Integrated active tracking detector for MRI-guided interventions

We present a fully integrated detector suitable for active tracking of interventional devices in MR-guided interventions. The single-chip microsystem consists of a detection coil, a tuning capacitor, an intermediate frequency downconversion receiver, and a phase-locked-loop-based frequency synthesizer. Thanks to the integrated mixer, the chip output stage delivers an analog frequency-downconverted NMR signal in the frequency range from 0 to 200 kHz. The microchip, realized in a standard complementary metal oxide semiconductor technology, has a size of 1 × 2 × 0.74 mm(3) and operates at a frequency of 63 MHz (i.e., in 1.5 T clinical scanners). Tests in a standard clinical scanner demonstrate the compatibility of the complementary metal oxide semiconductor microchip with clinical MRI systems. Using a solid sample of cis-polyisoprene having a size of 1 × 1.9 × 0.8 mm(3) as internal signal source, the detector achieves a three-dimensional isotropic spatial resolution of 0.15 mm in a measuring time of 100 ms.     

Screening for embryonic loss during in utero development of mice with a human 1.5 Tesla clinical MRI scanner

Purpose

To establish in utero MRI‐scanning of mouse implantation sites in a 1.5 Tesla whole‐body human clinical scanner for evaluation of impaired implantation, placental or developmental defects due to genetic alterations.

Materials and Methods

Pregnant C57Bl/6 wild‐type and Cx31‐deficient mice revealing placental defects were analyzed in utero using a 1.5 Tesla whole‐body clinical scanner in combination with a 3‐cm‐diameter single loop (slice thickness: 1.2 mm). Imaging of implantation sites was evaluated from 6.5–13.5 dpc and amount of implantation sites and in vivo development was analyzed during the critical phase of placentation from 10.5–13.5 dpc.

Results

This method provided high resolution in plane images permitting confident identification of all implantation sites from 6.5 dpc onward. A loss of 60% of Cx31‐deficient embryos was demonstrated compared with controls. Repeated anesthesia as well as imaging protocols produced no gross malformations in the surviving mice.

Conclusion

Using a human clinical MRI scanner high resolution imaging of the entire uterus of the mice and all the embryos inside could be performed. This method is well suited to noninvasively monitor and quantify embryo implantation and to follow this dynamic process in vivo without compromising pregnancy progression and embryonic development. J. Magn. Reson. Imaging 2010;32:1158–1165. © 2010 Wiley‐Liss, Inc.     

Performance of a miniature high‐temperature superconducting (HTS) surface coil for in vivo microimaging of the mouse in a standard 1.5T clinical whole‐body scanner

The performance of a 12‐mm high‐temperature superconducting (HTS) surface coil for in vivo microimaging of mice in a standard 1.5T clinical whole‐body scanner was investigated. Systematic evaluation of MR image quality was conducted on saline phantoms with various conductivities to derive the sensitivity improvement brought by the HTS coil compared with a similar room‐temperature copper coil. The observed signal‐to‐noise ratio (SNR) was correlated to the loaded quality factor of the radio frequency (RF) coils and is theoretically validated with respect to the noise contribution of the MR acquisition channel. The expected in vivo SNR gain was then extrapolated for different anatomical sites by monitoring the quality factor in situ during animal imaging experiments. Typical SNR gains of 9.8, 9.8, 5.4, and 11.6 were found for brain, knee, back, and subcutaneous implanted tumors, respectively, over a series of mice. Excellent in vivo image quality was demonstrated in 16 min with native voxels down to (59 μm)³ with an SNR of 20. The HTS coil technology opens the way, for the first time at the current field strength of clinical MR scanners, to spatial resolutions below 10^–3 mm³ in living mice, which until now were only accessible to specialized high‐field MR microscopes. Magn Reson Med 60:917–927, 2008. © 2008 Wiley‐Liss, Inc.     

High-resolution ultrashort echo time (UTE) imaging on human knee with AWSOS sequence at 3.0 T

Purpose:

To demonstrate the technical feasibility of high‐resolution (0.28–0.14 mm) ultrashort echo time (UTE) imaging on human knee at 3T with the acquisition‐weighted stack of spirals (AWSOS) sequence.

Materials and Methods:

Nine human subjects were scanned on a 3T MRI scanner with an 8‐channel knee coil using the AWSOS sequence and isocenter positioning plus manual shimming.

