Dependence of MR signal intensity on Gd tissue concentration over a broad dose range

GLIS1 regulates trabecular meshwork function and intraocular pressure and is associated with glaucoma in humans

Chronically elevated intraocular pressure (IOP) is the major risk factor of primary open-angle glaucoma, a leading cause of blindness. Dysfunction of the trabecular meshwork (TM), which controls the outflow of aqueous humor (AqH) from the anterior chamber, is the major cause of elevated IOP. Here, we demonstrate that mice deficient in the Krüppel-like zinc finger transcriptional factor GLI-similar-1 (GLIS1) develop chronically elevated IOP. Magnetic resonance imaging and histopathological analysis reveal that deficiency in GLIS1 expression induces progressive degeneration of the TM, leading to inefficient AqH drainage from the anterior chamber and elevated IOP. Transcriptome and cistrome analyses identified several glaucoma- and extracellular matrix-associated genes as direct transcriptional targets of GLIS1. We also identified a significant association between GLIS1 variant rs941125 and glaucoma in humans (P = 4.73 × 10-6), further supporting a role for GLIS1 into glaucoma etiology. Our study identifies GLIS1 as a critical regulator of TM function and maintenance, AqH dynamics, and IOP.

Extracellular diffusion quantified by magnetic resonance imaging during rat C6 glioma cell progression

Solution reflux and edema hamper the convection-enhanced delivery of the standard treatment for glioma. Therefore, a real-time magnetic resonance imaging (MRI) method was developed to monitor the dosing process, but a quantitative analysis of local diffusion and clearance parameters has not been assessed. The objective of this study was to compare diffusion into the extracellular space (ECS) at different stages of rat C6 gliomas, and analyze the effects of the extracellular matrix (ECM) on the diffusion process. At 10 and 20 days, after successful glioma modeling, gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA) was introduced into the ECS of rat C6 gliomas. Diffusion parameters and half-life of the reagent were then detected using MRI, and quantified according to the mathematical model of diffusion. The main ECM components [chondroitin sulfate proteoglycans (CSPGs), collagen IV, and tenascin C] were detected by immunohistochemical and immunoblot analyses. In 20-day gliomas, Gd-DTPA diffused more slowly and derived higher tortuosity, with lower clearance rate and longer half-life compared to 10-day gliomas. The increased glioma ECM was associated with different diffusion and clearance parameters in 20-day rat gliomas compared to 10-day gliomas. ECS parameters were altered with C6 glioma progression from increased ECM content. Our study might help better understand the glioma microenvironment and provide benefits for interstitial drug delivery to treat brain gliomas.

Novel ultrasound and DCE-MRI analyses after antiangiogenic treatment with a selective VEGF receptor inhibitor

We report a comparison between tumor perfusion estimates acquired using contrast-enhanced MRI and motion-corrected contrast-enhanced ultrasound before and after treatment with AG-028262, a potent vascular endothelial growth factor receptor tyrosine kinase inhibitor. Antiangiogenic activity was determined by assessing weekly ultrasound and MRI images of rats with bilateral hind flank mammary adenocarcinomas before and after treatment with AG-028262. Images were acquired with a spoiled gradient, 1.5 T magnetic resonance sequence and a destruction-replenishment ultrasound protocol. For ultrasound, a time to 80% contrast replenishment was calculated for each tumor voxel; for MR imaging, a measure of local flow rate was estimated from a linear fit of minimum to maximum intensities. AG-028262 significantly decreased tumor growth and increased the time required to replenish tumor voxels with an ultrasound contrast agent from 2.66 to 4.54 s and to fill with an MR contrast agent from 29.5 to 50.8 s. Measures of flow rate derived from MRI and ultrasound demonstrated a positive linear correlation of r2 = 0.86.

Quantification of myocardial blood flow using model based analysis of first-pass perfusion MRI: extraction fraction of Gd-DTPA varies with myocardial blood flow in human myocardium

For the absolute quantification of myocardial blood flow (MBF), Patlak plot-derived K1 need to be converted to MBF by using the relation between the extraction fraction of gadolinium contrast agent and MBF. This study was conducted to determine the relation between extraction fraction of Gd-DTPA and MBF in human heart at rest and during stress. Thirty-four patients (19 men, mean age of 66.5 ± 11.0 years) with normal coronary arteries and no myocardial infarction were retrospectively evaluated. First-pass myocardial perfusion MRI during adenosine triphosphate stress and at rest was performed using a dual bolus approach to correct for saturation of the blood signal. Myocardial K1 was quantified by Patlak plot method. Mean MBF was determined from coronary sinus flow measured by phase contrast cine MRI and left ventricle mass measured by cine MRI. The extraction fraction of Gd-DTPA was calculated as the K1 divided by the mean MBF. The extraction fraction of Gd-DTPA was 0.46 ± 0.22 at rest and 0.32 ± 0.13 during stress (P < 0.001). The relationship between extraction fraction (E) and MBF in human myocardium can be approximated as E = 1 - exp(-(0.14 × MBF + 0.56)/MBF). The current results indicate that MBF can be accurately quantified by Patlak plot method of first-pass myocardial perfusion MRI by performing a correction of extraction fraction.

