Inversion recovery EPI of bolus transit in rat myocardium using intravascular and extravascular gadolinium-based MR contrast media: dose effects on peak signal enhancement

DUSTER: dynamic contrast enhance up-sampled temporal resolution analysis method

Dynamic contrast enhanced (DCE) MRI using Tofts' model for estimating vascular permeability is widely accepted, yet inter-tissue differences in bolus arrival time (BAT) are generally ignored. In this work we propose a method, incorporating the BAT in the analysis, demonstrating its applicability and advantages in healthy subjects and patients. A method for DCE Up Sampled TEmporal Resolution (DUSTER) analysis is proposed which includes: baseline T1 map using DESPOT1 analyzed with flip angle (FA) correction; preprocessing; raw-signal-to-T1-to-concentration time curves (CTC) conversion; automatic arterial input function (AIF) extraction at temporal super-resolution; model fitting with model selection while incorporating BAT in the pharmacokinetic (PK) model, and fits contrast agent CTC while using exhaustive search in the BAT dimension in super-resolution. The method was applied to simulated data and to human data from 17 healthy subjects, six patients with glioblastoma, and two patients following stroke. BAT values were compared to time-to-peak (TTP) values extracted from dynamic susceptibility contrast imaging. Results show that the method improved the AIF estimation and allowed extraction of the BAT with a resolution of 0.8 s. In simulations, lower mean relative errors were detected for all PK parameters extracted using DUSTER compared to analysis without BAT correction (vp:5% vs. 20%, Ktrans: 9% vs. 24% and Kep: 8% vs. 17%, respectively), and BAT estimates demonstrated high correlations (r = 0.94, p < 1e− 10) with true values. In real data, high correlations between BAT values were detected when extracted from data acquired with high temporal resolution (2 s) and sub-sampled standard resolution data (6 s) (mean r = 0.85,p < 1e− 10). BAT and TTP values were significantly correlated in the different brain regions in healthy subjects (mean r = 0.72,p = < 1e− 3), as were voxel-wise comparisons in patients (mean r = 0.89, p < 1e− 10). In conclusion, incorporating BAT in DCE analysis improves estimation accuracy for the AIF and the PK parameters while providing an additional clinically important parameter.

Magnetic resonance imaging and multi-detector computed tomography assessment of extracellular compartment in ischemic and non-ischemic myocardial pathologies

Myocardial pathologies are major causes of morbidity and mortality worldwide. Early detection of loss of cellular integrity and expansion in extracellular volume (ECV) in myocardium is critical to initiate effective treatment. The three compartments in healthy myocardium are: intravascular (approximately 10% of tissue volume), interstitium (approximately 15%) and intracellular (approximately 75%). Myocardial cells, fibroblasts and vascular endothelial/smooth muscle cells represent intracellular compartment and the main proteins in the interstitium are types I/III collagens. Microscopic studies have shown that expansion of ECV is an important feature of diffuse physiologic fibrosis (e.g., aging and obesity) and pathologic fibrosis [heart failure, aortic valve disease, hypertrophic cardiomyopathy, myocarditis, dilated cardiomyopathy, amyloidosis, congenital heart disease, aortic stenosis, restrictive cardiomyopathy (hypereosinophilic and idiopathic types), arrythmogenic right ventricular dysplasia and hypertension]. This review addresses recent advances in measuring of ECV in ischemic and non-ischemic myocardial pathologies. Magnetic resonance imaging (MRI) has the ability to characterize tissue proton relaxation times (T1, T2, and T2*). Proton relaxation times reflect the physical and chemical environments of water protons in myocardium. Delayed contrast enhanced-MRI (DE-MRI) and multi-detector computed tomography (DE-MDCT) demonstrated hyper-enhanced infarct, hypo-enhanced microvascular obstruction zone and moderately enhanced peri-infarct zone, but are limited for visualizing diffuse fibrosis and patchy microinfarct despite the increase in ECV. ECV can be measured on equilibrium contrast enhanced MRI/MDCT and MRI longitudinal relaxation time mapping. Equilibrium contrast enhanced MRI/MDCT and MRI T1 mapping is currently used, but at a lower scale, as an alternative to invasive sub-endomyocardial biopsies to eliminate the need for anesthesia, coronary catheterization and possibility of tissue sampling error. Similar to delayed contrast enhancement, equilibrium contrast enhanced MRI/MDCT and T1 mapping is completely noninvasive and may play a specialized role in diagnosis of subclinical and other myocardial pathologies. DE-MRI and when T1-mapping demonstrated sub-epicardium, sub-endocardial and patchy mid-myocardial enhancement in myocarditis, Behcet’s disease and sarcoidosis, respectively. Furthermore, recent studies showed that the combined technique of cine, T2-weighted and DE-MRI technique has high diagnostic accuracy for detecting myocarditis. When the tomographic techniques are coupled with myocardial perfusion and left ventricular function they can provide valuable information on the progression of myocardial pathologies and effectiveness of new therapies.

Parameter optimization for quantitative signal-concentration mapping using spoiled gradient echo MRI

Rationale and Objectives. Accurate signal to tracer concentration maps are critical to quantitative MRI. The purpose of this study was to evaluate and optimize spoiled gradient echo (SPGR) MR sequences for the use of gadolinium (Gd-DTPA) as a kinetic tracer. Methods. Water-gadolinium phantoms were constructed for a physiologic range of gadolinium concentrations. Observed and calculated SPGR signal to concentration curves were generated. Using a percentage error determination, optimal pulse parameters for signal to concentration mapping were obtained. Results. The accuracy of the SPGR equation is a function of the chosen MR pulse parameters, particularly the time to repetition (TR) and the flip angle (FA). At all experimental values of TR, increasing FA decreases the ratio between observed and calculated signals. Conversely, for a constant FA, increasing TR increases this ratio. Using optimized pulse parameter sets, it is possible to achieve excellent accuracy (approximately 5%) over a physiologic range of concentration tracer concentrations. Conclusion. Optimal pulse parameter sets exist and their use is essential for deriving accurate signal to concentration curves in quantitative MRI.

Gadolinium-based contrast agents for magnetic resonance cancer imaging

Magnetic resonance imaging (MRI) is a clinical imaging modality effective for anatomical and functional imaging of diseased soft tissues, including solid tumors. MRI contrast agents (CA) have been routinely used for detecting tumor at an early stage. Gadolinium-based CA are the most commonly used CA in clinical MRI. There have been significant efforts to design and develop novel Gd(III) CA with high relaxivity, low toxicity, and specific tumor binding. The relaxivity of the Gd(III) CA can be increased by proper chemical modification. The toxicity of Gd(III) CA can be reduced by increasing the agents' thermodynamic and kinetic stability, as well as optimizing their pharmacokinetic properties. The increasing knowledge in the field of cancer genomics and biology provides an opportunity for designing tumor-specific CA. Various new Gd(III) chelates have been designed and evaluated in animal models for more effective cancer MRI. This review outlines the design and development, physicochemical properties, and in vivo properties of several classes of Gd(III)-based MR CA tumor imaging.

