine-MRT was performed in 8 healthy subjects and in 16 patients with myocardial infarction, in whom regional wall motion abnormalities had been demonstrated by left ventricular angiography. Short axis and long axis slices were obtained by angulation of the imaging planes according to the orientation of the cardiac axes. From the end-diastolic and end-systolic images, systolic wall thickening and radial wall motion were determined. In healthy subjects, regional variation in wall thickening was not significant. Patients with myocardial infarction demonstrated abnormal wall motion within the infarct region. Discrepancies between wall thickening and wall motion were observed in 8 patients.

[High-resolution MRT with increased magnetic field gradients and a 512(2)-data matrix: initial clinical experiences]

By increasing the strength of the magnetic field gradients of a whole-body imager to 9.4mT/m, it was possible to obtain spin-echo images with short echo delays and high spatial resolution. The reduction of the pixel size was effected either by decreasing the field of view or by increasing the scan matrix (512(2) pixel). To prevent loss of signal-to-noise ratio, the scan parameters had to be carefully selected. With the 512(2) matrix, the resolution could be increased without decreasing the field of view, so that backfolding artifacts were avoided. In comparison with the 256(2) matrix, the 256 x 512 matrix provided improved resolution in the frequency encoded direction without an increase in acquisition time.