Quantification of MRI measured myocardial perfusion reserve in healthy humans: a comparison with positron emission tomography

PURPOSE

To validate a noninvasive quantitative MRI technique, the K(i) perfusion method, for myocardial perfusion in humans using (13)N-ammonia PET as a reference method.

MATERIALS AND METHODS

Ten healthy males (64 +/- 8 years) were examined with combined PET and MRI perfusion imaging at rest and during stress induced by dipyridamole in order to determine the myocardial perfusion reserve. Myocardial and blood time concentration curves obtained by Gd-DTPA-enhanced MRI and (13)N-ammonia PET were fitted by a two-compartment perfusion model.

RESULTS

Mean perfusion values (+/-SD) derived from the MRI method at rest and at hyperemia were 80 +/- 20 and 183 +/- 56 mL/min/100 g, respectively. The same data for PET were 71 +/- 16 and 203 +/- 67 mL/min/100 g. A linear relationship was observed between MRI and PET-derived myocardial perfusion reserve for regional and global data. Linear regression for the global absolute perfusion reserve gave a correlation coefficient of 0.96 (P < 0.004, y=0.83x-6.9). A good agreement between the two methods to determine low or high perfusion reserves was found.

CONCLUSION

Our data provide validation of the perfusion marker K(i) derived by the MRI method as a quantitative marker for myocardial perfusion in healthy humans.

Quantitative assessment of macromolecular concentration during direct infusion into an agarose hydrogel phantom using contrast-enhanced MRI

Convection-enhanced delivery (CED), that is, direct tissue infusion, has emerged as a promising local drug delivery method for treating diseases of the nervous system. Determination of the spatial distribution of therapeutic agents after infusion is important in evaluating the efficacy of treatment, optimizing infusion protocols and improving the understanding of drug pharmacokinetics. In this study, we provide a methodology to determine the concentration distribution of Gd-labeled tracers during infusion using contrast-enhanced magnetic resonance imaging (MRI). To the best of our knowledge, MR studies that quantify concentration profiles for CED have not been previously reported. The methodology utilizes intrinsic material properties (T(1) and R(1)) and reduces the effect of instrumental factors (e.g., inhomogeneity of MR detection field). As a methodology investigation, this study used an agarose hydrogel phantom as a tissue substitute for infusion. An 11.1-T magnet system was used to image infusion of Gd-DTPA-labeled albumin (Gd-albumin) into the hydrogel. By using data from preliminary scans, Gd-albumin distribution was determined from the signal intensity of the MR images. As a validation test, MR-derived concentration profiles were found comparable to both results measured directly using quantitative optical imaging and results from a computational transport model in porous media. In future studies, the developed methodology will be used to quantitatively monitor the distribution of Gd tracer following infusion directly into tissues.

In vivo myocardial tissue kinetics of Gd(ABE-DTTA), a tissue-persistent contrast agent

The phenomenological tissue kinetics of Gd(ABE-DTTA) was investigated in myocardial infarction (MI). Reperfused infarction was generated by balloon catheter in closed-chest canines (N=11). Forty-eight hours thereafter, inversion-recovery (IR)-prepared fast gradient-echo control images were acquired with varying inversion times (TIs). Precontrast R(1) maps were calculated from the TI dependence of signal intensity (SI) using nonlinear curve fitting. Then 0.05 mmol/kg Gd(ABE-DTTA) was administered I.V. In 11 dogs postcontrast R(1) maps were generated at 24 hr and 48 hr postcontrast. In five dogs measurements were also repeated at 108 hr and 12 days. In one dog early measurement was carried out at 4 hr. Delta R(1) values for blood and viable and infarcted myocardium were calculated at each time point by subtracting the precontrast R(1) from the postcontrast R(1). Gd(ABE-DTTA) showed significant, progressive accumulation into infarcts during the first 2 days (k(in)=0.39 hr(-1)) and a delayed clearance (k(out) = 0.005 hr(-1)). Among the time points sampled, the maximum infarct Delta R(1) was detected at 48 hr (1.72 s(-1)). Contrast agent (CA) in infarcted tissue was detectable for 12 days. Clearance from blood and viable myocardium occurred in parallel and was completed by 108 hr. Gd(ABE-DTTA) displays slow, tissue-persistent kinetics and partly intravascular, partly extravascular characteristics. It demonstrates high affinity for infarcted myocardium and induces highlighting of infarcts between 4 hr and 12 days following administration.