Vessel size index measurements in a rat model of glioma: comparison of the dynamic (Gd) and steady-state (iron-oxide) susceptibility contrast MRI approaches

Vessel size index (VSI), a parameter related to the distribution of vessel diameters, may be estimated using two MRI approaches: (i) dynamic susceptibility contrast (DSC) MRI following the injection of a bolus of Gd-chelate. This technique is routinely applied in the clinic to assess intracranial tissue perfusion in patients; (ii) steady-state susceptibility contrast with USPIO contrast agents, which is considered here as the standard method. Such agents are not available for human yet and the steady-state approach is currently limited to animal studies. The aim is to compare VSI estimates obtained with these two approaches on rats bearing C6 glioma (n = 7). In a first session, VSI was estimated from two consecutive injections of Gd-Chelate (Gd(1) and Gd(2)). In a second session (4 hours later), VSI was estimated using USPIO. Our findings indicate that both approaches yield comparable VSI estimates both in contralateral (VSI{USPIO} = 7.5 ± 2.0 µm, VSI{Gd(1)} = 6.5 ± 0.7 µm) and in brain tumour tissues (VSI{USPIO} = 19.4 ± 7.1 µm, VSI{Gd(1)} = 16.6 ± 4.5 µm). We also observed that, in the presence of BBB leakage (as it occurs typically in brain tumours), applying a preload of Gd-chelate improves the VSI estimate with the DSC approach both in contralateral (VSI{Gd(2)} = 7.1 ± 0.4 µm) and in brain tumour tissues (VSI{Gd(2)} = 18.5 ± 4.3 µm) but is not mandatory. VSI estimates do not appear to be sensitive to T(1) changes related to Gd extravasation. These results suggest that robust VSI estimates may be obtained in patients at 3 T or higher magnetic fields with the DSC approach.

Toward local arterial input functions in dynamic contrast-enhanced MRI

PURPOSE

To present a method for estimating the local arterial input function (AIF) within a dynamic contrast-enhanced MRI scan, based on the alternating minimization with model (AMM) method.

MATERIALS AND METHODS

This method clusters a subset of data into representative curves, which are then input to the AMM algorithm to return a parameterized AIF and pharmacokinetic parameters. Computer simulations are used to investigate the accuracy with which the AMM is able to estimate the true AIF as a function of the input tissue curves.

RESULTS

Simulations show that a power law relates uncertainty in kinetic parameters and SNR and heterogeneity of the input. Kinetic parameters calculated with the measured AIF are significantly different from those calculated with either a global (P < 0.005) or a local input function (P = 0.0). The use of local AIFs instead of measured AIFs yield mean lesion-averaged parameter changes: K(trans): +24% [+15%, +70%], k(ep): +13% [-36%, +300%]. Globally estimated input functions yield mean lesion-averaged changes: K(trans): +9% [-38%, +65%], k(ep): +13% [-100%, +400%]. The observed improvement in fit quality with local AIFs was found to be significant when additional free parameters were accounted for using the Akaike information criterion.

CONCLUSION

Local AIFs result in significantly different kinetic parameter values. The statistically significant improvement in fit quality suggests that changes in parameter estimates using local AIFs reflect differences in underlying tissue physiology.

Comparison of quantitative parameters in cervix cancer measured by dynamic contrast-enhanced MRI and CT

Cervical tumors of 38 cervix cancer patients were scanned by T(1)-weighted dynamic contrast enhanced (DCE) MRI and then by DCE-CT on the same day. Gadodiamide and iohexol were respectively used as the low-molecular-weight contrast agent in DCE-MRI and DCE-CT. Under an extended Tofts model, DCE-MRI data were analyzed using either individual arterial input functions estimated by a multiple reference tissue method or a population arterial input function by Parker et al., whereas DCE-CT data were analyzed using the arterial input function directly measured from the external iliac arteries. The derived quantitative parameters of cervical tumors were compared between DCE-MRI and DCE-CT. When using the individual multiple reference tissue method arterial input functions to analyze the DCE-MRI data, the correlation coefficients between DCE-MRI- and DCE-CT-derived parameters were, respectively, back-flux rate constant (r = 0.80), extravascular extracellular fractional volume (r = 0.73), contrast agent transfer rate (r = 0.62), and blood plasma volume (r = 0.32); when using the Parker population arterial input function, the correlation coefficients were back-flux rate constant (r = 0.79), extravascular extracellular fractional volume (r = 0.77), contrast agent transfer rate (r = 0.63), and blood plasma volume (r = 0.58). Tumor parametric maps derived by DCE-MRI and DCE-CT had very similar morphologies. However, the means of most derived quantitative parameters were significantly different between the two imaging methods. Close correlation of quantitative parameters derived from two independent imaging modalities suggests both are measuring similar tumor physiologic variables.

Model-based blind estimation of kinetic parameters in dynamic contrast enhanced (DCE)-MRI

A method to simultaneously estimate the arterial input function (AIF) and pharmacokinetic model parameters from dynamic contrast-enhanced (DCE)-MRI data was developed. This algorithm uses a parameterized functional form to model the AIF and k-means clustering to classify tissue time-concentration measurements into a set of characteristic curves. An iterative blind estimation algorithm alternately estimated parameters for the input function and the pharmacokinetic model. Computer simulations were used to investigate the algorithm's sensitivity to noise and initial estimates. In 12 patients with sarcomas, pharmacokinetic parameter estimates were compared with "truth" obtained from model regression using a measured AIF. When arterial voxels were included in the blind estimation algorithm, the resulting AIF was similar to the measured input function. The "true" K(trans) values in tumor regions were not significantly different than the estimated values, 0.99 +/- 0.41 and 0.86 +/- 0.40 min(-1), respectively, P = 0.27. "True" k(ep) values also matched closely, 0.70 +/- 0.24 and 0.65 +/- 0.25 min(-1), P = 0.08. When only tissue curves free of significant vascular contribution are used (v(p) < 0.05), the resulting AIF showed substantial delay and dispersion consistent with a more local AIF such as has been observed in dynamic susceptibility contrast imaging in the brain.

Reproducibility assessment of a multiple reference tissue method for quantitative dynamic contrast enhanced-MRI analysis

Bone metastases of 16 prostate cancer patients were scanned twice 1 week apart by dynamic contrast enhanced (DCE)-MRI at 2-s resolution using a two-dimensional gradient-echo pulse sequence. With a multiple reference tissue method (MRTM), the local tissue arterial input function (AIF) was estimated using the contrast agent enhancement data from tumor subregions and muscle. The 32 individual AIFs estimated by the MRTM, which had considerable intra-patient and inter-patient variability, were similar to directly measured AIFs in the literature and using the MRTM AIFs in a pharmacokinetic model to derive estimated individual cardiac outputs provided physiologically reasonable results. The MRTM individual AIFs gave better fits with smaller sum of squared errors and equally reproducible estimate of kinetic parameters compared with a previous reported population AIF measured from remote arteries. The individual MRTM AIFs were also used to obtain a mean local tissue AIF for the unique population of this study, which further improved the reproducibility of the estimated kinetic parameters. The MRTM can be applied to DCE-MRI studies of bone metastases from prostate cancers to provide an AIF from which reproducible quantitative DCE-MRI parameters can be derived, thus help standardize DCE-MRI studies in cancer patients.