Results:

High‐resolution UTE images were obtained on the subject knees at TE = 0.6 msec with total acquisition time of 5.12 minutes for 60 slices at an in‐plane resolution of 0.28 mm and 10.24 minutes for 40 slices at an in‐plane resolution of 0.14 mm. Isocenter positioning, manual shimming, and the 8‐channel array coil helped minimize image distortion and achieve high signal‐to‐noise ratio (SNR).

Conclusion:

It is technically feasible on a clinical 3T MRI scanner to perform UTE imaging on human knee at very high spatial resolutions (0.28–0.14 mm) within reasonable scan time (5–10 min) using the AWSOS sequence. J. Magn. Reson. Imaging 2012;35:204‐210. © 2011 Wiley Periodicals, Inc.     

Diffusion tensor imaging (DTI) of rodent brains in vivo using a 1.5T clinical MR scanner

PURPOSE: To evaluate the feasibility of using a clinical 1.5T MR scanner to perform magnetic resonance (MR) diffusion tensor imaging (DTI) on in vivo rodent brains and to trace major rodent neuronal bundles with anatomical correlation. MATERIALS AND METHODS: Two normal adult Sprague Dawley (SD) rats were anesthetized and imaged in a 1.5T MR scanner with a microscopic coil. DTI was performed at a resolution of 0.94 mm x 0.94 mm x 0.5 mm (reconstructed to 0.47 mm x 0.47 mm x 0.5 mm, with b-factors of 600 seconds/mm2 and 1000 seconds/mm2) and a higher resolution of 0.63 mm x 0.63 mm x 0.5 mm (reconstructed to 0.235 mm x 0.235 mm x 0.5 mm, with a b-factor of 1500 seconds/mm2). The fiber-tracking results were correlated with corresponding anatomical sections stained to visualize neuronal fibers. The apparent diffusion coefficient (ADC) and fractional anisotropy (FA) of the neuronal fibers were measured and compared with results in published reports. RESULTS: Several major neuronal fiber tracts, including the corticospinal cord, corpus callosum, and anterior commissure, were identified in all DTI data sets. Stained anatomical sections obtained from the rats confirmed the location of these fibers. The ADC values (0.6-0.8 +/- 10(-3) mm2/second) of the fibers were similar to published figures. However, the FA values (0.3-0.35) were lower than those obtained in previous studies of white matter in rodent spinal cord. CONCLUSION: We have demonstrated the feasibility of using a 1.5T clinical MR scanner for neuronal fiber tracking in rodent brains. The technique will be useful in rodent neuroanatomy studies. Further investigation is encouraged to verify the FA values generated by DTI with such techniques.     

Evaluation of sodium T1 relaxation times in human heart

PURPOSE: To evaluate the sodium longitudinal relaxation (T(1)) characteristics for myocardium and blood in humans. MATERIALS AND METHODS: Eleven healthy volunteers were examined by using a (23)Na heart surface coil at a 1.5 T clinical scanner equipped with a broadband spectroscopy option. (23)Na MR measurements were performed by using a three-dimensional spoiled gradient echo sequence (in-plane resolution, 3.5 mm x 7 mm; slice thickness, 24 mm; TE, 3.1 msec; bandwidth, 65 Hz/pixel; TR, 21 to 150 msec). RESULTS: Longitudinal T(1) relaxation time components were 31.6+/-7.0 msec and 31.1+/-7.5 msec for myocardium and blood, respectively. CONCLUSION: (23)Na T(1) relaxation times of myocardium and blood can be determined in humans. The results are in agreement with values obtained from animal studies.     

First‐pass contrast‐enhanced myocardial perfusion MRI in mice on a 3‐T clinical MR scanner

First‐pass contrast‐enhanced myocardial perfusion MRI in rodents has so far not been possible due to the temporal and spatial resolution requirements. We developed a new first‐pass perfusion MR method for rodent imaging on a clinical 3.0‐T scanner (Philips Healthcare, Best, The Netherlands) that employed 10‐fold k‐space and time domain undersampling with constrained image reconstruction, using temporal basis sets (k‐t principle component analysis) to achieve a spatial resolution of 0.2 × 0.2 × 1.5mm³ and an acquisition window of 43 msec. The method was successfully tested in five healthy and four infarcted mice (C57BL/6J) at heart rates of 495.1 ± 45.8 beats/min. Signal‐intensity‐time profiles showed a percentage myocardial signal increase of 141.3 ± 38.9% in normal mice, compared with 44.7 ± 32.4% in infarcted segments. Mean myocardial blood flow by Fermi function for constrained deconvolution in control mice was 7.3 ± 1.5 mL/g/min, comparable to published literature, with no significant differences between three myocardial segments. In infarcted segments, myocardial blood flow was significantly reduced to 1.2 ± 0.8 mL/g/min (P < 0.01). This is the first report of first‐pass myocardial perfusion MR in a mouse model on a clinical 3‐T MR scanner and using a k‐t undersampling method. Data were acquired on a 3‐T scanner, using an approach similar to clinical acquisition protocols, thus facilitating translation of imaging findings between rodent and human studies. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

