Mohammed S. Shazeeb

MRI investigation of the linkage between respiratory motion of the heart and markers on patient's abdomen and chest: Implications for respiratory amplitude binning list-mode PET and SPECT studies

Respiratory motion of the heart impacts the diagnostic accuracy of myocardial-perfusion emission-imaging studies. Amplitude binning has come to be the method of choice for binning list-mode based acquisitions for correction of respiratory motion in PET and SPECT. In some subjects respiratory motion exhibits hysteretic behavior similar to damped non-linear cyclic systems. The detection and correction of hysteresis between the signals from surface movement of the patients body used in binning and the motion of the heart within the chest remains an open area for investigation. This study reports our investigation in nine volunteers of the combined MRI tracking of the internal respiratory motion of the heart using Navigators with stereo-tracking of markers on the volunteers chest and abdomen by a visual-tracking system (VTS). The respiratory motion signals from the internal organs and the external markers were evaluated for hysteretic behavior analyzing the temporal correspondence of the signals. In general, a strong, positive correlation between the external marker motion (AP direction) and the internal heart motion (SI direction) during respiration was observed. The average ± standard deviation in the Spearmans ranked correlation coefficient (ρ) over the nine volunteer studied was 0.92±0.1 between the external abdomen marker and the internal heart, and 0.87±0.2 between the external chest marker and the internal heart. However despite the good correlation on average for the nine volunteers, in three studies a poor correlation was observed due to hysteretic behavior between inspiration and expiration for either the chest marker and the internal motion of the heart, or the abdominal marker and the motion of the heart. In all cases we observed a good correlation of at least either the abdomen or the chest with the heart. Based on this result, we propose the use of marker motion from both the chest and abdomen regions when estimating the internal heart motion to detect and address hysteresis when binning list-mode emission data.

Targeted Signal-Amplifying Enzymes Enhance MRI of EGFR Expression in an Orthotopic Model of Human Glioma

Epidermal growth factor receptor (EGFR) imaging in brain tumors is essential to visualize overexpression of EGFRvIII variants as a signature of highly aggressive gliomas and to identify patients that would benefit from anti-EGFR therapy. Seeking imaging improvements, we tested a novel pretargeting approach that relies on initial administration of enzyme-linked anti-EGFR monoclonal antibodies (mAb; EMD72000) followed by administration of a low-molecular-weight paramagnetic molecule (diTyr-GdDTPA) retained at the site of EGFR mAb accumulation. We hypothesized that diTyr-GdDTPA would become enzyme activated and retained on cells due to binding to tissue proteins. In support of this hypothesis, mAb-enzyme conjugates reacted with both membrane-isolated wild-type (wt) EGFR and EGFRvIII, but they bound primarily to EGFRvIII-expressing cells and not to EGFRwt-expressing cells. In vivo analysis of magnetic resonance (MR) tumor signal revealed differences in MR signal decay following diTyr-GdDTPA substrate administration. These differences were significant in that they suggested differences in substrate elimination from the tissue which relied on the specificity of the initial mAb binding: a biexponential signal decay was observed in tumors only upon preinjection with EGFR-targeted conjugates. Endpoint MRI in this setting revealed detailed images of tumors which correlated with immunohistochemical detection of EGFR expression. Together, our findings suggest an improved method to identify EGFRvIII-expressing gliomas in vivo that are best suited for treatment with therapeutic EGFR antibodies.

Correction of hysteretic respiratory motion in SPECT myocardial perfusion imaging: Simulation and patient studies