Estimation of absolute myocardial blood flow during first-pass MR perfusion imaging using a dual-bolus injection technique: comparison to single-bolus injection method

PURPOSE

To compare the dual-bolus to single-bolus quantitative first-pass magnetic resonance myocardial perfusion imaging for estimation of absolute myocardial blood flow (MBF).

MATERIALS AND METHODS

Dogs had local hyperemia of MBF in the left anterior descending (LAD) coronary artery (intracoronary adenosine). Animals (n = 6) had sequential single- and dual-bolus perfusion studies with microsphere determination of absolute MBF. Perfusion imaging was performed using a saturation-recovery gradient-echo sequence. Absolute MBF was by Fermi function deconvolution and compared to transmural, endocardial, and epicardial microsphere values in the same region of interest (ROI).

RESULTS

Signal and contrast were significantly higher for the dual-bolus perfusion images. The correlation with MBF by microspheres was r = 0.94 for the dual-bolus method and r = 0.91 for the single-bolus method. There was no significant difference between MRI and microsphere MBF values for control or hyperemic zones for transmural segments for either technique. When the ROI was reduced to define endocardial and epicardial zones, single-bolus MR first-pass imaging significantly overestimated MBF and had a significantly larger absolute error vs. microspheres when compared to dual-bolus perfusion.

CONCLUSION

Both single-bolus and dual-bolus perfusion methods correlate closely with MBF but the signal and contrast of the dual-bolus images are greater. With smaller nontransmural ROIs where signal is reduced, the dual-bolus method appeared to provide slightly more accurate results.

Multiple reference tissue method for contrast agent arterial input function estimation

A precise contrast agent (CA) arterial input function (AIF) is important for accurate quantitative analysis of dynamic contrast-enhanced (DCE)-MRI. This paper proposes a method to estimate the AIF using the dynamic data from multiple reference tissues, assuming that their AIFs have the same shape, with a possible difference in bolus arrival time. By minimizing a cost function, one can simultaneously estimate the parameters and underlying AIF of the reference tissues. The method is computationally efficient and the estimated AIF is smooth and can have higher temporal resolution than the original data. Simulations suggest that this method can provide a reliable estimate of the AIF for DCE-MRI data with a moderate signal-to-noise ratio (SNR) and temporal resolution, and its performance increases significantly as the SNR and temporal resolution increase. As demonstrated by its clinical application, sufficient reference tissues can be easily obtained from normal tissues and subregions segmented from a tumor region of interest (ROI), which suggests this method can be generally applied to cancer-based DCE-MRI studies to estimate the AIF. This method is applicable to general kinetic models in DCE-MRI, as well as other CE imaging modalities.

Myocardial First Pass Perfusion Imaging With Gadobutrol

OBJECTIVE

To implement parallel imaging algorithms in fast gradient recalled echo sequences for myocardial perfusion imaging and evaluate image quality, signal-to-noise ratio (SNR), contrast-enhancement ratio (CER), and semiquantitative perfusion parameters.

MATERIALS AND METHODS

In 20 volunteers, myocardial perfusion imaging with gadobutrol was performed at rest using an accelerated TurboFLASH sequence (TR 2.3 milliseconds, TE 0.93 milliseconds, flip angle [FA] 15 degrees) with GRAPPA, R=2. A nonaccelerated TurboFLASH sequence with similar scan parameters served as standard of reference. Artifacts were assessed qualitatively. SNR, CER, and CNR were calculated and semiquantitative perfusion parameters were determined from fitted SI-time curves.

RESULTS

Phantom measurements yielded significant higher SNR for nonaccelerated images (P<0.001). CER was equal; differences in CNR were statistically nonsignificant. The evaluation of semiquantitative perfusion parameters yielded significantly higher peak signal intensities in nonaccelerated images (P<0.001). Differences in maximum upslope were statistically nonsignificant. A qualitative examination of all images for artifacts by 2 board-certified radiologists yielded a significant reduction in dark rim artifacts with GRAPPA, R=2 (P<0.001).

CONCLUSIONS

The application of GRAPPA with an acceleration factor of R=2 leads to a significant reduction of dark rim artifacts in fast gradient recalled echo sequences.

Usage of the T1 effect of an iron oxide contrast agent in an animal model to quantify myocardial blood flow by MRI

BACKGROUND

To present a new method for fully quantitative analysis of myocardial blood flow (MBF) using magnetic resonance imaging. The first pass of an intravascular iron oxide contrast medium can be used to quantify myocardial perfusion. The technique was validated in an animal model using colored microspheres.

MATERIALS AND METHODS

In six pigs, a tracking catheter was positioned in the left anterior descending artery (LAD). Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) was performed on a 1.5-T scanner using a hybrid gradient-echo/echoplanar imaging (GRE-EPI) sequence. Regional myocardial blood flow (rMBF) was altered by either inducing vasodilatation with adenosine or creating coronary artery obstruction. The T(1) effect of a superparamagnetic iron oxide-based contrast medium (Resovist) administered at a dose of 8 micromol/kg was used. Upslope, time-to-peak and peak intensity were calculated from the signal intensity-time curves and absolute rMBF using the Kety-Schmidt equation; results were compared to those obtained using colored microspheres.

RESULTS

The mean rMBF calculated by MRI was 1.49 (+/-6.91, quartile width) ml/min/g versus 3.21 (+/-1.61) ml/min/g measured by means of microspheres under resting conditions. rMBF increased to a mean of 6.21 (+/-2.83) ml/min/g versus 4.22 (+/-1.70) ml/min/g under adenosine and was reduced to zero flow in total occlusion. Linear regression showed the best correlation for upslope (R=0.714), time-to-peak (R=0.626) and the Kety-Schmidt equation (R=0.584).

CONCLUSIONS

The T(1) effect of an iron oxide-based contrast medium allows determination of rMBF when using the Kety-Schmidt equation. The results are similar to those obtained with the standard of reference, colored micropheres, but not better than the results of the semiquantitative approach.

Myocardial perfusion

Noninvasive cardiac magnetic resonance (CMR) imaging has progressed rapidly over the past few years and will most likely become an integral part of the diagnostic workup of patients with known or suspected coronary artery disease (CAD). In this article the rationale for using perfusion-CMR is discussed, followed by a summary of current state-of-the-art perfusion-CMR techniques that addresses pharmacological stress, monitoring, pulse sequences, and doses of contrast media (CM) for first-pass studies. In the second part, unresolved aspects of perfusion-CMR, such as the lack of fully established and validated imaging protocols, are discussed. The optimum pulse sequence parameters, required cardiac coverage, analysis algorithms, criteria for data quality, and other aspects remain to be defined. Furthermore, since expertise in perfusion-CMR is not yet widely available, training of physicians and technicians to perform perfusion-CMR according to recognized standards is an important future requirement. In the last part of the review, some ideas are proposed to improve the management of patients with known or suspected CAD. This involves making a shift from a "reactive" strategy, in which patients are typically approached when they are symptomatic, to an "active" strategy, in which perfusion-CMR is performed for early detection of high-risk patients so that revascularizations can be performed before potentially deadly infarcts occur. An ideal test for such an active strategy would be highly accurate, reliable, safe (and thus repeatable), and affordable. Large multicenter trials have shown that in experienced centers perfusion-CMR is reliable and repeatable, and it is hoped that future studies will demonstrate its cost-effectiveness as well.