High resolution SPECT imaging for visualization of intratumoral heterogeneity using a SPECT/CT scanner dedicated for small animal imaging

Objectives

Tumor interiors are never homogeneous and in vivo visualization of intratumoral heterogeneity would be an innovation that contributes to improved cancer therapy. But, conventional nuclear medicine tests have failed to visualize heterogeneity in vivo because of limited spatial resolution. Recently developed single photon emission computed tomographic (SPECT) scanners dedicated for small animal imaging are of interest due to their excellent spatial resolution of <1?mm, but few studies have focused on the evaluation of intratumoral heterogeneity. We investigated the optimal conditions related to high resolution imaging of heterogeneous tumor interiors using a small animal SPECT scanner.

Methods

The conditions related to SPECT/CT visualization of heterogeneous tumor interiors were investigated using phantoms with ¹¹¹In and simulations of actual small animal imaging. The optimal conditions obtained were validated by in vivo imaging of sarcoma 180-bearing mice.

Results

Larger number of counts must be obtained within limited acquisition time to visualize tumor heterogeneity in vivo in animal imaging, compared to cases that simply detect tumors. At an acquisition time of 30?min, better image quality was obtained with pinhole apertures diameter of 1.4?mm than of 1.0?mm. The obtained best spatial resolution was 1.3?mm, it was acceptable for our purpose, though a little worse than the best possible performance of the scanner (1.0?mm). Additionally, the reconstruction parameters, such as noise suppression, voxel size, and iteration/subset number, needed to be optimized under the limited conditions and were different from those found under the ideal condition. The minimal radioactivity concentration for visualization of heterogeneous tumor interiors was estimated to be as high as 0.2?C0.5?MBq/mL. Liposomes containing ¹¹¹In met this requirement and were administered to tumor-bearing mice. SPECT imaging successfully showed heterogeneous ¹¹¹In distribution within the tumors in vivo with good spatial resolution. A threshold of 0.2?MBq/g for clear visualization of tumor heterogeneity was validated. Autoradiograms obtained ex vivo of excised tumors confirmed that the in vivo SPECT images accurately depicted the heterogeneous intratumoral accumulation of liposomes.

Conclusion

Intratumoral heterogeneity was successfully visualized under the optimized conditions using a SPECT/CT scanner.     

Performance evaluation of the microPET R4 PET scanner for rodents

The microPET R4 scanner is a dedicated positron emission tomograph (PET) for studies of rodents. A number of scanner parameters such as spatial resolution, sensitivity, scatter, and count rate performance were determined in this work, which showed that the microPET R4 is a suitable PET scanner for small animals like mice and rats. In the center of the field of view (FOV) a maximal sensitivity of 43.66 cps/kBq for a centered point source was calculated from a measurement with a germanium-68 line source within an energy widow of 250-750 keV. A spatial resolution of 1.85 mm full-width at half-maximum (FWHM) in the axial direction and 1.66 mm FWHM in the transaxial direction was measured in the center with a 1-mm-diameter sodium-22 point source. Within the inner 20 mm of the FOV the volumetric resolution is better than 15.6 micro l, corresponding to a linear resolution of less than 2.5 mm in all three dimensions. Images of a high-resolution phantom and from mice and rat studies illustrate the good performance of the scanner. A maximal noise equivalent count rate (NECR) was reached at 174 kcps for a mouse phantom and at 93 kcps for a rat phantom (energy window 250-750 keV). Scatter fractions were measured between 0.30 and 0.42 for an energy window of 250-750 keV and phantom diameters similar to mice and rats. A comparison with the microPET P4 model for primates illustrates the gain in sensitivity due to a smaller detector ring diameter but also the changes in NECR.     

