PubMed Health⌕ Search

Biomedical subjects

G E Santyr

Publications and source records attributed to G E Santyr.

14 recordsLinked to original sources
PubMed1998

Rapid in vivo measurement of single-kidney extraction fraction and glomerular filtration rate with MR imaging.

PURPOSE: To compare the accuracy of a noninvasive magnetic resonance (MR) imaging method to rapidly determine single-kidney glomerular filtration rate (GFR) relative to findings with an accepted standard of reference. MATERIALS AND METHODS: Simultaneous inulin and MR imaging measurements of renal excretory function were performed in six small swine. Renal extraction fraction of gadopentetate dimeglumine was determined with T1 measurements of flowing blood in the renal vein and in a systemic vessel 10-300 minutes after administration. These data were correlated with standard inulin-derived measurements of single-kidney GFR obtained during the same intervals as the MR imaging measurements. Gradient-echo (GRE) and echo-planar imaging were investigated for in vivo measurement of extraction fraction. RESULTS: The MR imaging-derived measurements of extraction fraction and single-kidney GFR were not significantly different from the inulin-determined measures (P > .18). The MR imaging extraction fraction and single-kidney GFR measurements correlated with inulin-derived measurements of the same parameters (r = .74 to .89, P < .0007). CONCLUSION: T1 measurement methods with GRE and echo-planar imaging are acceptable techniques with which to measure renal excretory function in a rapid and noninvasive manner in this model. Rapid measurements of single-kidney GFR should enable studies of the response of renal hemodynamics to pharmaceutical manipulation.

Animals↗
PubMed1997

Rapid measurement of Gd-DTPA extraction fraction in a dialysis system using echo-planar imaging.

Gd-DTPA (Magnevist, Berlex, Wayne, NJ) extraction fractions (EF) have been measured for three dialysis filter types using an echo-planar imaging (EPI) Look-Locker T1 measurement technique under conditions of fast and slow flow. The mean EF measured in Fresenius (Bad Homburg, Germany) F3, F6, and F8 dialysis filters were 0.015 +/- 0.005, 0.053 +/- 0.004, and 0.084 +/- 0.003, respectively, under conditions of fast flow which provided complete refreshment of spins in the intervals between read-out pulse samples of the T1 relaxation recovery. Data acquisition and post-processing techniques were developed to extend the accuracy of the LL technique to systems with slow flow which did not provide complete refreshment of spins between samples of the T1 recovery. A multi-shot EPI LL interleaved acquisition of relaxation recovery space (IRRS) provided T1 measurement accuracy comparable to the refreshed system, +/- 10, but at the expense of increased scan times (factor of 2 or 3). Discarding the first few non-equilibrium relaxation recovery samples from the T1 fit allowed accurate T1 estimation (+/- 10) with a single-shot EPI LL method under conditions of slow flow. These EPI LL EF measurement methods may provide useful techniques for evaluating renal function in vivo.

Dialysis↗
PubMed1996

Measurement of Gd-DTPA dialysis clearance rates by using a look-locker imaging technique.

The exponential clearance rate constant, (kappa), and filtration fraction (FF) have been measured for dialysis of Gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA) (Magnevist, Berlex, Wayne, NJ) solutions by using a Look-Locker imaging technique under conditions of flow. The measured values of kappa for the Baxter CA-50, CA-110, and CA-210 filters were 0.0037 +/- 0.0003, 0.0057 +/- 0.0017, and 0.0092 +/- 0.0018 min-1, respectively, for dialysis of 4.0 liters of aqueous Gd-DTPA solutions. The measured values of FF for the Baxter CA-110 and CA-210 filters were 0.060 +/- 0.013 and 0.089 +/- 0.015, respectively, for dialysis of aqueous Gd-DTPA at 350 ml/min. The kappa and FF measurements agree with values that use inversion recovery (IR) on static samples obtained by drawing aliquots of solution during the course of dialysis. This in vitro experiment suggests that accurate in vivo measurements of filtration fraction and glomerular filtration rate (GFR) may be possible.

Filtration↗
PubMed1994

Analysis of discrete T2 components of NMR relaxation for aqueous solutions in hollow fiber capillaries.