Myocardial perfusion imaging by cardiac magnetic resonance

Cardiovascular magnetic resonance (CMR) has been shown to provide high quality data on cardiac and valvular function, perfusion, viability, blood flow, and potentially, on cardiac metabolism as well. Several of these CMR applications (eg, function and viability assessment) matured during the past years and are now established components of a cardiac workup. Perfusion-CMR is close to this status and is already a major contributor to cardiac examinations in a growing number of expert centers. Large multicenter perfusion-CMR trials comparing the diagnostic performance of CMR with other techniques were recently reported yielding areas under the receiver-operator-characteristics curve as a high as 0.85 for coronary artery disease detection (MR-IMPACT). Anticipating a growing role for perfusion-CMR in cardiology in the near future, this article discusses the principles of perfusion-CMR and its integration into the workup of patient with coronary artery disease (CAD). In addition to a functional study, this integration is mainly composed of a perfusion-CMR part, followed by a viability assessment by late enhancement CMR techniques. The principal characteristics of these CMR techniques are compared with those of single photon emission computed tomography (SPECT) and positron emission tomography (PET). After introduction into principles and techniques of perfusion-CMR, some open questions in perfusion-CMR and challenges for the future are addressed. Finally, newer CMR applications are shortly mentioned utilizing hyperpolarized carbon-13 compounds in experimental models for quantification of myocardial perfusion and for real-time assessment of metabolic pathways in postischemic myocardium.

Contrast–dose relation in first-pass myocardial MR perfusion imaging

PURPOSE

To determine the regime of linear contrast enhancement in human first-pass perfusion cardiovascular magnetic resonance (CMR) imaging to improve accuracy in myocardial perfusion quantification.

MATERIALS AND METHODS

A total of 10 healthy subjects were studied on a clinical 1.5T MR scanner. Seven doses of Gd-DTPA ranging from 0.00125 to 0.1 mmol/kg of body weight (b.w.) were administered as equal volumes by rapid bolus injection (6 mL/second). Resting periods of 15 minutes were introduced after delivery of Gd doses >0.01 mmol/kg b.w. For each subject, two series of rest perfusion scans were performed using two different multislice saturation-recovery perfusion sequences. Maximum contrast enhancement and maximum upslope were obtained in the blood pool of the left ventricular (LV) cavity and in the myocardium. The range of linear contrast-dose relation was determined by linear regression analysis.

RESULTS

MR signal intensity increased linearly for contrast agent concentrations up to 0.01 mmol/kg b.w. in the LV blood pool and up to 0.05 mmol/kg b.w. in the myocardium. For Gd concentrations exceeding these thresholds the signal intensity response was not linear with respect to the contrast agent dose.

CONCLUSION

Quantitative evaluation of cardiac MR perfusion data needs to account for signal saturation in both the LV blood pool and the myocardium.

Novel approach to the measurement of absolute cerebral blood volume using vascular-space-occupancy magnetic resonance imaging

Quantitative determination of cerebral blood volume (CBV) is important for understanding brain physiology and pathophysiology. In this work, a novel approach is presented for accurate measurement of absolute CBV (aCBV) using vascular-space-occupancy (VASO) MRI, a blood-nulling pulse sequence, in combination with the T(1) shortening property of Gd-DTPA. Two VASO images with identical imaging parameters are acquired before and after contrast agent injection, resulting in a subtracted image that reflects the amount of blood present in the brain, i.e., CBV. With an additional normalizing factor, aCBV in units of milliliters of blood per 100 mL of brain can be estimated. Experimental results at 1.5 and 3 T systems showed that aCBV maps with high spatial resolution can be obtained with high reproducibility. The averaged aCBV values in gray and white matter were 5.5 +/- 0.2 and 1.4 +/- 0.1 mL of blood/100 mL of brain, respectively. Compared to dynamic susceptibility contrast techniques, VASO MRI is based upon a relatively straightforward theory and the calculation of CBV does not require measurement of an arterial input function. In comparison with previous pre/postcontrast difference approaches, VASO MRI provides maximal signal difference between pre- and postcontrast situation and does not require the use of whole blood for signal normalization.

In vivo quantification of regional myocardial blood flow: Validity of the fast-exchange approximation for intravascularT1 contrast agent and long inversion time

In the present study we investigated the effects of water exchange between intra- and extravascular compartments on absolute quantification of regional myocardial blood flow (rMBF) using a saturation-recovery sequence with a rather long inversion time (TI, 176 ms) and a T1-shortening intravascular contrast agent (CMD-A2-Gd-DOTA). Data were acquired in normal and ischemically injured pigs, with radiolabeled microsphere flow measurements used as the gold standard. Five water exchange rates (fast, 6 Hz, 3 Hz, 1 Hz, and no exchange) were tested. The results demonstrate that the fast-exchange approximation may be appropriate for rMBF quantification using the described experimental setting. Relaxation rate change (DeltaR1) analysis improved the accuracy of the analysis of rMBF compared to the MR signal. In conclusion, the current protocol could provide sufficient accuracy for estimating rMBF assuming fast exchange and a linear relationship between signal and tissue concentration when quantification of precontrast T1 is not an option.

Changes in First-Pass Interstitial Kinetics of DTPA in Myocardium Submitted to Low-Flow Ischemia

OBJECTIVES

This study aimed to determine the changes during ischemia in the myocardial first-pass kinetics of DTPA, an extracellular tracer that is currently used for assessing myocardial perfusion with magnetic resonance imaging (Magnevist).

MATERIALS AND METHODS

Using an indicator-dilution technique, first-pass kinetics of DTPA were compared between normoxia (n=11) and low-flow ischemia (n=11) in isolated rabbit hearts perfused with red blood cell-enhanced perfusate.

RESULTS

There was no difference between ischemia and normoxia in the interstitial extraction and clearance rates of DTPA. Interstitial distribution volume of DTPA was, however, lower in ischemia than in normoxia (in percent of myocardial volume: 15+/-11% vs 25+/-11%, P=0.02) as a result of a relationship with coronary flow (P<0.001).

CONCLUSIONS

During low-flow myocardial ischemia, DTPA kinetics are unchanged, except for the interstitial distribution volume that is decreased, presumably because of the shrinkage of extracellular fluid. These kinetic properties are favorable for detecting myocardial ischemia at rest with magnetic resonance imaging.

Optimization of a blood pool contrast agent injection protocol for MR angiography

PURPOSE

To design an ideal first-pass profile for MR angiography (MRA) by optimizing a multiphasic injection protocol based on two experimental animal models.

MATERIALS AND METHODS

An equivalent contrast-enhanced (CE) MRA injection protocol was developed with controlled injection modalities (injection rate, volume, and dose) in rabbits and pigs. P792, a blood pool contrast agent, was injected in 17 male New Zealand rabbits and five farm pigs with variable injection schemes (mono- and multiphasic). From the gadolinium (Gd) blood concentration data, a simulation of an MR acquisition was performed to evaluate the impact of such an injection protocol on MR arterial signal and to select the best injection protocol.

