Laura Pisani

SETUP ERROR IN RADIOTHERAPY: ON-LINE CORRECTION USING ELECTRONIC KILOVOLTAGE AND MEGAVOLTAGE RADIOGRAPHS

PURPOSE We hypothesize that the difference in image quality between the traditional kilovoltage (kV) prescription radiographs and megavoltage (MV) treatment radiographs is a major factor hindering our ability to accurately measure, thus correct, setup error in radiation therapy. The objective of this work is to study the accuracy of on-line correction of setup errors achievable using either kV- or MV-localization (i.e., open-field) radiographs. METHODS AND MATERIALS Using a gantry mounted kV and MV dual-beam imaging system, the accuracy of on-line measurement and correction of setup error using electronic kV- and MV-localization images was examined based on anthropomorphic phantom and patient imaging studies. For the phantom study, the users ability to accurately detect known translational shifts was analyzed. The clinical study included 14 patients with disease in the head and neck, thoracic, and pelvic regions. For each patient, 4 orthogonal kV radiographs acquired during treatment simulation from the right lateral, anterior-to-posterior, left lateral, and posterior-to-anterior directions were employed as reference prescription images. Two-dimensional (2D) anatomic templates were defined on each of the 4 reference images. On each treatment day, after positioning the patient for treatment, 4 orthogonal electronic localization images were acquired with both kV and 6-MV photon beams. On alternate weeks, setup errors were determined from either the kV- or MV-localization images but not both. Setup error was determined by aligning each 2D template with the anatomic information on the corresponding localization image, ignoring rotational and nonrigid variations. For each set of 4 orthogonal images, the results from template alignments were averaged. Based on the results from the phantom study and a parallel study of the inter- and intraobserver template alignment variability, a threshold for minimum correction was set at 2 mm in any direction. Setup correction was applied by translating the treatment couch in the lateral, superior-to-inferior and vertical directions only. During treatment, kV open-field images were acquired for off-line treatment verification and analysis. Each patient study spanned 2-6 weeks. The 14 patient studies were completed with 8248 electronic images acquired and analyzed. RESULTS Results from the phantom studies showed that the users were able to detect the applied translational shift to better than 2 mm, and mostly to within 1 mm. The intraobserver variability of template alignment was on the order of 1 mm using a sample of either MV or kV patient images. The difference between using MV or kV images was significant for only a few cases. However in most cases, interobserver alignment variability was larger when using MV images than kV. For on-line setup correction, the study procedure added 10 min. to conventional treatment time. Setup variation measured with either kV- or MV-localization images was similar. The initial magnitude of setup error was appreciable, with a mean displacement of about 6.6 +/- 2.4 mm for the 14 patients. On-line correction using either kV- or MV-localization images improved setup accuracy. Over all study patients, setup errors occurred with standard deviations greater than 2 mm in any direction with a frequency of 48% before correction, and were reduced to 16% after correction. On average, kV image-based correction reduced radial setup variation to 2.6 +/- 1.6 mm compared to the 3.3 +/- 1.8 mm attained using MV images. The difference detected between the kV and MV data was not statistically significant when averaged over all patients. However, for on-line corrections in the neck and thoracic regions, using kV-localization images reduced setup error significantly more than using MV images. CONCLUSIONS In our anatomic template alignment study, interobserver variability was smaller using kV images than MV images. Intraobserver variability was smaller for alignments on kV images

Ferumoxytol: a new, clinically applicable label for stem-cell tracking in arthritic joints with MRI

AIM To develop a clinically applicable MRI technique for tracking stem cells in matrix-associated stem-cell implants, using the US FDA-approved iron supplement ferumoxytol. MATERIALS & METHODS Ferumoxytol-labeling of adipose-derived stem cells (ADSCs) was optimized in vitro. A total of 11 rats with osteochondral defects of both femurs were implanted with ferumoxytol- or ferumoxides-labeled or unlabeled ADSCs, and underwent MRI up to 4 weeks post matrix-associated stem-cell implant. The signal-to-noise ratio of different matrix-associated stem-cell implant was compared with t-tests and correlated with histopathology. RESULTS An incubation concentration of 500 µg iron/ml ferumoxytol and 10 µg/ml protamine sulfate led to significant cellular iron uptake, T2 signal effects and unimpaired ADSC viability. In vivo, ferumoxytol- and ferumoxides-labeled ADSCs demonstrated significantly lower signal-to-noise ratio values compared with unlabeled controls (p < 0.01). Histopathology confirmed engraftment of labeled ADSCs, with slow dilution of the iron label over time. CONCLUSION Ferumoxytol can be used for in vivo tracking of stem cells with MRI.