Determination of macromolecular concentration following direct infusion into hydrogel using contrast-enhanced MRI

Direct tissue infusion has emerged as a promising drug delivery method for treating diseases of the nervous system because the blood-brain or blood-spinal cord barriers are circumvented. Determination of the spatial distribution of therapeutic agents after infusion is important in evaluating the efficacy of treatment and optimizing infusion protocols. In this study, we provide a methodology to determine the concentration distribution of Gd-labeled tracers using contrast-enhanced MRI. An 11.1 T magnet system was used to image infusion of Gd-DTPA labeled albumin (Gd-albumin) into an agarose-based hydrogel. By using data from preliminary scans, Gd-albumin distribution was determined from the signal intensity of the MR images. As an initial validation test, these concentration profiles were compared with distribution profiles predicted for porous media transport by convection and diffusion. Comparison of model results show good correlation between predicted distributions. In future studies, the presented methodology may be used to estimate the distribution of Gd-tracer following infusion directly into tissue.

Perfusion MRI with radial acquisition for arterial input function assessment

Quantification of myocardial perfusion critically depends on accurate arterial input function (AIF) and tissue enhancement curves (TECs). Except at low doses, the AIF is inaccurate because of the long saturation recovery time (SRT) of the pulse sequence. The choice of dose and SRT involves a trade-off between the accuracy of the AIF and the signal-to-noise ratio (SNR) of the TEC. Recent methods to resolve this trade-off are based on the acquisition of two data sets: one to accurately estimate the AIF, and one to find the high-SNR TEC. With radial k-space sampling, a set of images with varied SRTs can be reconstructed from the same data set, allowing an accurate assessment of the AIF and TECs, and their conversion to contrast agent (CA) concentration. This study demonstrates the feasibility of using a radial acquisition for quantitative myocardial perfusion imaging.

Effect of Gd-DTPA-BMA on blood and myocardial T1 at 1.5T and 3T in humans

Purpose

To compare T₁ values of blood and myocardium at 1.5T and 3T before and after administration of Gd‐DTPA‐BMA in normal volunteers, and to evaluate the distribution of contrast media between myocardium and blood during steady state.

Materials and Methods

Ten normal subjects were imaged with either 0.1 mmol/kg (N = 5) or 0.2 mmol/kg (N = 5) of Gd‐DTPA‐BMA contrast agent at 1.5T and 3T. T₁ measurements of blood and myocardium were performed prior to contrast injection and every five minutes for 35 minutes following contrast injection at both field strengths. Measurements of biodistribution were calculated from the ratio of ΔR₁ (ΔR_1myo/ΔR_1blood).

Results

Precontrast blood T₁ values (mean ± SD, N = 10) did not significantly differ between 1.5T and 3T (1.58 ± .13 sec, and 1.66 ± .06 sec, respectively; P > 0.05), but myocardium T₁ values were significantly different (1.07 ± .03 sec and 1.22 ± .07 sec, respectively; P < 0.05). The field‐dependent difference in myocardium T₁ postinjection (T₁@3T – T₁@1.5T) decreased by approximately 72% relative to precontrast T₁ values, while the field‐dependent difference of blood T₁ decreased only 30% postcontrast. Measurements of ΔR_1myo/ΔR_1blood were constant for 35 minutes postcontrast, but changed between 1.5T and 3T (0.46 ± .06 vs. 0.54 ± .06, P < 0.10).

Conclusion

T₁ is significantly longer for myocardium (but not blood) at 3T compared to 1.5T. The differences in T₁ due to field strength are reduced following contrast administration, which may be attributed to changes in ΔR_1myo/ΔR_1blood with field strength. J. Magn. Reson. Imaging 2006. © 2006 Wiley‐Liss, Inc.

Magnetic resonance perfusion measurements for the noninvasive detection of coronary artery disease

BACKGROUND

With MRI, an index of myocardial perfusion reserve (MPRI) can be determined. We assessed the value of this technique for the noninvasive detection of coronary artery disease (CAD) in patients with suspected CAD.

METHODS AND RESULTS

Eighty-four patients referred for a primary diagnostic coronary angiography were examined with a 1.5 T MRI tomograph (Philips-ACS). For each heartbeat, 5 slices were acquired during the first pass of 0.025 mmol gadolinium-diethylenetriamine pentaacetic acid/kg body weight before and during adenosine vasodilation by using a turbo-gradient echo/echo-planar imaging-hybrid sequence. MPRI was determined from the alteration of the upslope of the myocardial signal intensity curves for 6 equiangular segments per slice. Receiver operating characteristics were performed for different criteria to differentiate ischemic and nonischemic segments. Prevalence of CAD was 51%. Best results were achieved when only the 3 inner slices were assessed and a threshold value of 1.1 was used for the second smallest value as a marker for significant CAD. This approach yielded a sensitivity of 88%, specificity of 90%, and accuracy of 89%.