Radial multigradient‐echo DCE‐MRI for 3D Ktrans mapping with individual arterial input function measurement in mouse tumor models

The purpose of this study was to provide proof of concept for a new three‐dimensional (3D) radial dynamic contrast enhanced MRI acquisition technique, called “Radial Entire Tumor with Individual Arterial input function dynamic contrast‐enhanced MRI” (RETIA dynamic contrast‐enhanced MRI), which allows for the simultaneous measurement of an arterial input function in the mouse heart at 2 s temporal resolution and coverage of the whole tumor. Alternating 2D and 3D projections contribute to the 2D heart image or 3D tumor data with a 3‐cm field of view. Sixty‐four 2D images of the heart are obtained during acquisition of each 3D tumor dataset. In a pilot study, global K^trans and v_e values were measured in four mice, in a respiratory motion‐animated subcutaneously implanted breast tumor model. This technique is expected to be most useful for the characterization of microvasculature in motion‐animated orthotopic tumors. Magn Reson Med 70:823–828, 2013. © 2012 Wiley Periodicals, Inc.     

Sonographic evaluation of orthotopic bladder tumors in mice treated with TNP-470, an angiogenic inhibitor

RATIONALE AND OBJECTIVES: The purpose of this study was to determine the feasibility of using transabdominal ultrasonography (US) to monitor tumor growth and response to therapy in a mouse model of orthotopic bladder carcinoma. MATERIALS AND METHODS: Human bladder carcinoma cell suspensions were injected into the bladders of 18 SCID mice, allowed to grow for 3 weeks, and monitored weekly with gray-scale US. After 23 days, five animals were treated with TNP-470, an angiogenic inhibitor, and five control animals were treated with saline solution. US images were evaluated for tumor location, size, and neovascularity. All untreated animals (n = 8) were imaged and sacrificed at 25 days. Eight of the treated animals were imaged and sacrificed after 14 days of treatment. US findings for both groups were compared with autopsy findings. RESULTS: While saline-treated tumors continued to grow, the growth of TNP-470-treated tumors was arrested within 7 days of therapy (P < .02). Tumors as small as 1.5 mm were identified prospectively with US. US volume estimates correlated well with autopsy volume measurements (r2 = 1.0, P < .0001). Although tumor neovascularity was identified in every animal, the pattern of neovascularity did not correlate with tumor volume or therapy. CONCLUSION: US can provide accurate intermediate end points for monitoring experimental intraabdominal tumor growth and response to therapy in the mouse model.     

Microcalcifications in clinically normal breast: the value of high field,surface coil,Gd-DTPA-enhanced MRI

State-of-the-art screening mammography allows the detection of nonpalpable breast lesions in approximately 30 % of patients. The presence of clustered microcalcifications without evidence of solid tumors usually requires further investigations, mainly biopsy. A 1.5-T magnet with a single breast coil was used to evaluate 32 patients with indeterminate mammography suggestive of microcalcifications prior to surgery. Both spin-echo (SE) and gradient-echo (GE; 2D fast low-angle short [FLASH]) techniques were utilized before and after injection of 0.2 mmol/kg Gd-DTPA. Upon surgery tumor diameters ranged between 3 and 10 mm. Use of MRI demonstrated 87.5 % overall accuracy, 83.3 % sensitivity, and 92.9 % specificity. False-negative MRI results were in situ carcinomas less than 5 mm in size. All the correctly diagnosed carcinomas measured between 5 and 10 mm. Partial volume is probably the greatest limit of this technique and lesions equal to or smaller than 5 mm are only rarely detected. The GE and SE sequences demonstrated comparable results. Correspondence to: J.DD. Tesoro-Tess     

High spatial resolution contrast-enhanced MR angiography of the supraaortic arteries using the quadrature body coil at 3.0T: A feasibility study

