Regional 1H transverse magnetization studies in perfused rabbit kidney

Triexponential decomposition of 1H spin-lattice relaxation decay curves of paramagnetically doped red cell suspensions at 7 T

The spin-lattice relaxation behaviour of protons in paramagnetically doped red cell suspensions at two haematocrit levels has been studied using high-field (7 T) inversion recovery data. The resulting relaxation decay curves were found to be best characterized by triexponential functions. The two-site exchange model, generally applied to biexponential relaxation data in calculating red cell water exchange parameters, was applied to the two fastest-relaxing fractions of the triexponential fits. The intracellular to extracellular water exchange rates so obtained were in good agreement with literature values. Additional exchange parameters including intracellular and extracellular water volume fractions and extracellular relaxation rates were also calculated directly from the relaxation data and found to be consistent with sample haematocrits and with independent relaxation rate measurements of the resuspension medium. The small-amplitude, slowly relaxing third component recovered from the triexponential fits of the relaxation data is attributed to intracellular haemoglobin protons.

In vivo characterization of cytotoxic intracellular edema by multicomponent analysis of transverse magnetization decay curves

RATIONALE AND OBJECTIVES

We investigated the multicompartmental nature of T2 decay in a specific white matter edema model.

METHODS

Triethyltin (TET) intoxication was produced in six male New Zealand White rabbits. Images were obtained over the 23-day study duration using a 64-echo Carr-Purcell-Meiboom-Gill (CPMG) sequence (repetition time = 3000 msec, echo time = 20 msec). T2 decay curves were extracted from 0.7 x 0.7 x 3.0 mm3 voxels in the corpus callosum and contiguous white matter tracts, cortex, thalamic nuclei, hypothalamic nuclei, and the masseter muscles. The curves were fit with biexponential functions.

RESULTS

Increased signal intensity in the corpus callosum was evident 2-3 days after the first TET injection. At this time, a substantial slowly relaxing component appeared in the decay curves of the corpus callosum and, to a lesser extent, in the thalamus and hypothalamus. Changes in the rabbits' body weight, general physical condition, and neurologic state paralleled the growth and regression of the second, slowly relaxing component.

CONCLUSION

The appearance and regression of a slowly decaying second component in the T2 decay curve is consistent with the formation and shrink-age of intracellular vesicles in the intramyelin sheaths of central white matter.

In vivo NMR imaging and spectroscopic investigation of renal pathology in lean and obese rat kidneys

Diabetic nephropathy is a major cause of end-stage renal failure. While our understanding of the pathogenesis of nephropathy is incomplete, progressive glomerular injury appears to play a significant role in the decline of renal function. Proton NMR spectroscopy and imaging techniques were used to address changes in renal pathology associated with glomerular mesangial expansion in vivo in kidneys from spontaneously obese and lean (control) littermate Zucker rats. Fully functioning rat kidneys were surgically exposed and externalized for direct NMR signal detection via a coil placed around the organ. High-resolution (78 microns in plane) proton images were obtained at 4.7 T magnetic field strength revealing fine structure within the well-defined cortical and medullary regions. The obese rat kidney images were distinct in appearance from the lean kidney images and exhibited marked cortical expansion as well as increased overall kidney size. Enlargement of mean glomerular diameter was verified histologically in the obese kidneys as compared with the lean kidneys. Proton T1 and T2 relaxation times were determined from the entire kidney using standard spectroscopic techniques, and from specific regions within the kidney from multiple T1- and T-2 weighted images. Additionally, image contrast enhancement resulting from saturation transfer between protons in restricted-mobility environments and mobile water protons within the kidney was investigated in the lean and obese rat kidneys using magnetization-transfer imaging techniques. At the early stage of renal injury examined in this study, diseased and healthy kidneys could not be differentiated on the basis of relaxation times alone. The magnitude of saturation transfer obtained in cortical tissue in the lean and obese kidneys was also not statistically significantly different. However, the magnitude of saturation transfer achieved in the medullary tissue of obese kidneys was statistically significantly less than that achieved in lean kidneys.