CONCLUSIONS

The determination of MPRI with MRI yields a high diagnostic accuracy in patients with suspected CAD.

Quantitative assessment of Gd-DTPA contrast agent from signal enhancement: an in-vitro study

Quantitative determination of in-vivo gadolinium diethylenetriamine-pentaacid (Gd-DTPA) concentration is attractive in various studies involving perfusion, tracer kinetics and permeability constants. Using a 1.5 T clinical system and a 7 T small-bore system, we evaluated a method for absolute determination of Gd-DTPA concentrations in plasma solutions. Different solutions of Gd-DTPA and (99m)Tc-DTPA were mixed in human plasma and concentrations in the range of 0-5.0 mmol/l (1.5 T system) or 0-3.0 mmol/l (7 T system) of Gd-DTPA were divided into thirteen tubes. All MRI measurements were carried out using conventional sequences (SE, FLASH and GRASS). The MR measured intensity was converted to Gd-DTPA concentration by mathematical interpretation of the sequences. All MRI sequences showed, that the measured concentrations of Gd-DTPA revealed a slight non-linear difference compared with the calculated Gd-DTPA concentrations determined by the plasma (99m)Tc-DTPA using gamma counting. This non-linearity was most pronounced at high Gd-DTPA concentrations, suggesting that the discrepancy could be a result of an increased plasma relaxivity at higher concentrations. Adjustment of measured Gd-DTPA concentration was therefore performed using a selected power function, A[Gd-DTPA](a), which yielded the best linear relationship. Regression analysis showed that the scaling constant (A) varied from 0.11 to 97.45 and the power constant (a) varied from 0.83 to 1.6. Based on these constants, the MRI measured concentrations of Gd-DTPA did not differ from the calculated concentrations of Gd-DTPA obtained from reference measurements of (99m)Tc-DTPA. In the 1.5 T system, a linear relationship (r(2) > or = 0.95) was demonstrated in the range of 0-5.0 mmol/l Gd-DTPA, and in the 7 T system, a linear relationship (r(2) > or = 0.92) was demonstrated in the range of 0-3.0 mmol/l Gd-DTPA. Additionally, the effect of signal-to-noise on measured concentrations of Gd-DTPA was simulated using MR data of the mixed solutions of Gd-DTPA in plasma and the analytical expression of the pulse sequences. The simulations showed that the concentrations were most sensitive to noise in the GRASS sequence. In conclusion, this study demonstrates a novel approach to quantify accurately the Gd-DTPA concentration directly from MRI signal data using different routine sequences.

Determination of in vivo rat muscle Gd-DTPA relaxivity at 6.3 T

For the in vivo relaxivity of Gd-DTPA at 6.3 T in rat muscle a value of 2.7+/-0.5 (mM s)(-1) was found, and for the in vitro value in water 3.00+/-0.56 (mM s)(-1) at 37 degrees C. The temperature dependence of the in vitro relaxivity was -0.087 (mM s degrees C)(-1). The relation between 1/T1 and the tissue Gd-DTPA concentration is linear for the normally used in vivo Gd-DTPA concentration range.

Contrast-enhanced hepatic MRI: comparison of half-dose and standard-dose gadolinium DTPA administration in lesion characterization with T1-weighted gradient echo sequences

The objective of this article was to compare half-dose (0.05 mm/kg) gadolinium-enhanced dynamic hepatic MR imaging to standard doses (0.10 mm/kg). Eighteen patients for follow-up hepatic MR received 0.05 mm/kg of gadolinium DTPA dynamically with gradient-echo imaging. Imaging parameters were identical to a 0.10-mm/kg study; patients were imaged during multiple phases of contrast enhancement. Two readers assessed for enhancement patterns and characterization. Quantitative signal-to-noise ratios (S/N) were obtained for abdominal viscera and contrast-to-noise ratios (C/N) were obtained on up to three lesions. No significant difference for the arterial dominant phase (P > 0.05) was found. Significant differences were found in all categories during the portal venous phase (except pancreas) and equilibrium phase (except liver). Lesion C/N ratios were not significant at any point (P > 0.05). Sixty-two out of 64 lesions (97%) were identically characterized. Therefore, half-dose dynamic gadolinium-enhanced MR may have diagnostic value.