Effects of spatial and temporal resolution for MR image‐guided thermal ablation of prostate with transurethral ultrasound

To describe approaches for determining optimal spatial and temporal resolutions for the proton resonance frequency shift method of quantitative magnetic resonance temperature imaging (MRTI) guidance of transurethral ultrasonic prostate ablation.

Intravenous ferumoxytol allows noninvasive MR imaging monitoring of macrophage migration into stem cell transplants

PURPOSE To develop a clinically applicable imaging technique for monitoring differential migration of macrophages into viable and apoptotic matrix-associated stem cell implants (MASIs) in arthritic knee joints. MATERIALS AND METHODS With institutional animal care and use committee approval, six athymic rats were injected with intravenous ferumoxytol (0.5 mmol iron per kilogram of body weight) to preload macrophages of the reticuloendothelial system with iron oxide nanoparticles. Forty-eight hours later, all animals received MASIs of viable adipose-derived stem cells (ADSCs) in an osteochondral defect of the right femur and mitomycin-pretreated apoptotic ADSCs in an osteochondral defect of the left femur. One additional control animal each received intravenous ferumoxytol and bilateral scaffold-only implants (without cells) or bilateral MASIs without prior ferumoxytol injection. All knees were imaged with a 7.0-T magnetic resonance (MR) imaging unit with T2-weighted fast spin-echo sequences immediately after, as well as 2 and 4 weeks after, matrix-associated stem cell implantation. Signal-to-noise ratios (SNRs) of viable and apoptotic MASIs were compared by using a linear mixed-effects model. MR imaging data were correlated with histopathologic findings. RESULTS All ADSC implants showed a slowly decreasing T2 signal over 4 weeks after matrix-associated stem cell implantation. SNRs decreased significantly over time for the apoptotic implants (SNRs on the day of matrix-associated stem cell implantation, 2 weeks after the procedure, and 4 weeks after the procedure were 16.9, 10.9, and 6.7, respectively; P = .0004) but not for the viable implants (SNRs on the day of matrix-associated stem cell implantation, 2 weeks after the procedure, and 4 weeks after the procedure were 17.7, 16.2, and 15.7, respectively; P = .2218). At 4 weeks after matrix-associated stem cell implantation, SNRs of apoptotic ADSCs were significantly lower than those of viable ADSCs (mean, 6.7 vs 15.7; P = .0013). This corresponded to differential migration of iron-loaded macrophages into MASIs. CONCLUSION Iron oxide loading of macrophages in the reticuloendothelial system by means of intravenous ferumoxytol injection can be utilized to monitor differential migration of bone marrow macrophages into viable and apoptotic MASIs in a rat model.

Improvements in parallel imaging accelerated functional MRI using multiecho echo-planar imaging†

Multiecho echo‐planar imaging (EPI) was implemented for blood‐oxygenation‐level‐dependent functional MRI at 1.5 T and compared to single‐echo EPI with and without parallel imaging acceleration. A time‐normalized breath‐hold task using a block design functional MRI protocol was carried out in combination with up to four echo trains per excitation and parallel imaging acceleration factors R = 1–3. Experiments were conducted in five human subjects, each scanned in three sessions. Across all reduction factors, both signal‐to‐fluctuation‐noise ratio and the total number of activated voxels were significantly lower using a single‐echo EPI pulse sequence compared with the multiecho approach. Signal‐to‐fluctuation‐noise ratio and total number of activated voxels were also considerably reduced for nonaccelerated conventional single‐echo EPI when compared to three‐echo measurements with R = 2. Parallel imaging accelerated multiecho EPI reduced geometric distortions and signal dropout, while it increased blood‐oxygenation‐level‐dependent signal sensitivity all over the brain, particularly in regions with short underlying T*2. Thus, the presented method showed multiple advantages over conventional single‐echo EPI for standard blood‐oxygenation‐level‐dependent functional MRI experiments. Magn Reson Med 63:959–969, 2010.