An analysis is presented of proton NMR T2 relaxation times measured for aqueous solutions in simple bundles of hollow fibers. The relaxation times are calculated with a two-compartment diffusive exchange model using the known relaxation times of the aqueous solutions and the fiber geometry. When the relaxation time outside the fibers is short (approximately 1 ms), three or more relaxation components are observed from this two compartment system, in agreement with the calculation. The amplitude and relaxation times of the third component are consistent with those of a diffusion-mediated mode, as suggested theoretically by Brownstein and Tarr (Phys. Rev. A 19, 2446 (1979)). The possible contribution of such modes to the multicomponent relaxation observed in tissues is discussed.

Animals↗
PubMed1994

Off-resonance spin locking for MR imaging.

Off-resonance spin locking is investigated as a low power method for achieving low field spin-lattice relaxation contrast using high field clinical MR imaging systems (e.g., 1.5 tesla). Spin-lattice relaxation times and equilibrium magnetizations in the off-resonance rotating frame (T1 rho(off), beta) were measured for tissue-mimicking phantom materials as a function of the ratio of the amplitude to the resonance offset of the spin-locking pulse (f1/delta). The phantom materials consisted of vegetable oil to simulate fat and two different gels containing 2% and 4% agar to simulate nonfatty tissues with different macromolecular compositions. These measurements were used to verify a signal strength equation for a multislice off-resonance spin-locking technique implemented on a clinical MR imaging system operating at 1.5 tesla. Although the oil showed little change in image contrast with increasing f1/delta, the two gels demonstrated a strong variation which provided improved discrimination compared to T1-weighted imaging. Off-resonance spin locking is suggested as a method for improving delineation of breast lesions and a preliminary clinical example from a patient volunteer is presented.

Breast Neoplasms↗
PubMed1994

MR imaging of the breast. Imaging and tissue characterization without intravenous contrast.

The development of noninvasive techniques should always be an important goal of diagnostic radiology. Despite the recent excitement regarding intravenous MR imaging contrast agents (e.g., Gd-DTPA), the role of noncontrast MR imaging for the breast should not be overlooked. First, it must be kept in mind that administration of an intravenous contrast material is an invasive procedure that poses some health concern and may not be acceptable to certain patients. Second, injection of intravenous contrast media is an expensive procedure that incurs costs related to the contrast media itself as well as from the personnel required to administer the media and monitor any adverse reactions. Third, there may be specific imaging situations in which a lesion must be located prior to administration of contrast media, for example, when dynamic contrast enhanced imaging is to be performed and temporal resolution limits the volume of coverage to only a few slices. Finally, noncontrast MR imaging techniques also may be useful in their own right for obtaining information not otherwise available from a contrast-enhanced study, particularly quantitative relaxometric and spectroscopic measurements that may be useful for tissue characterization. It should also be noted that optimized noncontrast MR methods also might be used in conjunction with contrast agents to improve overall clinical utility. The heterogeneous nature of the breast and the overlap in T1 and T2 between different normal and abnormal breast tissue types frequently renders conventional noncontrast breast MR images confusing and ambiguous. Hybrid imaging approaches combining T1 weighting, T2 weighting, and fat suppression techniques as well as multivariate image analysis may be helpful for improving the sensitivity of MR imaging to breast disease; however, specificity remains a problem. Recent advances in clinical MR imaging have provided the opportunity to explore other approaches which may shed new light on the characterization of breast disease including NMRD, spin locking, magnetization transfer, and multicomponent relaxation analysis. MR imaging derived morphologic factors (i.e., lesion spiculation, parenchymal patterns) also may have a role to play in breast disease diagnosis and risk assessment. Several avenues of future development of noncontrast breast MR imaging would seem fruitful. NMRD and spin locking results suggest the potential contrast benefit of low field imaging of the breast, particularly, with the probable development of open concept low field MR imaging devices, which will provide improved access and low cost. In vivo relaxation time measurements, particularly multi-component T2 analysis and incorporation of MTC and chemical shift imaging techniques, may improve breast MR image specificity.(ABSTRACT TRUNCATED AT 400 WORDS)

Breast↗
PubMed1993

Magnetization transfer effects in multislice MR imaging.