Measurement of the distribution volume of gadopentetate dimeglumine at echo-planar MR imaging to quantify myocardial infarction: comparison with 99mTc-DTPA autoradiography in rats

PURPOSE

To measure the fractional distribution volume of gadopentetate dimeglumine in normal and reperfused infarcted myocardium at magnetic resonance (MR) imaging by using the fractional distribution volume of technetium 99m-diethylenetriaminepentaacetic acid (DTPA) as an independent reference.

MATERIALS AND METHODS

Rats were subjected to 1 hour of coronary artery occlusion and 1 hour of reperfusion before inversion-recovery echo-planar imaging or autoradiography. Regional change in relaxation rate (delta R1) ratios for myocardium over blood were compared with radioactivity ratios for myocardium over blood after the injection of 99mTc-DTPA.

RESULTS

Both delta R1 and radioactivity ratios demonstrated equilibrium distribution and hence represent partition coefficients (lambda). The fractional distribution volumes were greater in infarcted myocardium (0.90 +/- 0.05 for gadopentetate dimeglumine and 0.89 +/- 0.04 for 99mTc-DTPA) than in normal myocardium (0.23 +/- 0.02 for gadopentetate dimeglumine and 0.16 +/- 0.01 for 99mTc-DTPA). Area at risk at autoradiography was not significantly different from that at histomorphometry. The infarction size defined by using triphenyltetrazolium chloride was 13% +/- 4 smaller than that defined by using autoradiography.

CONCLUSION

The fractional distribution volumes of gadopentetate dimeglumine and 99mTc-DTPA are similar and indicate extracellular distribution in normal myocardium and intracellular as well as extracellular distribution in reperfused infarction. Because the failure of cells to exclude these agents is indicative of necrosis, contrast medium-enhanced MR imaging may be useful to quantify myocardial infarction.

Effects of water exchange on the measurement of myocardial perfusion using paramagnetic contrast agents

To investigate the effects of water exchange on quantification of perfusion, data were acquired in isolated hearts (n = 11) and used to develop a model of exchange. Myocardial T1 was measured 3 times/sec during step changes in concentration of intravascular (polylysine-gadolinium-diethylene-triamine-pentaacetic acid) and extracellular (gadoteridol) agents. For the intravascular agent, the change in 1/T1 (deltaR1) was lower than predicted by fast exchange (2.7+/-0.5 vs. 7.8 sec(-1), respectively), and suggested an intra-extravascular exchange rate of 3 Hz. For the extracellular agent, contrast kinetics were similar to those of similarly sized molecules (wash-in time constant 38+/-5 sec), and the data suggested fast interstitial-cellular exchange. Modeling showed that perfusion is underestimated for both agents if exchange is ignored, although the relationships of measured to actual perfusion were monotonic. We conclude that myocardial water exchange strongly affects first-pass enhancement but that ignoring the effects of exchange may still provide reasonable estimates of regional perfusion differences.

Capillary transfer constant of Gd-DTPA in the myocardium at rest and during vasodilation assessed by MRI

The purpose of this study was to determine whether the capillary transfer constant (Ki) of gadolinium-DTPA was sensitive to perfusion changes and whether ischemic regions in the myocardium could be identified using the modified Kety formula. Ki was measured at rest and during dipyridamole-induced vasodilation in 10 healthy volunteers and in 10 patients with ischemic heart disease. Ki increased by a factor of 2.5+/-1.2 (mean +/- SD) from 55+/-16 ml 100 g(-1)min(-1) at rest to 136+/-46 ml 100 g(-1)min(-1) (P < 0.01) during vasodilation in the healthy subjects. In the patients, there were no changes in Ki during vasodilation in ischemic regions (50+/-18 versus 49+/-30 ml 100 g(-1)min(-1) (P > 0.4)). Ki increased in nonischemic regions by a factor of 2.0+/-0.8 from 44+/-17 to 81+/-32 ml 100 g(-1)min(-1) during vasodilation (P < 0.02). It is concluded that the capillary transfer constant is sensitive to perfusion changes and that regional ischemia can be detected with MRI. This noninvasive and quantitative method may prove useful in the evaluation of patients with ischemic heart disease.

Quantification of gadolinium-DTPA concentrations for different inversion times using an IR-turbo flash pulse sequence: a study on optimizing multislice perfusion imaging