Purpose: Amplitude‐based respiratory gating is known to capture the extent of respiratory motion (RM) accurately but results in residual motion in the presence of respiratory hysteresis. In our previous study, we proposed and developed a novel approach to account for respiratory hysteresis by applying the Bouc–Wen (BW) model of hysteresis to external surrogate signals of anterior/posterior motion of the abdomen and chest with respiration. In this work, using simulated and clinical SPECT myocardial perfusion imaging (MPI) studies, we investigate the effects of respiratory hysteresis and evaluate the benefit of correcting it using the proposed BW model in comparison with the abdomen signal typically employed clinically. Methods: The MRI navigator data acquired in free‐breathing human volunteers were used in the specially modified 4D NCAT phantoms to allow simulating three types of respiratory patterns: monotonic, mild hysteresis, and strong hysteresis with normal myocardial uptake, and perfusion defects in the anterior, lateral, inferior, and septal locations of the mid‐ventricular wall. Clinical scans were performed using a Tc‐99m sestamibi MPI protocol while recording respiratory signals from thoracic and abdomen regions using a visual tracking system (VTS). The performance of the correction using the respiratory signals was assessed through polar map analysis in phantom and 10 clinical studies selected on the basis of having substantial RM. Results: In phantom studies, simulations illustrating normal myocardial uptake showed significant differences (P < 0.001) in the uniformity of the polar maps between the RM uncorrected and corrected. No significant differences were seen in the polar map uniformity across the RM corrections. Studies simulating perfusion defects showed significantly decreased errors (P < 0.001) in defect severity and extent for the RM corrected compared to the uncorrected. Only for the strong hysteretic pattern, there was a significant difference (P < 0.001) among the RM corrections. The errors in defect severity and extent for the RM correction using abdomen signal were significantly higher compared to that of the BW (severity = −4.0%, P < 0.001; extent = −65.4%, P < 0.01) and chest (severity = −4.1%, P < 0.001; extent = −52.5%, P < 0.01) signals. In clinical studies, the quantitative analysis of the polar maps demonstrated qualitative and quantitative but not statistically significant differences (P = 0.73) between the correction methods that used the BW signal and the abdominal signal. Conclusions: This study shows that hysteresis in respiration affects the extent of residual motion left in the RM‐binned data, which can impact wall uniformity and the visualization of defects. Thus, there appears to be the potential for improved accuracy in reconstruction in the presence of hysteretic RM with the BW model method providing a possible step in the direction of improvement.

A novel paramagnetic substrate for detecting myeloperoxidase activity in vivo

Bis-phenylamides and bis-hydroxyindolamides of diethylenetriaminepentaacetic acid-gadolinium (DTPA(Gd)) are paramagnetic reducing substrates of peroxidases that enable molecular imaging of peroxidase activity in vivo. Specifically, gadolinium chelates of bis-5-hydroxytryptamide-DTPA (bis-5HT-DTPA(Gd)) have been used to image localized inflammation in animal models by detecting neutrophil-derived myeloperoxidase (MPO) activity at the inflammation site. However, in other preclinical disease models, bis-5HT-DTPA(Gd) presents technical challenges due to its limited solubility in vivo. Here we report a novel MPO-sensing probe obtained by replacing the reducing substrate serotonin (5-HT) with 5-hydroxytryptophan (HTrp). Characterization of the resulting probe (bis-HTrp-DTPA(Gd)) in vitro using nuclear magnetic resonance spectroscopy and enzyme kinetic analysis showed that bis-HTrp-DTPA(Gd) (1) improves solubility in water; (2) acts as a substrate for both horseradish peroxidase and MPO enzymes; (3) induces cross-linking of proteins in the presence of MPO; (4) produces oxidation products, which bind to plasma proteins; and (5) unlike bis-5HT-DTPA(Gd), does not follow first-order reaction kinetics. In vivo magnetic resonance imaging (MR!) in mice demonstrated that bis-HTrp-DTPA(Gd) was retained for up to 5 days in MPO-containing sites and cleared faster than bis-5HT-DTPA(Gd) from MPO-negative sites. Bis-HTrp-DTPA(Gd) should offer improvements for MR! of MPO-mediated inflammation in vivo, especially in high-field MR!, which requires a higher dose of contrast agent.