RESULTS

An empirical relationship between the arterial peak concentration and the injection parameters was found in the rabbits and pigs, allowing precise prediction of the first-pass profile. Of the four injection scheme strategies tested (standard bolus and bi-, tri-, and multiphasic injection protocols), the multiphasic "ramp" injection protocol provided the most optimal contrast agent pharmacokinetics with a durable plateau of concentration.

CONCLUSION

Ramp injection protocol provides an optimized first-pass profile for CE-MRA.

In and Ex Vivo MR Evaluation of Acute Myocardial Ischemia in Pigs by Determining R1 in Steady State After the Administration of the Intravascular Contrast Agent NC100150 Injection

RATIONALE AND OBJECTIVES

To study the dose response in perfused and nonperfused myocardium by measuring relaxation rate (R1) in a steady-state situation after injection of the intravascular contrast agent NC100150 Injection in pigs and whether the dose response differs in vivo and ex vivo.

MATERIALS AND METHODS

The left anterior descending artery was occluded. R1 was measured using a Look-Locker sequence for 2 dose groups (2 mg Fe/kg bw, n = 4, and 5 mg Fe/kg bw, n = 5) and a control group (n = 3).

RESULTS

A significant increase in R1 was found in perfused myocardium after contrast agent injection, in contrast to nonperfused myocardium. There was a significantly larger difference in R1 between perfused and nonperfused myocardium in the 5 mg Fe/kg bw dose group compared with the other 2 groups. The difference in R1 between perfused and nonperfused myocardium was significantly higher in vivo than ex vivo.

CONCLUSION

A nearly linear R1 dose response was found in perfused myocardium in vivo. The dose response ex vivo was less steep possibly due to larger water exchange limitations.

Absolute myocardial perfusion in canines measured by using dual-bolus first-pass MR imaging

PURPOSE

To compare fluorescent microsphere measurements of myocardial blood flow (MBF) with qualitative, semiquantitative, and fully quantitative measurements of first-pass perfusion at magnetic resonance (MR) imaging.

MATERIALS AND METHODS

Coronary artery occlusion or intracoronary adenosine infusion was successfully performed in 16 beagles; both procedures were performed simultaneously in one animal. MBF was assessed at microsphere analysis. First-pass myocardial perfusion MR imaging was performed during a dual-bolus administration of gadopentetate dimeglumine (0.0025 mmol/kg followed by 0.10 mmol/kg). The absolute myocardial perfusion at MR imaging was calculated by using Fermi function deconvolution methods. Qualitative, semiquantitative, and absolute myocardial perfusion MR imaging measurements were compared with microsphere MBF measurements by using paired t tests, linear correlation, and Bland-Altman analysis.

RESULTS

Fully quantitative (ie, absolute) analysis of MBF at MR imaging correlated with microsphere MBF measurement (r = 0.95, P <.001) across the full range of blood flow rates encountered (from 0 to >5.0 mL/min/g). Similar close correlations were observed in endocardial and epicardial segments (representing approximately 0.85 g of the myocardium). With modest increases in MBF, qualitative measurements plateaued in the hyperemic zones. Semiquantitative measurements did not correlate with MBF as well (r = 0.69-0.89); they plateaued around 3.0 mL/min/g.

CONCLUSION

Dual-bolus MR imaging enabled accurate measurement of absolute epicardial and endocardial perfusion across a wide range of blood flow rates (0 to >5.0 mL/min/g). Use of qualitative MR imaging measures such as the contrast enhancement ratio led to substantially underestimated hyperemic blood flow measurements.

Assessment of myocardial blood volume and water exchange: theoretical considerations and in vivo results

The measured signal response in contrast-enhanced myocardial perfusion imaging has been shown to be affected by the rate of water exchange between the intravascular and extravascular compartments, the effect being particularly significant when intravascular contrast agents are used. In the present study, the T(1) relaxation rates were measured in eight pigs in blood and myocardium using a Look-Locker sequence after repeated injections of the intravascular contrast agent NC100150. The selection of myocardial region of interest was automated based on a minimum chi-square method. The intra- and extravascular water exchange rates and the myocardial blood volume were calculated from the measured relaxation rates by applying a two-compartment water exchange limited model that accounts for biexponential longitudinal relaxation. The following (mean +/- SD) values were obtained for the exchange frequency (f), the extravascular residence time (tau(e)), the intravascular residence time (tau(i)) and blood volume (BV), respectively: f = 1.39 +/- 0.52 s(-1), tau(e) = 708 +/- 264 ms, tau(i) = 107 +/- 63 ms, and BV = 11.2 +/- 2.1 mL/100 g. The mean value of f was found to be about 15% higher if biexponential relaxation was not accounted for, supporting the hypothesis that significant biexponential relaxation in tissues with large blood volume can lead to an overestimation of water exchange rates unless corrected for.

MR imaging of myocardial perfusion and viability

CMR is a rapidly developing new modality with applications in clinical cardiology for detection and assessment of myocardial ischemia and viability. CMR perfusion results for the detection of ischemia in comparison with stress echocardiography and scintigraphic techniques are reasonable, but all the studies reported to date have been conduced in selected patients. Larger studies in patient populations reflecting a broader spectrum of disease are necessary before perfusion CMR can be envisaged as a clinically reliable and robust diagnostic tool. Other CMR techniques provide a variety of novel methods of obtaining information on postischemic viability. Signs of viability that can be observed by CMR are the absence of late gadolinium-based contrast enhancement in a myocardial region involved in a recent infarct, any sign of wall thickening at rest (which is detectable with high accuracy by CMR), wall thickening after stimulation by low-dose dobutamine, and preserved wall thickness. Conversely, myocardial necrosis is characterized by signal enhancement of the infarct area after injection of Gd-DTPA, reduced wall thickness in chronic infarcts, and absence of a contractile reserve during dobutamine stimulation. Dobutamine CMR and late enhancement contrast-enhanced CMR predict contractile improvement after revascularization.

Pharmacokinetic modeling of Gd-DTPA extravasation in brain tumors

RATIONALE AND OBJECTIVES

The endothelial permeability of brain tumors affecting the blood-brain barrier can be quantified using dynamic contrast enhanced MR imaging.

MATERIALS AND METHODS

The dynamics of contrast medium (CM) exchange was investigated in 31 brain tumors (intra- and extra-axial) by fast repeated T1-weighted imaging in order to determine the exchange rates and saturation concentrations.

RESULTS

The analysis of CM exchange reveals two different transport processes from the blood into two separate interstitial subcompartments for intra- and extra-axial tumors, a rapid one with a transfer constant of 1/ =7.0 seconds and a slow one with 1/ =133.7 seconds. Highly significant differences exhibit gliomas and meningiomas with respect to the saturation concentrations of fast and slowly enhancing compartments.

CONCLUSIONS

The fast component is probably caused by extravasation into viable tissue (enhanced in meningiomas) while the slow one probably reflects increased diffusion distances into poorly perfused tissue such as necrotic areas in glioblastomas.