The purpose of this study was to optimize an inversion-recovery (IR) turbo fast low-angle shot (FLASH) for multislice imaging by evaluating the accuracy of calculated the relaxation-rate (R1) for different inversion times (TI). This is important for tracer kinetic modeling because it requires a system responding linearly to input. R1 are linearly related to changes in the concentration of gadolinium (Gd)-diethylenetriaminepentaacetic acid (DTPA), and R1 is a parameter that can be derived from the magnetic resonance (MR) signal. The accuracy of calculated R1 using an IR turbo fast low-angle shot was evaluated in phantoms and for increasing TIs using spectroscopically measured R1 values as reference. Signal curves, obtained in vivo after a bolus injection of Gd-DTPA, were used in an analytical computer program to study the effect of different TI-values on accurate calculation of R1. Results show that TIeff should be <200 ms to measure the bolus-passage of Gd-DTPA in blood accurately, whereas the myocardial response can be measured correctly for TIeff < 870 ms at 1.5 T. The initial slope of the myocardial signal enhancement curve becomes steeper for larger TI values, whereas the calculated R1 curves were similar, indicating that these curves, rather than signal curves, are more suitable even for qualitative perfusion evaluation. It is concluded that the results can be incorporated in a multislice IR turbo fast low-angle shot using the first slice (with a short TI) for assessment of both the arterial input function and the tissue response and the second slice in another position for assessment of the tissue response alone.

Microvascular injury in reperfused infarcted myocardium: noninvasive assessment with contrast-enhanced echoplanar magnetic resonance imaging

OBJECTIVES

The purpose of this study was to measure the accumulation of labeled albumin and to visualize its distribution pattern in reperfused infarcted myocardium as a function of time between onset of reperfusion and administration of the tracer.

BACKGROUND

Myocardial microvascular injury leads to leakage of albumin from the intravascular space. Quantitative measurements of GdDTPA-albumin with inversion recovery echoplanar imaging (IR-EPI) may allow noninvasive monitoring of microvascular injury.

METHODS

After 1 h of coronary artery occlusion, 56 rats were injected with GdDTPA-albumin or 123I-GdDTPA-albumin either immediately before reperfusion or 1/2, 1 or 24 h after reperfusion. GdDTPA-albumin in blood, normal myocardium and reperfused infarction was dynamically measured with IR-EPI during 1 h postinjection (PI). Autoradiograms were obtained at 15 min PI. Accumulation of labeled albumin in myocardium was expressed as the ratio of myocardial to blood content.

RESULTS

In normal myocardium, the ratio of changes of relaxation rate-ratio (deltaR1-ratio) was 0.12+/-0.01 and did not change over 1 h. In reperfused infarction, however, the deltaR1-ratio increased after administration. Animals given GdDTPA-albumin before reperfusion exhibited fastest accumulation (deltaR1-ratio 15 min PI: 0.56+/-0.03) and essentially homogeneous distribution. The accumulation was slower when administered at 1/2, 1 and 24 h after reperfusion (deltaR1-ratios 15 min PI: 0.39+/-0.03; 0.31+/-0.04; 0.16+/-0.01; p < 0.001 compared to administration before reperfusion). Moreover, the tracer accumulated predominantly in the periphery of the injury zone.

CONCLUSIONS

Amount and distribution pattern of labeled albumin in reperfused infarction are modulated by duration of reperfusion. The accumulation of GdDTPA-albumin can be quantified by IR-EPI. Thus, IR-EPI may be useful to noninvasively monitor myocardial microvascular injury in reperfused infarction.

Measurement of the arterial concentration of Gd-DTPA using MRI: a step toward quantitative perfusion imaging

A noninvasive method using an inversion recovery turbo-FLASH for dynamic measurement of the arterial input function represented by the bolus passage of Gd-DTPA in the descending aorta is presented, and the results are compared with the input function obtained by arterial blood samples. A good accordance between the two input functions was found, indicating that it is possible to measure the input function to the myocardium using MRI. A variation between the two concentration curves of 5% at upslope, 2.7% at peak point, and < 7% at downslope was found. The study also indicates that a short inversion time < 250 ms has to be used to ensure correct measurement of peak concentration.

Physiological basis of myocardial contrast enhancement in fast magnetic resonance images of 2-day-old reperfused canine infarcts

BACKGROUND

Contrast-enhanced fast magnetic resonance (MR) images of acute, reperfused human infarcts demonstrate regions of hypoenhancement and hyperenhancement. The relations between the spatial extent and time course of these enhancement patterns to myocardial risk, infarct, and no-reflow regions have not been well characterized.

METHODS AND RESULTS

The proximal left anterior descending coronary artery was occluded in 11 closed-chest dogs for 90 minutes followed by 2 days of reperfusion. Regional blood flow was determined by use of radioactive microspheres. The animals were studied at the 2-day time point with contrast-enhanced fast MRI (Signa 1.5 T, General Electric). Thioflavin-S was administered to demarcate no-reflow regions. The hearts were then excised, sectioned into five base-to-apex slices, stained with 2,3,5-triphenyltetrazolium chloride (TTC), and photographed under room light (for TTC) and ultraviolet light (for thioflavin). The spatial extents of thioflavin-negative, TTC-negative, and risk regions were compared planimetrically with MRI hypoenhanced and hyperenhanced regions. The spatial locations of subendocardial hypoenhancement in MR images correlated closely with those of thioflavin-negative regions. Microsphere blood flow in these regions was significantly reduced compared with remote regions (0.37 +/- 0.09 versus 0.88 +/- 0.10 mL/min per gram, respectively, P < .001) and with baseline (0.37 +/- 0.09 versus 0.87 +/- 0.15 mL/min per gram, P < .01). The spatial extent of hyperenhancement was smaller than the risk region (r = .64, slope = 0.48, P < .001) but highly correlated with TTC-negative regions and were, on average, 12% larger (r = .93, slope = 1.12, P = .035).