Dose dependence and temporal evolution of the T1 relaxation time and MRI contrast in the rat brain after subcutaneous injection of manganese chloride

Divalent manganese ion (Mn2+) is a widely used T1 contrast agent in manganese‐enhanced MRI studies to visualize functional neural tracts and anatomy in the brain in vivo. In animal studies, Mn2+ is administered at a dose that will maximize the contrast, while minimizing its toxic effects. In rodents, systemic administration of Mn2+ via intravenous injection has been shown to create unique MRI contrast in the brain at a maximum dose of 175 mg kg−1. However, intravenous administration of Mn2+ results in faster bioelimination of excess Mn2+ from the plasma due to a steep concentration gradient between plasma and bile. By contrast, following subcutaneous injection (LD50 value = 320 mg kg−1), Mn2+ is released slowly into the bloodstream, thus avoiding immediate hepatic elimination resulting in prolonged accumulation of Mn2+ in the brain via the choroid plexus than that obtained via intravenous administration. The goal of this study was to investigate MRI dose response of Mn2+ in rat brain following subcutaneous administration of Mn2+. Dose dependence and temporal dynamics of Mn2+ after subcutaneous injection can prove useful for longitudinal in vivo studies that require brain enhancement to persist for a long period of time to visualize neuroarchitecture like in neurodegenerative disease studies. Magn Reson Med, 2012.

Liposome-encapsulated superoxide dismutase mimetic: theranostic potential of an MR detectable and neuroprotective agent

Endogenous manganese based superoxide dismutase (Mn-SOD) provides the primary defense against excess production of potentially toxic superoxide anion (O2 (-) ). M40401 is a synthetic enzyme mimetic that has a catalytic activity rate exceeding that of the native SOD enzymes. The presence of a paramagnetic Mn(II) cation in M40401 suggests that the delivery and spatial distribution of this enzyme mimetic in vivo may be directly detectible using magnetic resonance imaging (MRI); however, the cardiotoxicity of Mn(II) severely limits the use of free M40401 in living systems. To deliver M40401 in vivo in amounts sufficient for MRI detection and to limit potential cardiotoxicity, we encapsulated M40401 into 170 nm liposomes composed of phosphatidylcholine and PEGylated phosphatidylethanolamine to achieve extended circulation in the bloodstream. The obtained liposomes efficiently catalyzed superoxide dismutation in vitro. Using 3 T MRI we investigated the biokinetics of liposome-encapsulated M40401 in mice and found that, in addition to catalyzing superoxide dismutation in vitro, M40401 caused differential and region-specific enhancement of mouse brain after systemic administration. Thus, liposome encapsulated M40401 is an ideal candidate for development as a theranostic compound useful for simultaneous MRI-mediated tracking of delivery as well as for neuroprotective treatment of ischemic brain.

Accounting for the hysteresis of respiratory motion of the heart in cardiac SPECT and PET using the Bouc-Wen model of hysteresis

Respiratory motion causes artifacts and blurring of cardiac structures in reconstructed slices in SPECT and PET studies. Respiratory motion correction methods based on amplitude binning of list-mode data have been observed to reduce but not eliminate the residual blurring when hysteresis of cardiac motion is present. The Bouc-Wen model of hysteresis is able to describe non-linear hysteretic systems and match the behavior of a hysteretic system to a range of hysteretic cycles. Our hypothesis is that the impact of hysteresis on the respiratory motion of heart can be accounted by developing hysteresis model employing modified Bouc-Wen equation. This study describes our investigation of our hypothesis through combined use of MRI and Visual Tracking System (VTS).

Combined collimator/reconstruction optimization for myocardial perfusion SPECT imaging using polar map-based LROC numerical observer