Multislice first-pass cardiac perfusion MRI: validation in a model of myocardial infarction

The purpose of this study was to validate a first-pass MRI method for imaging myocardial perfusion with multislice coverage and relatively small analyzable regions of interest (ROIs). A fast gradient-echo (FGRE) sequence with an echo-train (ET) readout was used to achieve multislice coverage, and a high dose of a contrast agent (CA) was used to achieve a high signal-to-noise ratio (SNR). Dogs (N = 6) were studied 1 day after reperfused myocardial infarction, and fluorescent microspheres were used as a standard for perfusion. First-pass MRI correlated well vs. microsphere flow, achieving mean R values of 0.87 (range = 0.82-0.93), 0.71 (range = 0.46-0.85), and 0.72 (range = 0.49-0.95) for subendocardial ROIs, transmural ROIs, and the endocardial-epicardial ratio, respectively. Additionally, analysis of myocardial time-intensity curves (TICs) indicated that 15.8 +/- 6 sectors, corresponding to 260 microl of endocardium, can be analyzed (R(2) > 0.95).

Assessment of hepatic perfusion parameters with dynamic MRI

Quantification of hepatic perfusion parameters greatly contributes to the assessment of liver function. The purpose of this study was to describe and validate the use of dynamic MRI for the noninvasive assessment of hepatic perfusion parameters. The signal from a fast T(1)-weighted spoiled gradient-echo sequence preceded by a nonslice-selective 90 degrees pulse and a spoiler gradient was calibrated in vitro with tubes filled with various gadolinium concentrations. Dynamic images of the liver were obtained after intravenous bolus administration of 0.05 mmol/kg of Gd-DOTA in rabbits with normal liver function. Hepatic, aortic, and portal venous signal intensities were converted to Gd-DOTA concentrations according to the in vitro calibration curve and fitted with a dual-input one-compartmental model. With MRI, hepatic blood flow was 100 +/- 35 mL min(-1) 100 mL(-1), the arterial fraction 24 +/- 11%, the distribution volume 13.0 +/- 3.7%, and the mean transit time 8.9 +/- 4.1 sec. A linear relationship was observed between perfusion values obtained with MRI and with radiolabeled microspheres (r = 0.93 for hepatic blood flow [P < 0.001], r = 0.79 for arterial blood flow [P = 0.01], and r = 0.91 for portal blood flow [P < 0.001]). Our results indicate that hepatic perfusion parameters can be assessed with dynamic MRI and compartmental modeling.

Quantification of the effect of water exchange in dynamic contrast MRI perfusion measurements in the brain and heart

Measurement of myocardial and brain perfusion when using exogenous contrast agents (CAs) such as gadolinium-DTPA (Gd-DTPA) and MRI is affected by the diffusion of water between compartments. This water exchange may have an impact on signal enhancement, or, equivalently, on the longitudinal relaxation rate, and could therefore cause a systematic error in the calculation of perfusion (F) or the perfusion-related parameter, the unidirectional influx constant over the capillary membranes (K(i)). The aim of this study was to quantify the effect of water exchange on estimated perfusion (F or K(i)) by using a realistic simulation. These results were verified by in vivo studies of the heart and brain in humans. The conclusion is that water exchange between the vascular and extravascular extracellular space has no effect on K(i) estimation in the myocardium when a normal dose of Gd-DTPA is used. Water exchange can have a significant effect on perfusion estimation (F) in the brain when using Gd-DTPA, where it acts as an intravascular contrast agent.

Pharmacokinetic analysis of glioma compartments with dynamic Gd-DTPA-enhanced magnetic resonance imaging

Dynamic contrast-enhanced magnetic resonance imaging (MRI) is widely used for measuring perfusion and blood volume, especially cerebral blood volume (CBV). In case of blood-brain barrier (BBB) disruption, the conventional techniques only partially determine the pharmacokinetic parameters of contrast medium (CM) exchange between different compartments. Here a modified pharmacokinetic model is applied, which is based on the bidirectional CM exchange between blood and two interstitial compartments in terms of the fractional volumes of the compartments and the vessel permeabilities between them. The evaluation technique using this model allows one to quantify the fractional volumes of the different compartments (blood, cells, slowly and fast enhancing interstitium) as well as the vessel permeabilities and cerebral blood flow (CBF) with a single T1-weighted dynamic MRI measurement. The method has been successfully applied in 25 glioma patients for generating maps of all of these parameters. The fractional volume maps allow for the differentiation of glioma vascularization types. The maps show a good correlation with the histological grading of these tumors. Furthermore, regions with enhanced interstitial volumes are found in high-grade gliomas. Differences in permeability maps of Gd-DTPA apart from BBB disruption do not exist between different tissue types. CBF measured in high-grade glioma is less pronounced than it would be expected from their blood volume. Therefore pharmacokinetic imaging provides an additional tool for glioma characterization.

Does a higher concentration of gadolinium chelates improve first-pass cardiac signal changes?

The purpose of this study was to evaluate first-pass cardiac signal changes with a higher concentrated gadolinium-chelate (gadobutrol) and its influence on bolus geometry. Phantom studies and in vivo first-pass cardiac studies were performed in rabbits (n = 8 experiments) under general anesthesia at 1.0 T using an ultrafast T1-weighted Turbo-fast low-angle shot (FLASH) sequence (TR/TE 4.7/1. 6 msec, alpha = 90 degrees ) with a time resolution of 870 msec. Gadobutrol was injected as an intravenous bolus at two concentrations (0.5 and 1.0 mol Gd/L) and five doses (0.3, 0.15, 0.1, 0.055, and 0.03 mmol Gd/kg bw). The blood-pool gadolinium compound gadopentetate dimeglumine-polylysine (0.15, 0.075, 0.05, and 0.015 mmol Gd/kg bw, 0.5 mol Gd/L) and the standard extracellular gadopentetate dimeglumine (0.1 and 0.05 mmol Gd/kg bw, 0.5 mol Gd/L) served as reference agents. Cardiac signal changes were calculated from serial signal intensity measurements. Maximum signal intensity changes and best peak profiles during first pass of the right and left ventricle were observed with a dose of 0.03 mmol Gd/kg bw gadobutrol using T1-weighted Turbo-FLASH. At the low application volumes used, the higher concentration of 1.0 mol Gd/L gadobutrol did not increase the degree of signal intensity changes or sharpen the bolus profile. First-pass cardiac signal changes using T1-weighted Turbo-FLASH with the new extracellular contrast agent gadobutrol are best observed at a dose of 0.03 mmol Gd/kg bw. There is no advantage to the concentrated formulation (1 mol Gd/L gadobutrol) when using small injection volumes. J. Magn. Reson. Imaging 1999;10:806-812.

Blood pool contrast agents for cardiovascular MR imaging

The distribution and elimination of contrast agents is mainly determined by their size. First-pass perfusion with the use of blood pool contrast agents (BPCAs) and/or rapid clearance blood-pool-like contrast agents may allow quantitative myocardial perfusion evaluation in patients. This requires contrast bolus injection with a very fast injection speed. A major profit from BPCAs is expected for magnetic resonance angiography (MRA). The persistent signal-enhancing effects of BPCAs allow for a longer acquisition time window, which may be used to increase both the signal-to-noise ratio and/or image resolution. This is of paramount importance for coronary imaging, in which high-resolution imaging is desired. Moreover, the improved acquisition time window can be used to make multiple scans after one contrast injection. The role of ultrasmall paramagnetic iron oxide particles (USPIOs) for MRA is not clear yet, as they are limited by T2* effects at higher doses. Several safety aspects have to be taken into account before BPCAs are applied in humans, for whom toxicity caused by the injection speed is a concern.

