Jane Sykes

Simultaneous MRI Measurement of Blood Flow, Blood Volume, and Capillary Permeability in Mammary Tumors Using Two Different Contrast Agents

A technique for the simultaneous measurement of three vascular parameters: blood flow (Fρ), blood volume (vb), and the capillary permeability‐surface area product (PSρ) in breast tumors using dynamic contrast‐enhanced magnetic resonance imaging (MRI) is presented. Features of the technique include measurement of precontrast tumor T1, rapid temporal sampling, measurement of the arterial input function, and use of a distributed parameter tracer kinetic model. Parameter measurements are compared that were determined using two contrast agents of different molecular weights, gadolinium‐diethylene triamine pentaacetic acid (Gd‐DTPA; 0.6 kDa) and Gadomer‐17 (17 kDa), in 18 spontaneous canine mammary tumors. Measurements of Fρ and vb corresponded well with literature values, and the mean PSρ measured using Gd‐DTPA was a factor of 15 higher than that measured using Gadomer‐17. J. Magn. Reson. Imaging 2000;12:991–1003.

Detection and Quantification of Myocardial Reperfusion Hemorrhage Using T2*-Weighted CMR

OBJECTIVES The purpose of this study was to validate T2*-weighted cardiac magnetic resonance (T2*-CMR) for the detection and quantification of reperfusion hemorrhage in vivo against an ex vivo gold standard, and to investigate the relationship of hemorrhage to microvascular obstruction, infarct size, and left ventricular (LV) functional parameters. BACKGROUND Hemorrhage can contribute to reperfusion injury in myocardial infarction and may have significant implications for patient management. There is currently no validated imaging method to assess reperfusion hemorrhage in vivo. T2*-CMR appears suitable because it can create image contrast on the basis of magnetic field effects of hemoglobin degradation products. METHODS In 14 mongrel dogs, myocardial infarction was experimentally induced. On day 3 post-reperfusion, an in vivo CMR study was performed including a T2*-weighted gradient-echo imaging sequence for hemorrhage, standard sequences for LV function, and post-contrast sequences for microvascular obstruction and myocardial necrosis. Ex vivo, thioflavin S imaging and triphenyl-tetrazoliumchloride (TTC) staining were performed to assess microvascular obstruction, hemorrhage, and myocardial necrosis. Images were analyzed by blinded observers, and comparative statistics were performed. RESULTS Hemorrhage occurred only in the dogs with the largest infarctions and the greatest extent of microvascular obstruction, and it was associated with more compromised LV functional parameters. Of 40 hemorrhagic segments on TTC staining, 37 (92.5%) were positive for hemorrhage on T2*-CMR (kappa = 0.96, p < 0.01 for in vivo/ex vivo segmental agreement). The amount of hemorrhage in 13 affected tissue slices as determined by T2*-CMR in vivo correlated strongly with ex vivo results (20.3 ± 2.3% vs. 17.9 ± 1.6% per slice; Pearson r = 0.91; r(2) = 0.83, p < 0.01 for both). Hemorrhage size was not different between in vivo T2*-CMR and ex vivo TTC (mean difference 2.39 ± 1.43%; p = 0.19). CONCLUSIONS T2*-CMR accurately quantified myocardial reperfusion hemorrhage in vivo. Hemorrhage was associated with more severe infarct-related injury.

Assessment of myocardial viability using MRI during a constant infusion of Gd‐DTPA: Further studies at early and late periods of reperfusion

It was previously shown in a canine model of ischemia/reperfusion injury that the partition coefficient of gadolinium‐diethylene triamine pentaacetic acid (Gd‐DTPA) (λ) increases in infarcted tissue. That previous study used a non‐magnetic resonance imaging (MRI) method to measure λ and only investigated reperfusion times from 2 hr to 3 weeks. This study presents evidence suggesting that λ starts to increase as early as 1 min after reperfusion of a 2 hr occlusion and continues to rise for up to 2 hr or more; λ stays increased as late as 8 weeks, reaching peak values at 1–11 days and subsequently decreasing. It was also demonstrated that λ can be accurately measured in vivo using a saturation recovery turbo fast low‐angle shot (FLASH) sequence. The results of this study show that MRI during a constant infusion of Gd‐DTPA has great potential for the non‐invasive determination of myocardial viability as early as 1 min to as late as 8 weeks following reperfusion of acute myocardial infarction. Magn Reson Med 42:60–68, 1999.