CONCLUSIONS

In contrast-enhanced MR images of 2-day-old reperfused canine infarcts, myocardial regions of hypoenhancement are related to the no-reflow phenomenon. Approximately 90% of the myocardium within hyperenhanced regions is nonviable.

Regional heterogeneity of human myocardial infarcts demonstrated by contrast-enhanced MRI. Potential mechanisms

BACKGROUND

Myocardial reperfusion is pivotal to the prognosis of patients with acute myocardial infarction. In these patients, coronary flow is generally assessed by angiography and tissue perfusion by tracer scintigraphy. This study was designed to examine whether magnetic resonance imaging (MRI) provides information on myocardial perfusion and damage beyond that supplied by angiography and thallium scintigraphy after acute myocardial infarction.

METHODS AND RESULTS

Twenty-two patients with recent myocardial infarction had ECG, echocardiography, coronary angiography, and fast contrast-enhanced MRI. Twelve patients also had exercise thallium scintigraphy. Time-intensity curves obtained from infarcted and noninfarcted regions were correlated with coronary anatomy and left ventricular function. Two perfusion patterns were observed in infarcted regions by comparison with the normal myocardial pattern. All patients but 1 had persistent myocardial hyperenhancement within the infarcted region up to 10 minutes after contrast. In 10 patients, this hyperenhanced region surrounded a subendocardial area of decreased signal at the center of the infarcted region associated with coronary occlusion at angiography, Q waves on ECG, and greater regional dysfunction by echocardiography. Moreover, the extent and location of the MRI abnormalities correlated well with the extent and location of the fixed single-photon emission computed tomography thallium defects.

CONCLUSIONS

Large human infarcts, associated with prolonged obstruction of the infarct-related artery, are characterized by central dark zones surrounded by hyperenhanced regions on MRI. Conversely, reperfused infarcts with less regional dysfunction have uniform signal hyperenhancement. The MRI hyperenhanced segment correlates well with the fixed scintigraphic defect in patients with acute myocardial infarction.

A magnetization-driven gradient echo pulse sequence for the study of myocardial perfusion

A T1-weighted imaging pulse sequence for contrast-based studies of myocardial perfusion is presented and evaluated in phantoms and in vivo. The sequence is similar to spoiled gradient-recalled echo sequences except that nonselective preparatory RF pulses drive magnetization to steady state prior to image acquisition. Steady state is thus obtained in both tissue and blood resulting in a stable, homogeneous, and dark pre-contrast baseline. Tip angles and timings are chosen so that pixel intensity approximates a linear relation to 1/T1. The dynamic range of signal response to contrast agent concentration is greater than that of an inversion-recovery fast low angle shot sequence. The sequence proposed should be useful for myocardial perfusion studies.

Effects of myocardial water exchange on T1 enhancement during bolus administration of MR contrast agents

Interpretation of first-pass myocardial perfusion studies employing bolus administration of T1 magnetic resonance (MR) contrast agents requires an understanding of the relationship between contrast concentration and image pixel intensity. The potential effects of myocardial water exchange rates among the intravascular, interstitial, and cellular compartments on this relationship are controversial. We directly studied these issues in isolated, nonbeating canine interventricular septa. Myocardial T1 was measured three times/s during bolus transit of intravascular (albumin-Gd-DTPA and polylysine-Gd-DTPA) and extracellular (gadoteridol) contrast agents. For polylysine-Gd-DTPA, the peak changes in myocardial 1/T1 (delta R1) scaled nonlinearly with perfusate contrast concentration whereas a linear relationship would be expected for fast water exchange among the vascular, interstitial, and cellular compartments. For all agents, the peak delta R1 were much smaller than the values expected on the basis of fast myocardial water exchange. The data demonstrate that in isolated myocardial tissue, myocardial T1 enhancement during bolus administration of contrast can be strongly affected by myocardial water exchange for both intravascular and extracellular MR contrast agents.