Magnetic resonance perfusion imaging in ischemic heart disease

This review explores the present status of contrast media available for myocardial perfusion studies, the magnetic resonance (MR) sequences adapted to multi-slice first-pass acquisitions, and the issue of myocardial perfusion quantification. To date, only low molecular weight paramagnetic gadolinium chelates have been used in clinical protocols for myocardial perfusion. With the availability of fast MR acquisition techniques to follow the first-pass distribution of the contrast agent in the myocardium, the bolus tracking technique represents the more widely used protocol in MR perfusion studies. On T1-weighted imaging, the ischemic zone appears with a delayed and lower signal enhancement compared with normally perfused myocardium. Visual analysis of the image series can be greatly improved by image post-processing to obtain relative myocardial perfusion maps. With an intravascular tracer, myocardial kinetics are in theory easier to analyze in terms of perfusion. In experimental studies, different intravascular or blood pool MR contrast agents have been tested to measure quantitative perfusion parameters. If a simple flow-limited kinetic model is developed with MR contrast agents, one important clinical application will be the evaluation of the functional consequence of coronary stenoses, ie, non-invasive evaluation of the coronary reserve.

Blood pool contrast agent CMD-A2-Gd-DOTA-enhanced MR imaging of infarcted myocardium in pigs

The purpose of this study was to assess the ability of the new blood pool contrast agent meglumine-carboxymethyldextran-ethylenediamino-gadoterate (CMD-A2-Gd-DOTA) to depict acute occlusive myocardial infarction (AMI). First-pass gradient-echo and delayed spin-echo magnetic resonance imaging (MRI) was performed 5 days after induction of AMI in a pig model. MRI was correlated with pathology. First-pass imaging with CMD-A2-Gd-DOTA allowed detection of infarcted myocardium in all pigs (n = 7). The infarction was recognized as a black spot on MRI as well as on a parametric image. The signal intensity (SI) amplitudes of normal versus infarcted myocardium were 80.55 +/- 18.61 versus 8.48 +/- 15.50 on MRI and 81.62 +/- 18.50 versus 1.61 +/- 3.73 on the parametric image (both P values < 0.001. The contrast ratio between normal and infarcted myocardium was not significantly improved on spin-echo MRI, suggesting largely intact vascular integrity outside the occluded area. CMD-A2-Gd-DOTA is useful for depicting occlusive myocardial infarction by first-pass MRI. Spin-echo imaging is promising in assessing vascular integrity. J. Magn. Reson. Imaging 1999;10:170-177.

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.

Use of the mean transit time of an intravascular contrast agent as an exchange-insensitive index of myocardial perfusion

A simple two-compartment model was used to study the effects of water exchange on the signal produced by an inversion recovery prepared rapid gradient-echo sequence during the first passage of a low dose of an intravascular contrast agent. Water exchange at intermediate rates of exchange (1-10 Hz) between the vascular and extravascular spaces caused the form of the signal changes during the first pass to be dependent on both the fractional sizes of the vascular and extravascular compartments and on the exchange rate. Unless the effects of exchange are minimized by using a very short inversion time, parameters such as the peak height and area under the curve will be affected by regional and/or pathological variations in the exchange rate and the size of the vascular fraction. The mean transit time (MTT) is, however, less affected by water exchange. Experimental first-pass data produced by intravascular low-dose injections of iron oxide particles were studied in five pigs at 0.5 T. The MTT as derived from the first-pass curves, without deconvolution with the arterial input function, was well correlated with the myocardial blood flow (MBF) as measured using radioactive microspheres (r = 0.70, n = 52, P < 0.01). Other first-pass parameters such as the peak height or area under the curve exhibited either a poorer, or no, correlation with the MBF. The data suggest that the MTT of the first pass of an intravascular contrast agent may be a robust, quantitative method for assessing myocardial blood flow in patients.

Assessment of myocardial function and perfusion in a canine model of non-occlusive coronary artery stenosis using fast magnetic resonance imaging

Magnetic resonance (MR) functional and perfusion imaging were employed in a canine model of coronary artery stenosis (n = 6) for the quantification of functional and perfusion deficits before and after dipyridamole administration. Left anterior descending and circumflex (LCX) coronary blood flow were measured continuously after placing Doppler flowmeters. Inversion recovery gradient echo images during the transit of MR contrast medium gadolinium-benzyloxypropionictetraacetate dimeglumine (Gd-BOPTA/Dimeg) and fast breath-hold cine MR images were acquired at baseline, during LCX stenosis in basal state, and during LCX stenosis with vasodilation (dipyridamole 0.5 mg/kg). The extent of the functional defect and perfusion defect was expressed as percent of left ventricle (LV) circumference. During stenosis (LCX flow: 62.6 +/- 5.6% of baseline) the extent of the functional defect was slightly larger than the perfusion defect (11.0 +/- 1.8% versus 6.3 +/- 1.70% of LV circumference, respectively; P < 0.01). During vasodilation the extent of the functional defect was considerably smaller than the perfusion defect (25.3 +/- 2.5% versus 35.3 +/- 3.5%; P < 0.01). Thus, the sizes of ischemic regions displayed by MR perfusion defect and functional defect differ from each other.

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.

Quantitative regional blood volume studies in rat myocardium in vivo

Many pathophysiological processes in the myocardium are in close relation to changes of the regional blood volume and regional myocardial blood flow or perfusion. Only few methods exist to obtain quantitative values for these parameters. Quantitative regional blood volume (RBV) studies in rat myocardium are presented using snapshot fast low angle shot (FLASH) inversion recovery T1 measurements with two different blood pool contrast agents, gadolinium diethylenetriaminopentaacetic acid (Gd-DTPA) albumin and Gd-DTPA polylysine. In contrast to previous attempts, each snapshot FLASH image acquisition was ECG-triggered under breathhold conditions. To measure relaxation times shorter than a heart cycle, each T1 sequence was repeated two times with different delays between inversion pulse and first image acquisition. The experiments were performed on a Bruker Biospec 70/21 using a homogeneous transmitter coil and a circularly polarized surface receiver coil, a special ECG trigger unit, and a respirator that is controlled by the pulse program. Based on a fast exchange model RBVm maps were calculated from the relaxation time maps for different concentrations of the two blood pool contrast agents. A significant dependence of the RBVm values on blood T1 was found. This is in accordance with a model that has been developed recently relating the dependence of RBVm on T1 of blood to perfusion. For Gd-DTPA albumin, the application of the model to the experimental data yields realistic values for RBV and perfusion. The values, which are in accordance with literature data, were obtained at highest contrast agent concentrations i.e., lowest relaxation times of blood (ca. 200 ms).

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.