A theoretical model is presented which describes the effects of magnetization transfer in multislice MR imaging of a tissue-mimicking phantom composed of cross-linked agar gel. The model is successful in explaining differences between single and multislice image signal intensities observed for the agar gel but not seen in a simple aqueous solution. Magnetization transfer leads to a reduction in the image signal intensity of a slice of interest due to off-resonance RF irradiation arising from 90 degrees and 180 degrees pulses intended for neighboring slices. The contribution of magnetization transfer to multislice MR imaging depends on the amount of off-resonance RF irradiation during the imaging sequence repetition interval. For the tissue-mimicking agar gel, conventional spin-echo multislice imaging gave rise to a negligible image signal intensity reduction (< or = 2%); however, fast spin-echo (FSE) imaging, which employs up to 16 times as many RF pulses per slice, exhibited as much as a 13% reduction in image signal intensity (13 slices). The reduction in multislice image signal intensity due to magnetization transfer is sample specific and is shown to be more dramatic for in vivo human leg muscle (10% for conventional spin echo, 40% for FSE) where magnetization transfer rates are greater than in the cross-linked agar gel.

Agar↗
PubMed1989

Spin locking for magnetic resonance imaging with application to human breast.

The dependence of rotating frame spin-lattice relaxation, T1 rho on locking field frequency, f1, was measured for phantom materials and human breast tissues. These data were used to predict the relative signal strengths obtainable in a spin-locking imaging sequence. This imaging sequence was implemented on a 0.15-T imaging system and measurements of phantom and tissue signal strength for various imaging parameters agreed with predicted signal strengths. Compared to T1 and T2, T1 rho appears to have unique capability to distinguish tumor from normal fat and fibrous breast tissues. The applications of T1 rho to tissue characterization and imaging at high static field strengths are discussed.

Adipose Tissue↗
PubMedDate not supplied

Pulsed magnetization transfer contrast for MR imaging with application to breast.

The relative populations and transverse relaxation times of the solid-like hydrogen pool (PB and T2B) and the magnetization transfer (MT) rates between the solid-like and liquid-like hydrogen pools (kappa) have been determined for three different agar gel concentrations (2%, 4%, and 8% by weight) as well as excised fibroglandular breast tissue specimens. PB was determined to be .003(.001), .01(.002), .02(.01), and .06(.01); T2B was determined to be 13.0(.2), 14.0(.1), 14.5(.1) and 15.2(1.3) microseconds; and kappa was determined to be 0.78(.01), 1.15(.02), 2.00(.02), and 3.55(1.5) sec-1 for the 2%, 4%, and 8% agar gels and the fibroglandular tissue, respectively. The image signal intensities of a pulsed MTC-prepared gradient-echo imaging technique are predicted using these MT parameters and are shown to agree well with experimental data obtained from a clinical MR imaging system. This technique is shown to suppress signal intensity of fibroglandular breast tissue by 40%-50% without exceeding SAR limits (< or = 8 W/kg) and is helpful for visualizing lesions and silicone implants.

Breast↗
PubMedDate not supplied

Application of a quantitative model to differentiate benign from malignant breast lesions detected by dynamic, gadolinium-enhanced MRI.

Both benign and malignant breast lesions may exhibit intense contrast enhancement when imaged using gadolinium-enhanced MRI. We propose a quantitative approach for fitting dynamic signal intensity (SI) data that may distinguish benign from malignant lesions. We studied 78 lesions in 75 women (18 malignancies, 16 fibroadenomas, and 44 other benign breast lesions) to determine the potential of this model for decreasing false-positive MR results. Twenty-eight lesions showed no enhancement; all were benign. One lesion showed a complex pattern not amenable to region-of-interest analysis and was considered a false positive. SI versus time data for the remaining 49 lesions were fit to the proposed model. We found that one parameter, M, the normalized slope of the SI enhancement profile evaluated at half the maximal signal intensity, seemed to be highly correlated with malignancy and offered improved discrimination between malignant and benign lesions compared to a previously published two-point slope method.

Breast Diseases↗
PubMedDate not supplied

Limitations of the keyhole technique for quantitative dynamic contrast-enhanced breast MRI.

The effect of keyhole data acquisition on quantitative analysis of dynamic MRI was examined. Experiments were performed retrospectively on raw data obtained from clinical dynamic contrast-enhanced breast imaging procedures. The effects of keyhole phase-encoding acquisition and type of reconstruction algorithm on the accuracy of derived quantitative parameters was assessed. Results indicate that the minimum keyhole size used should be restricted by the approximate minimum size of the expected lesions. Furthermore, reconstruction algorithms that offer improved image resolution do not circumvent this restriction.

Algorithms↗