To examine whether the the increased signal-to-noise (S/N) available at 3.0T would permit the use of the quadrature body coil for high spatial resolution contrast-enhanced (CE) MR angiography (MRA), and whether the large FOV that was used in our routine 1.5T protocol would also be feasible at 3.0T. In a prospective study, 43 patients and five volunteers were examined on a clinical whole-body 3.0T MR unit (Intera, Philips Medical Systems, Best, The Netherlands) after institutional review board approval and informed consent. Three-dimensional CE MRA (T1 gradient echo-sequence with TR/TE = 5.7/1.93 msec.; acquisition time, 1:54 min.) using randomly segmented central k-space ordering (CENTRA) was acquired with the quadrature body coil, using over a FOV of 350 mm. A high-image matrix of 432×432 yielded a non-zero filled voxel size of 0.81 mm × 0.81 mm × 1.0 mm (0.66 mm³). For quantitative analysis, contrast ratios (CR) between vessels (S) and signal in surrounding tissue (ST) were calculated [(S−ST)/(S+ST)]. For qualitative analysis, image quality and presence of artifacts were rated by two radiologists in consensus on a five-point scale (1=excellent to 5=nondiagnostic). Digital subtraction angiography (DSA) served as the standard of reference in patients with vascular disease. In the five volunteers, 1.5T CE MRA using a phased array neurovascular coil was available for intraindividual comparison. 3.0T CE MRA was successfully performed in 48/48 subjects (100%). Mean CR± SD were 0.76 (139.30/182.42) and 0.87 (235.18/270.14) at 3.0T and 1.5T respectively .Mean image quality was 3.82±0.86. Intraindividual comparison between 1.5T and 3.0T CE MRA in the volunteers revealed no significant difference in image quality (4.2±0.74 vs 4.6±0.80; p>0.05). Vascular disease was correctly identified in 13/13 patients with DSA correlation. CE MRA of the supraaortic arteries is feasible at 3.0T using a large FOV of 350 mm. The signal gain at 3.0T enables high spatial resolution contrast-enhanced MR angiography by using the built-in quadrature body coil only.     

3 Tesla magnetic resonance imaging of the prostate with combined pelvic phased-array and endorectal coils; Initial experience(1)

RATIONALE AND OBJECTIVES: High-resolution magnetic resonance imaging of the prostate at 1.5T has gained acceptance for pretherapeutic staging of prostate cancer. The aim of this study was to evaluate the potential clinical utility of combined pelvic phased-array and endorectal coils at 3T. MATERIALS AND METHODS: Six volunteers were examined on 1.5T and 3T scanners with pelvic phased-array surface coil combined with a disposable endorectal prostate coil. RESULTS: We were able to acquire T2-W fast spin echo images with 1.5 mm slices, field of view 12, matrix 320 x 192, (voxel size 0.35 mm(3)), with excellent anatomic detail and good T2 contrast. A 1.5 mm axial slice thickness permitted high-quality multiplanar reconstructions with clear visualization of small patho-anatomic structures. Dynamic contrast-enhanced gradient echo images showed excellent spatial resolution (voxel size, 0.38 mm(3)) and temporal resolution. With this level of anatomic information in dynamic images we could clearly distinguish between intracapsular and extracapsular contrast enhancement. CONCLUSION: Using modified T2-fast spin echo and dynamic contrast-enhanced gradient echo sequences, we obtained whole gland coverage with 35-38 microm(3) resolution, without interfering artifacts, in reasonable acquisition times and staying well below the specific absorption rate guidelines. The high spatial resolution in the axial plane allowed meaningful multiplanar reconstructions. The initial results show the clinical utility of endorectal 3T for the noninvasive evaluation of the prostate with image features and quality not achievable at 1.5 T.     

In vivo diffusion-weighted imaging of liver tumor necrosis in the VX2 rabbit model at 1.5 Tesla

OBJECTIVES: We sought to demonstrate the feasibility of using single-shot spin-echo echo-planar imaging for imaging liver tumor necrosis in the in vivo VX2 rabbit model at 1.5 T. MATERIALS AND METHODS: VX2 liver tumors were grown in 4 rabbits. Diffusion-weighted images (DWIs) were acquired during breath-hold using twice refocused SE-EPI (b = 0, 700, 1400 seconds/mm). Anatomic images for tumor size measurements were acquired using T2W TSE. Rabbits were euthanized for subsequent necropsy. Viable and necrotic tumor tissue ADC measurements were performed with reference to hematoxylin and eosin pathology. RESULTS: A total of 8 tumors were grown with diameters ranging from 1.2 to 5.3 cm. Viable and necrotic tumor compartments were clearly differentiated. Apparent diffusion coefficient in necrotic tumor cores, 1.26 +/- 0.11 x 10 mm/s, were significantly greater than those in surrounding viable tumor tissues, 0.74 +/- 0.06 x 10 mm/s (mean +/- SD, P < 0.05). CONCLUSIONS: In vivo DWI of liver tumor necrosis in the VX2 rabbit model is feasible using a 1.5 T clinical magnetic resonance imaging scanner. DWI may permit longitudinal assessment of liver tumor therapies in both preclinical and clinical studies.     