Histologic confirmation of microvascular hyperpermeability to macromolecular MR contrast medium in reperfused myocardial infarction

A macromolecular MR contrast medium (MMCM) designed to permit histochemical staining and specific tissue localization, albumin-(biotin)10-(Gd-DTPA)25 (Bio-Alb-Gd), was used in a rat model of reperfused myocardial infarction to confirm the presence and distribution of microvascular hyperpermeability. T1-weighted spin-echo images were acquired before and after administration of Bio-Alb-Gd. An avidin-biotin-complex (ABC) stain, specific for the biotinylated MR contrast medium, was used to define the MMCM distribution and to detect any regional change in microvascular permeability related to infarction. Immediately after Bio-Alb-Gd administration, the infarcted region was enhanced, with greatest signal intensity noted at the rim and less at the center. There was a gradual increase in signal intensity of the initially hypointense central region. The steady increase in signal intensity of the central region suggested convection transport of MMCM through the interstitial space and its influx into cellular compartment after leakage from the vascular compartment. Histologic findings confirmed regional microvascular hyperpermeability corresponding to the site of infarction and a predominant rim distribution of the MMCM. Bio-Alb-Gd was identified at high microscopic power in the intravascular, interstitial, and intracellular spaces at the periphery of reperfused infarcted myocardium. Bio-Alb-Gd can be used as an MR contrast medium in reperfused infarcted myocardium to confirm the existence and to localize altered microvascular permeability to macromolecules. Bio-Alb-Gd contrast technique removes all the ambiguity between the distribution of the MR or other imaging contrast agent and the distribution of the substrate for histochemical staining.

Imaging perfusion deficits in ischemic heart disease with susceptibility-enhanced T2-weighted MRI: preliminary human studies

AIM

This feasibility study explores relative myocardial perfusion characterization with an investigational T2/T2 contrast agent.

METHODS

Dysprosium-DTPA bis (methylamide) was administered peripherally in six patients with thallium defects. Rest and stress multi-section, gated, T2-weighted images were acquired with a 1.5 T echo-planar imager. Change in transverse relaxation rate was calculated in four segments for each subject.

RESULTS

Magnetic resonance (MR) identified five of five instances of ischemia or infarction, at a dose of agent (0.25 mmol/kg) that was comparable to that currently used with clinically approved gadolinium agents. Injection at twice this dose resulted in saturation of the signal change, and the one ischemic segment corresponding to the higher dose was not identified by MR. MR was negative in two segments which, on final diagnosis, were determined to manifest thallium attenuation artifact.

CONCLUSION

MR perfusion imaging with high susceptibility agents has the potential to characterize myocardial perfusion deficits.

Contrast-agent phase effects: an experimental system for analysis of susceptibility, concentration, and bolus input function kinetics

A system is presented for experimental arterial input function (AIF) simulation and for accurate measurement of the concentration, susceptibility effects, and magnetic moment of paramagnetic MR contrast agents. Signal effects of contrast agents are evaluated with a stable, well-characterized, and precise experimental setup. A cylindrical phantom and a closed-loop circulating flow system were designed for AIF simulation, assessment of the physical determinants of contrast-agent phase effects, and measurement of contrast-agent properties under controlled conditions. A mathematical model of the AIF dynamics is proposed. From the experimental phase shift (delta phi), either the concentration or molar susceptibility, chiM, is determined. The linear dependence of delta phi on concentration and echo time (TE), the orientation dependence, and the lack of dependence on T1, T2, and diffusion time are proven precisely for water solutions under a wide variety of conditions. The measured effective magnetic moment of Gd+3, mu(eff), was 7.924 +/- 0.015 Bohr magnetons in agreement with the theoretical value of 7.937.

MRI quantitative myocardial perfusion with compartmental analysis: a rest and stress study

K1 (first-order transfer constant from arterial plasma to myocardium for Gd-DTPA) and Vd (distribution volume of Gd-DTPA in myocardium) were measured in vivo in a canine model (n = 5) using MRI-derived myocardial perfusion curves and a compartmental model. Perfusion curves were obtained after a bolus injection of Gd-DTPA (0.04 mM/kg) with an inversion-prepared fast gradient echo sequence. Myocardium and blood signal intensity were converted to a concentration of Gd-DTPA, according to a model appropriate for short (<1 s) interimage intervals characteristic of cardiac-triggered acquisitions. Before dipyridamole-induced stress, K1 and Vd, obtained from the fit of the MRI-derived perfusion curves, were 6.2 +/- 1.4 (mHz) and 17.5 +/- 4.2%, respectively. After dipyridamole infusion, a K1 increase of a factor of 2.82 +/- 0.72 was measured (P = 0.003). No change was observed in Vd (P = 0.98). These results suggest that the K1 increase after dipyridamole reflects a flow-related effect that can be useful to quantify the MRI-derived perfusion curves.

MRI for the evaluation of regional myocardial perfusion in an experimental animal model

Myocardial perfusion was assessed in nine pigs using ultrafast gradient-echo MRI (.5 T, 15-mT/m gradients) at different levels of myocardial blood flow (range, .005-1.84 ml/min/g), generated either by adenosine infusion or by a mechanical occluder, and measured independently using radiolabeled microspheres. Sixty-four consecutive, ECG-triggered, diastolic, short axis images of the left ventricle were obtained during intravenous bolus injections (n = 30) of .05 mmol/kg of gadopentetate dimeglumine. Relative changes in peak intensity, time to peak intensity, washin slope, correlation coefficient, and cross-correlation coefficient were computed from the time-intensity curves obtained from four regions of interest, namely septal, anterior, lateral, and inferior walls. The values from the inferior wall acted as reference for evaluating relative changes in the other three regions. The cross-correlation coefficient (P < .001, rho = .60) and the peak intensity (P < .001, r = .72) showed the best correlation with myocardial blood flow. The washin slope showed a weak positive trend (P < .05), but the low value of r (r = .28) indicated that the use of this parameter to predict flow was invalid; the correlation coefficient and time to peak intensity were not correlated (P = ns). In conclusion, this study shows that it is possible to evaluate relative myocardial perfusion after the first pass of a an intravenously injected bolus of gadopentetate dimeglumine, using dynamic MRI on a conventional medium field MRI system. The cross-correlation coefficient and the peak intensity resulted in more efficient parameters to evaluate relative inhomogeneity of regional myocardial perfusion.

Alterations in T1 of normal and reperfused infarcted myocardium after Gd-BOPTA versus GD-DTPA on inversion recovery EPI

This study tested whether Gd-BOPTA/Dimeg or Gd-DTPA exerts greater relaxation enhancement for blood and reperfused infarcted myocardium. Relaxivity of Gd-BOPTA is increased by weak binding to serum albumin. Thirty-six rats were subjected to reperfused infarction before contrast (doses = 0.05, 0.1, and 0.2 mmol/kg). delta R1 was repeatedly measured over 30 min. Gd-BOPTA caused greater delta R1 for blood and myocardium than did Gd-DTPA; clearance of both agents from normal- and infarcted myocardium was similar to blood clearance; plots of delta R1 myocardium/delta R1 blood showed equilibrium phase contrast distribution. Fractional contrast agent distribution volumes were approximately 0.24 for both agents in normal myocardium, 0.98 and 1.6 for Gd-DTPA and Gd-BOPTA, respectively, in reperfused infarction. The high value for Gd-BOPTPA was ascribed to greater relaxivity in infarction versus blood. It was concluded that Gd-BOPTA/Dimeg causes a greater delta R1 than Gd-DTPA in regions which contain serum albumin.