Determining the minimum number of detectable cardiac-transplanted 111In-tropolone-labelled bone-marrow-derived mesenchymal stem cells by SPECT

In this work, we determined the minimum number of detectable 111In-tropolone-labelled bone-marrow-derived stem cells from the maximum activity per cell which did not affect viability, proliferation and differentiation, and the minimum detectable activity (MDA) of 111In by SPECT. Canine bone marrow mesenchymal cells were isolated, cultured and expanded. A number of samples, each containing 5x10(6) cells, were labelled with 111In-tropolone from 0.1 to 18 MBq, and cell viability was measured afterwards for each sample for 2 weeks. To determine the MDA, the anthropomorphic torso phantom (DataSpectrum Corporation, Hillsborough, NC) was used. A point source of 202 kBq 111In was placed on the surface of the heart compartment, and the phantom and all compartments were then filled with water. Three 111In SPECT scans (duration: 16, 32 and 64 min; parameters: 128x128 matrix with 128 projections over 360 degrees) were acquired every three days until the 111In radioactivity decayed to undetectable quantities. 111In SPECT images were reconstructed using OSEM with and without background, scatter or attenuation corrections. Contrast-to-noise ratio (CNR) in the reconstructed image was calculated, and MDA was set equal to the 111In activity corresponding to a CNR of 4. The cells had 100% viability when incubated with no more than 0.9 MBq of 111In (80% labelling efficiency), which corresponded to 0.14 Bq per cell. Background correction improved the detection limits for 111In-tropolone-labelled cells. The MDAs for 16, 32 and 64 min scans with background correction were observed to be 1.4 kBq, 700 Bq and 400 Bq, which implies that, in the case where the location of the transplantation is known and fixed, as few as 10,000, 5000 and 2900 cells respectively can be detected.

Comparison of Initial Cell Retention and Clearance Kinetics After Subendocardial or Subepicardial Injections of Endothelial Progenitor Cells in a Canine Myocardial Infarction Model

Neither intravenous nor intracoronary routes provide targeted stem cell delivery to recently infarcted myocardium in sufficient quantities. Direct routes appear preferable. However, most prior studies have used epicardial injections, which are not practical for routine clinical use. The objective of this study was to compare cell retention and clearance kinetics between a subepicardial and a subendocardial technique. Methods: We evaluated 7 dogs with each technique, using 111In-tropolone–labeled endothelial progenitor cells and serial SPECT/CT for 15 d after injection. Results: In vivo indium imaging demonstrated comparable degrees of retention: 57% ± 15% for the subepicardial injections and 54% ± 26% for the subendocardial injections. Clearance half-lives were also similar at 69 ± 26 and 60 ± 21 h, respectively. Conclusion: This study demonstrates that subendocardial injections, clinically more practical, show clearance kinetics comparable to those of subepicardial injections and will facilitate the ultimate clinical use of this treatment modality.

The use of Gd-DTPA as a marker of myocardial viability in reperfused acute myocardial infarction

At present, accurate assessment of the extent of myocardial viability after acute myocardial infarction is limited due to the spatial resolution of currently available imaging modalities. MR cardiac imaging, with its superior spatial resolution, would be used if viable and infarcted tissue could be separated based on signal intensity. In infarcted tissue, cell membrane breakdown allows the entry of the MR contrast agent Gd-DTPA which is normally extracellular. The increased space for Gd-DTPA distribution (partition coefficient, λ) in this infarcted tissue results in increased Gd-DTPA concentration and hence increased signal intensity on T1-weighted MR images. In a canine model of ischemia/reperfusion injury, the partition coefficient in infarcted tissue increased as early as 1 min post reperfusion. λ in infarcted tissue stayed increased over that in normal tissue for at least 8 weeks. The accuracy of contrast-enhanced MRI was confirmed by results of 201Tl SPECT and a cine MRI dobutamine wall motion study in a patient 1 week after an acute myocardial infarction. Thus, contrast-enhanced MRI shows great promise for the non-invasive determination of myocardial viability after acute myocardial infarction.

Myocardial viability imaging using Gd-DTPA: physiological modeling of infarcted myocardium, and impact on injection strategy and imaging time.

Results of simulations are shown which illustrate how the concentration‐time curves of an extravascular extracellular (EVEC) contrast agent, such as Gd‐DTPA, vary in myocardial tissue. The simulations show that the variable permeability of dead myocytes within a recent myocardial infarction will significantly alter delayed enhancement patterns following a bolus injection, invariably reducing the sensitivity of this technique for the detection of permanently damaged tissue. It is further predicted that if the bolus injection is followed by a suitably selected constant infusion, the infarct size and infarct volume of distribution may be more accurately determined, even though the degree of enhancement of an infarcted region (with normal flow) above normal tissue is slightly higher for the bolus technique within the first 30 min following the injection. The degree of enhancement of an infarcted region (with normal flow) above normal tissue was comparable between the two techniques at the point in the constant infusion at which the volume of contrast injected was the same as in the bolus case, i.e., at approximately 30 min after the bolus injection. The constant infusion approach became superior thereafter as overall tissue concentrations became greater in both normal and infarcted tissue, and these concentrations remained more stable with the constant infusion approach. Preliminary experimental results in a canine model of infarction/reperfusion illustrated a delayed wash‐in of contrast agent in infarcted tissue, which may be explained by a physiological model in which dead myocytes in infarcted myocardium have non‐infinite permeability. Magn Reson Med 48:791–800, 2002.