Measurement of kinetic perfusion parameters of gadoteridol in intact myocardium: effects of ischemia/reperfusion and coronary vasodilation

Quantitation of myocardial perfusion is feasible using contrast enhanced magnetic resonance imaging. A method to quantitate myocardial blood flow is provided by the Kety model modified to account for a diffusable tracer such as gadoteridol. In the present study, perfusion parameters of the modified Kety model (partition coefficient and extraction efficiency) were determined for gadoteridol in intact myocardium using a constant flow, isolated, perfused heart model. Perfusion conditions included hearts with normal perfusion, hearts made globally ischemic for 20 min then perfused normally, and hearts whose coronary flow was more than doubled with 9 microM adenosine. T1 relaxation times were rapidly measured at 0.5 T following step increases in perfusate gadoteridol concentration and at steady state. Both the partition coefficient and extraction efficiency were found to be significantly increased in ischemic/reperfused hearts compared to normal. While flow rates in adenosine hearts were too high for accurate extraction efficiency determination using this technique, the partition coefficient was no different between adenosine and normally perfused hearts. The method described in this article allowed the kinetic parameters of the modified Kety model to be determined in intact heart using NMR relaxation time measurements as the basis of the calculation.

Dynamics of tumor imaging with Gd-DTPA-polyethylene glycol polymers: dependence on molecular weight

Macromolecular contrast media offer potential advantages over freely diffusible agents in magnetic resonance (MR) imaging outside the central nervous system. To identify an optimum molecular weight for macromolecular contrast media, the authors studied a novel macromolecular contrast agent, gadolinium diethylenetriaminepentaacetic acid polyethylene glycol (DTPA-PEG), synthesized in seven polymer (average) molecular weights ranging from 10 to 83 kd. Twenty-eight rabbits bearing V2 carcinoma in thighs underwent T1-weighted spin-echo imaging before injection and 5-60 minutes and 24 hours after injection of the Gd-DTPA-PEG polymers or Gd-DTPA at a gadolinium dose of 0.1 mmol/kg. Tumor region-of-interest measurements were obtained at each time point to determine contrast enhancement dynamics. Blood-pool enhancement dynamics were observed for the Gd-DTPA-PEG polymers larger than 20 kd. Polymers smaller than 20 kd displayed dynamics similar to those of the freely diffusible agent Gd-DTPA. Above the 20 kd threshold, tumor enhancement was more rapid for smaller polymers. The authors conclude that the 21.9-kd Gd-DTPA-PEG polymer is best suited for clinical MR imaging.

A multi-organ, axially distributed model of capillary permeability for a magnetic resonance imaging contrast agent

Capillary permeability was resolved from uptake data for eight rat organs with gadoteridol, which is a stable, well-tolerated, nonionic, highly water soluble, gadolinium-containing, magnetic resonance imaging contrast agent. The extracellular kinetics were elucidated with an axially distributed, plasma interstitial fluid model and measured plasma flow, organ plasma volume, and interstitial fluid volume. The molecular and biological properties of gadoteridol and this kinetic model provide magnetic resonance imaging with a tool to begin measuring physiologic processes.

Dual effects of gadodiamide injection in depiction of the region of myocardial ischemia

The high safety index of the nonionic magnetic resonance (MR) contrast agent gadodiamide injection permits a wide dose range without induction of hemodynamic effects. The wide dose range might confer a diagnostic advantage by providing both T1 enhancing and magnetic susceptibility effects for demarcating pathologic regions. The dual utility of this contrast agent for T1 and T2 enhancement in creating differential contrast between normal and acutely ischemic myocardium was explored. MR imaging was initiated 20-40 minutes after occlusion of the left coronary artery in two groups of rats: Group 1 (n = 8) received 0.3 mmol/kg gadodiamide injection before and after acquisition of T1-weighted images; group 2 (n = 10) received 0.5 mmol/kg before and after acquisition of T2-weighted images. Postcontrast T1-weighted images showed clear delineation of the ischemic region as a relatively low-signal-intensity area for 15 minutes. On postcontrast T2-weighted images, the ischemic region appeared as a relatively high-signal-intensity area for 60 minutes. These effects were obtained with doses of gadodiamide injection that have safety indexes greater than those of gadopentetate dimeglumine.

Preretinal neovascularization in diabetic retinopathy: a preliminary investigation using contrast-enhanced magnetic resonance imaging

Preretinal neovascularization is a well-described feature of advanced diabetic retinopathy. In this study, contrast-enhanced magnetic resonance imaging was used to examine blood-retinal barrier breakdown associated with preretinal neovascularization in three subjects with proliferative diabetic retinopathy. Using a standard imaging protocol, a varying degree of vitreous enhancement was observed in these eyes. The location and severity of enhancement, judged by visual inspection of the images, corresponded to the fluorescein angiographic and/or clinical appearance of preretinal neovascularization. This result suggests that contrast-enhanced magnetic resonance imaging may prove a reasonable approach to the identification of preretinal neovascularization in eyes with significant media opacities.