Evaluation of the novel USPIO GEH121333 for MR imaging of cancer immune responses.

Tumor-associated macrophages (TAM) maintain a chronic inflammation in cancers, which is associated with tumor aggressiveness and poor prognosis. The purpose of this study was to: (1) evaluate the pharmacokinetics and tolerability of the novel ultrasmall superparamagnetic iron oxide nanoparticle (USPIO) compound GEH121333; (2) assess whether GEH121333 can serve as a MR imaging biomarker for TAM; and (3) compare tumor MR enhancement profiles between GEH121333 and ferumoxytol. Blood half-lives of GEH121333 and ferumoxytol were measured by relaxometry (n = 4 each). Tolerance was assessed in healthy rats injected with high dose GEH121333, vehicle or saline (n = 4 each). Animals were monitored for 7 days regarding body weight, complete blood counts and serum chemistry, followed by histological evaluation of visceral organs. MR imaging was performed on mice harboring MMTV-PyMT-derived breast adenocarcinomas using a 7 T scanner before and up to 72 h post-injection (p.i.) of GEH121333 (n = 10) or ferumoxytol (n = 9). Tumor R1, R2* relaxation rates were compared between different experimental groups and time points, using a linear mixed effects model with a random effect for each animal. MR data were correlated with histopathology. GEH121333 showed a longer circulation half-life than ferumoxytol. Intravenous GEH121333 did not produce significant adverse effects in rats. All tumors demonstrated significant enhancement on T1, T2 and T2*-weighted images at 1, 24, 48 and 72 h p.i. GEH121333 generated stronger tumor T2* enhancement than ferumoxytol. Histological analysis verified intracellular compartmentalization of GEH121333 by TAM at 24, 48 and 72 h p.i. MR imaging with GEH121333 nanoparticles represents a novel biomarker for TAM assessment. This new USPIO MR contrast agent provides a longer blood half-life and better TAM enhancement compared with the iron supplement ferumoxytol. Copyright

Imaging Tumor Necrosis with Ferumoxytol

Objective Ultra-small superparamagnetic iron oxide nanoparticles (USPIO) are promising contrast agents for magnetic resonance imaging (MRI). USPIO mediated proton relaxation rate enhancement is strongly dependent on compartmentalization of the agent and can vary depending on their intracellular or extracellular location in the tumor microenvironment. We compared the T1- and T2-enhancement pattern of intracellular and extracellular USPIO in mouse models of cancer and pilot data from patients. A better understanding of these MR signal effects will enable non-invasive characterizations of the composition of the tumor microenvironment. Materials and Methods Six 4T1 and six MMTV-PyMT mammary tumors were grown in mice and imaged with ferumoxytol-enhanced MRI. R1 relaxation rates were calculated for different tumor types and different tumor areas and compared with histology. The transendothelial leakage rate of ferumoxytol was obtained by our measured relaxivity of ferumoxytol and compared between different tumor types, using a t-test. Additionally, 3 patients with malignant sarcomas were imaged with ferumoxytol-enhanced MRI. T1- and T2-enhancement patterns were compared with histopathology in a descriptive manner as a proof of concept for clinical translation of our observations. Results 4T1 tumors showed central areas of high signal on T1 and low signal on T2 weighted MR images, which corresponded to extracellular nanoparticles in a necrotic core on histopathology. MMTV-PyMT tumors showed little change on T1 but decreased signal on T2 weighted images, which correlated to compartmentalized nanoparticles in tumor associated macrophages. Only 4T1 tumors demonstrated significantly increased R1 relaxation rates of the tumor core compared to the tumor periphery (p<0.001). Transendothelial USPIO leakage was significantly higher for 4T1 tumors (3.4±0.9x10-3 mL/min/100cm3) compared to MMTV-PyMT tumors (1.0±0.9x10-3 mL/min/100 cm3). Likewise, ferumoxytol imaging in patients showed similar findings with high T1 signal in areas of tumor necrosis and low signal in areas of intracellularly compartmentalized iron. Conclusion Differential T1- and T2-enhancement patterns of USPIO in tumors enable conclusions about their intracellular and extracellular location. This information can be used to characterize the composition of the tumor microenvironment.