Polar maps have been used to assist clinicians diagnose coronary artery diseases (CAD) in single photon emission computed tomography (SPECT) myocardial perfusion imaging. Herein, we investigate the optimization of collimator design for perfusion defect detection in SPECT imaging when reconstruction includes modeling of the collimator. The optimization employs an LROC clinical model observer (CMO), which emulates the clinical task of polar map detection of CAD. By utilizing a CMO, which better mimics the clinical perfusion-defect detection task than previous SKE based observers, our objective is to optimize collimator design for SPECT myocardial perfusion imaging when reconstruction includes compensation for collimator spatial resolution. Comparison of lesion detection accuracy will then be employed to determine if a lower spatial resolution hence higher sensitivity collimator design than currently recommended could be utilized to reduce the radiation dose to the patient, imaging time, or a combination of both. As the first step in this investigation, we report herein on the optimization of the three-dimensional (3D) post-reconstruction Gaussian filtering of and the number of iterations used to reconstruct the SPECT slices of projections acquired by a low-energy generalpurpose (LEGP) collimator. The optimization was in terms of detection accuracy as determined by our CMO and four human observers. Both the human and all four CMO variants agreed that the optimal post-filtering was with sigma of the Gaussian in the range of 0.75 to 1.0 pixels. In terms of number of iterations, the human observers showed a preference for 5 iterations; however, only one of the variants of the CMO agreed with this selection. The others showed a preference for 15 iterations. We shall thus proceed to optimize the reconstruction parameters for even higher sensitivity collimators using this CMO, and then do the final comparison between collimators using their individually optimized parameters with human observers and three times the test images to reduce the statistical variation seen in our present results.

Abstract 2443: MR imaging in human glioma tumor xenograft models using enzyme-conjugated anti-EGFRvIII-specific antibody fragments

A truncated and constitutively active form of the EGF receptor variant III (EGFRvIII) is a major determinant of tumor growth and poor prognosis in glioblastoma multiforme [1]. To test a system for targeted imaging of EGFRvIII we investigated the retention of peroxidase-generated products of a paramagnetic substrate diTyr-DTPAGd in U87αEGFR orthotopic human glioma xenografts. F(ab’)2 fragments of EMD72000 mAb were conjugated to deglycosylated horseradish peroxidase (HRP) and glucose oxidase (GOX) and used as a self-complementing enzymatic signal amplification system [2] for EGFRvIII targeted imaging. We anticipated that receptor expression sites will exhibit a prolonged MR signal enhancement due to the formation of polymerized products of diTyr-DTPAGd oxidation by mAb conjugates [2]. F(ab’)2 fragments of mAb were linked to enzymes using bisaromatic hydrazone bonds and purified by HPLC. The conjugates were characterized in U87αEGFR cell culture and optimal ratios of HRP and GOX conjugates were determined to provide the maximum signal with low cytotoxicity. F(ab’)2-GOX and F(ab’)2-HRP were also modified with NHS-MAG3 for radiolabeling. U87αEGFR cells were stereotaxically implanted in the brains of RNU rats. Ten days after tumor implantation, each animal was imaged using T1-weighted spin-echo MRI (TR/TE=700ms/8.2ms) at 3T on two occasions: Day 1 - images were acquired after IV injection of 0.1 mmol/kg diTyr-DTPAGd over a 1.5h period; Day 2 - anti-EGFRvIII conjugates (100 µg mAb/animal) were injected IV, and then 4h later 0.1 mmol/kg diTyr-DTPAGd was injected IV followed by image acquisition for 2h. Both 99mTc-labeled conjugates showed specific binding to U87αEGFR cells in vitro and in vivo as demonstrated using SPECT/CT. Cell-binding and internalization studies showed that 80% conjugates were internalized at 37°C. MR T1-weighted images showed significantly higher enhancement and longer retention of the MR signal on Day 2 in rats pre-injected with conjugates compared to Day 1 over the same time period due to the conjugate co-localization at the EGFRvIII target sites. The washout of the contrast agent was best modeled using a biexponential decay as compared to Gli36αEGFR-bearing rats [4]. U87αEGFR tumors showed biexponential MR signal decay on both days. Administration of EGFRvIII-targeted mAb conjugates resulted in specific binding to U87αEGFR cells of which at least ∼20% remained on the surface enabling the reaction with the contrast agent. The biexponential decay constants on Day 2 for both Gli36αEGFR and U87αEGFR animals were not significantly different indicating a similarity of conjugate accumulation in both tumor models which resulted in a similar bimodal washout of the contrast agent. References: [1] Hu, J et al. (2011) PNAS 108:15984 [2] Bogdanov, A., et al. (2007). Bioconjug Chem 18: 1123 [3] Shazeeb, M.S., et al. (2011). Cancer Res 71: 2230. Supported by NIH 5R01EB000858-09. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2443. doi:1538-7445.AM2012-2443