In Vivo SPECT Quantification of Transplanted Cell Survival After Engraftment Using 111In-Tropolone in Infarcted Canine Myocardium

Current investigations of cell transplant therapies in damaged myocardium are limited by the inability to quantify cell transplant survival in vivo. We describe how the labeling of cells with 111In can be used to monitor transplanted cell viability in a canine infarction model. Methods: We experimentally determined the contribution of the 111In signal associated with transplanted cell (TC) death and radiolabel leakage to the measured SPECT signal when 111In-labeled cells were transplanted into the myocardium. Three groups of experiments were performed in dogs. Radiolabel leakage was derived by labeling canine myocardium in situ with free 111In-tropolone (n = 4). To understand the contribution of extracellular 111In (e.g., after cell death), we developed a debris impulse response function (DIRF) by injecting lysed 111In-labeled cells within reperfused (n = 3) and nonreperfused (n = 5) myocardial infarcts and within normal (n = 3) canine myocardium. To assess the application of the modeling derived from these experiments, 111In-labeled cells were transplanted into infarcted myocardium (n = 4; 3.1 × 107 ± 5.4 × 106 cells). Serial SPECT images were acquired after direct epicardial injection to determine the time-dependent radiolabel clearance. Clearance kinetics were used to correct for 111In associated with viable TCs. Results: 111In clearance followed a biphasic response and was modeled as a biexponential with a short (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{T}_{1/2}^{\mathrm{s}}\) \end{document}) and long (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{T}_{1/2}^{\mathrm{l}}\) \end{document}) biologic half-life. The \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{T}_{1/2}^{\mathrm{s}}\) \end{document} was not significantly different between experimental groups, suggesting that initial losses were due to transplantation methodology, whereas the \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{T}_{1/2}^{\mathrm{l}}\) \end{document} reflected the clearance of retained 111In. DIRF had an average \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{T}_{1/2}^{\mathrm{l}}\) \end{document} of 19.4 ± 4.1 h, and the \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{T}_{1/2}^{\mathrm{l}}\) \end{document} calculated from free 111In-tropolone injected in situ was 882.7 ± 242.8 h. The measured \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{T}_{1/2}^{\mathrm{l}}\) \end{document} for TCs was 74.3 h and was 71.2 h when corrections were applied. Conclusion: A new quantitative method to assess TC survival in myocardium using SPECT and 111In has been introduced. At the limits, method accuracy is improved if appropriate corrections are applied. In vivo 111In imaging most accurately describes cell viability half-life if \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{T}_{1/2}^{\mathrm{l}}\) \end{document} is between 20 h and 37 d.

Gd-DTPA bolus tracking in the myocardium using T1 fast acquisition relaxation mapping (T1 FARM).

MRI methods currently used for bolus tracking in the myocardium, such as saturation recovery turbo‐fast low‐angle shot (FLASH) (srTFL), are limited by signal intensity (SI) saturation at high contrast agent (CA) concentrations. By using T1 fast acquisition relaxation mapping (T1 FARM), a Gd‐DTPA bolus (0.075 vs. 0.025 mmol/kg) may be injected without causing saturation. This study tested the feasibility of in vivo T1 FARM bolus tracking under rest/stress conditions in seven beagles with multiple permanently occluded branches of the left anterior descending (LAD) coronary artery. Although it underestimated the myocardial perfusion reserve (MPR) measured ex vivo using radioactive microspheres (mean ± SEM; 3.60 ± 0.26), the MPR determined upon application of the modified Kety model (1.86 ± 0.10) enabled distinction between normal and infarcted tissue. The partition coefficient (λ) estimated at rest and stress using the modified Kety model underestimated ex vivo radioactive measurements in infarcted tissue (0.25 ± 0.01 vs. 0.26 ± 0.01 vs. 0.79 ± 0.08 ml/g, P < 0.0001) yet was accurate in normal tissue (0.28 ± 0.01 vs. 0.30 ± 0.01 vs. 0.33 ± 0.01 ml/g, P = NS). Thus, although unsuitable for myocardial viability assessment, T1 FARM bolus tracking shows potential for assessment of myocardial perfusion. Magn Reson Med 46:555–564, 2001.

Determining the extent to which delayed-enhancement images reflect the partition-coefficient of Gd-DTPA in canine studies of reperfused and unreperfused myocardial infarction

MRI after a constant infusion (CI) of Gd‐DTPA has been used to identify the extent of myocardial infarction (MI). However, Gd‐DTPA‐enhanced “viability” imaging is more commonly performed with a bolus (for “delayed‐enhancement” (DE) imaging). This study sought to determine how image delay time and time postinfarction influence the assessment of necrosis by DE. Both infusion and DE imaging was performed in dogs with reperfused (N = 6) or unreperfused (N = 4) MI. Estimates of the partition‐coefficient of Gd‐DTPA (λ) with DE were compared with those calculated after 60 min of infusion, and the comparisons were repeated until 4 (reperfused) or 8 (unreperfused) weeks postinfarction. In reperfused animals, the concordance (Rc) between DE and infusion estimates of λ was > 0.90 for most image delays > 8 min postinjection, for day 0 through week 3, with Rc at day 0 greater than at week 4 (P = 0.022). In unreperfused animals, there was an interaction between image delay time and time postinfarction (P < 0.001): Rc > 0.90 corresponded to longer image delays at week 1 than at weeks 4–8. Therefore, when image delays are selected appropriately, DE images can strongly reflect λ and identify irreversibly injured myocardium. Magn Reson Med 52:1069–1079, 2004.