Restricted field of view magnetic resonance imaging of a dynamic time series

A restricted field of view (rFOV) approach for imaging a dynamic time series of volumes of limited spatial extent within a larger subject is described. The shorter readout with rFOV‐MRI can be exploited to either limit image artifacts or increase spatial resolution. To accomplish rFOV imaging of a multislice volume for a dynamic series, an outer volume suppression (OVS) preparation that saturates signal external to a cylinder through the subject is followed by slice‐selective excitation and a spiral readout. The pass‐ and stopband efficiencies of the OVS in an agar gel phantom were 97% (±1.5%) and 3% (±1%), respectively. Profiles of the temporal signal‐to‐noise ratio (SNR) were measured in a phantom and an adult brain. The rFOV sequence reduced distortions from off‐resonance signal and T  2* ‐induced blurring compared to a conventional sequence. Sequence utility is demonstrated for high‐resolution rFOV functional MRI (fMRI) in the visual cortex. The rFOV sequence may prove to be useful for other multislice dynamic and high‐resolution imaging applications. Magn Reson Med 57:297–307, 2007.

Bioluminescence and Magnetic Resonance Imaging of Macrophage Homing to Experimental Abdominal Aortic Aneurysms

Macrophage infiltration is a prominent feature of abdominal aortic aneurysm (AAA) progression. We used a combined imaging approach with bioluminescence (BLI) and magnetic resonance imaging (MRI) to study macrophage homing and accumulation in experimental AAA disease. Murine AAAs were created via intra-aortic infusion of porcine pancreatic elastase. Mice were imaged over 14 days after injection of prepared peritoneal macrophages. For BLI, macrophages were from transgenic mice expressing luciferase. For MRI, macrophages were labeled with iron oxide particles. Macrophage accumulation during aneurysm progression was observed by in situ BLI and by in vivo 7T MRI. Mice were sacrificed after imaging for histologic analysis. In situ BLI (n = 32) demonstrated high signal in the AAA by days 7 and 14, which correlated significantly with macrophage number and aortic diameter. In vivo 7T MRI (n = 13) at day 14 demonstrated T2* signal loss in the AAA and not in sham mice. Immunohistochemistry and Prussian blue staining confirmed the presence of injected macrophages in the AAA. BLI and MRI provide complementary approaches to track macrophage homing and accumulation in experimental AAAs. Similar dual imaging strategies may aid the study of AAA biology and the evaluation of novel therapies.

Assessment of Elastin Deficit in a Marfan Mouse Aneurysm Model Using an Elastin-Specific Magnetic Resonance Imaging Contrast Agent

Background—Ascending aortic dissection and rupture remain a life-threatening complication in patients with Marfan syndrome. The extracellular matrix provides strength and elastic recoil to the aortic wall, thereby preventing radial expansion. We have previously shown that ascending aortic aneurysm formation in Marfan mice (Fbn1C1039G/+) is associated with decreased aortic wall elastogenesis and increased elastin breakdown. In this study, we test the feasibility of quantifying aortic wall elastin content using MRI with a gadolinium-based elastin-specific magnetic resonance contrast agent in Fbn1C1039G/+ mice. Methods and Results—Ascending aorta elastin content was measured in 32-week-old Fbn1C1039G/+ mice and wild-type (n=9 and n=10, respectively) using 7-T MRI with a T1 mapping sequence. Significantly lower enhancement (ie, lower R1 values, where R1=1/T1) was detected post–elastin-specific magnetic resonance contrast agent in Fbn1C1039G/+ compared with wild-type ascending aortas (1.15±0.07 versus 1.36±0.05; P<0.05). Post–elastin-specific magnetic resonance contrast agent R1 values correlated with ascending aortic wall gadolinium content directly measured by inductively coupled mass spectroscopy (P=0.006). Conclusions—Herein, we demonstrate that MRI with elastin-specific magnetic resonance contrast agent accurately measures elastin bound gadolinium within the aortic wall and detects a decrease in aortic wall elastin in Marfan mice compared with wild-type controls. This approach has translational potential for noninvasively assessing aneurysm tissue changes and risk, as well as monitoring elastin content in response to therapeutic interventions.