Preliminary study on potential of the jPET-D4 human brain scanner for small animal imaging

Objective  One trend in positron emission tomography (PET) instrumentation over the last decade has been the development of scanners dedicated to small animals such as rats and mice. Thicker crystals, which are necessary to obtain higher sensitivity, result in degraded spatial resolution in the peripheral field-of-view (FOV) owing to the parallax error. On the other hand, we are developing the jPET-D4, which is a dedicated human brain PET scanner that has a capability for depth-of-interaction (DOI) measurement. Although its crystal width is about twice that of commercially available small animal PET scanners, we expect the jPET-D4 to have a potential for small animal imaging by making full use of the DOI information. In this article, we investigate the jPET-D4’s potential for small animal imaging by comparing it with the microPET Focus220, a state-of-the-art PET scanner dedicated to small animals. Methods  The jPET-D4 uses four-layered GSO crystals measuring 2.9 mm × 2.9 mm × 7.5 mm, whereas the microPET Focus220 uses a single layer of LSO crystals measuring 1.5 mm × 1.5 mm × 10.0 mm. First, the absolute sensitivity, counting rate performance and spatial resolution of both scanners were measured. Next a small hot-rod phantom was used to compare their imaging performance. Finally, a rat model with breast tumors was imaged using the jPET-D4. Results  Thanks to the thicker crystals and the longer axial FOV, the jPET-D4 had more than four times higher sensitivity than the microPET Focus220. The noise equivalent counting-rate performance of the jPETD4 reached 1,024 kcps for a rat-size phantom, whereas that of the microPET Focus220 reached only 165 kcps. At the center of the FOV, the resolution was 1.7 mm for the microPET Focus220, whereas it was 3.2 mm for the jPET-D4. On the other hand, the difference of resolution became smaller at the off-center position because the radial resolution degraded faster for the microPET Focus220. The results of phantom imaging showed that the jPET-D4 was comparable to the microPET Focus220 at the off-center position even as the microPET Focus220 outperformed the jPET-D4 except for the peripheral FOV. Conclusions  The jPET-D4 human brain PET scanner, which was designed to achieve not only high resolution but also high sensitivity by measuring DOI information, was proven to have a potential for small animal imaging.     

Real-time MR temperature mapping of rabbit liver in vivo during thermal ablation.

It has been shown that quantitative MRI thermometry using the proton resonance frequency (PRF) method can be used to noninvasively monitor the evolution of tissue temperature, and to guide minimally-invasive tumor ablation based on local hyperthermia. Although hepatic tumors are among the main targets for thermal ablation, PRF-based temperature MRI of the liver is difficult to perform because of motion artifacts, fat content, and low T(*) (2). In this study the stability of real-time thermometry was tested on a clinical 1.5 T scanner for rabbit liver in vivo. The fast segmented EPI principle was used together with respiratory gating to limit respiratory motion artifacts. Lipid signal suppression was achieved with a binomial excitation pulse. Saturation slabs were applied to suppress artifacts due to flowing blood. The respiratory-gated MR thermometry in the rabbit liver in vivo showed a standard deviation (SD) of 1-3 degrees C with a temporal resolution of 3 s per slice and 1.4 mm x 1.9 mm spatial resolution in plane (slice thickness = 5 mm). The method was used to guide thermal ablation experiments with a clinical infrared laser. The estimated size of the necrotic area, based on the thermal dose calculated from MR temperature maps, corresponded well with the actual lesion size determined by histology and conventional MR images obtained 5 days posttreatment. These results show that quantitative MR temperature mapping can be obtained in the liver in vivo, and can be used for real-time control of thermal ablation and for lesion size prediction.     

Ultra-fast and accurate assessment of cardiac function in rats using accelerated MRI at 9.4 Tesla.

MRI can accurately and reproducibly assess cardiac function in rodents but requires relatively long imaging times. Therefore, parallel imaging techniques using a 4-element RF-coil array and MR sequences for cardiac MRI in rats were implemented at ultra-high magnetic fields (9.4 Tesla [T]). The hypothesis that these developments would result in a major reduction in imaging time without loss of accuracy was tested on female Wistar rats under isoflurane anesthesia. High-resolution, contiguous short-axis slices (thickness 1.5 mm) were acquired covering the entire heart. Two interleaved data sets (i) with the volume coil (eight averages) and (ii) with the four-element coil array (one average) were obtained. In addition, two-, three-, and fourfold accelerated data sets were generated through postprocessing of the coil array data, followed by a TGRAPPA reconstruction, resulting in five data sets per rat (in-plane voxel size 100 x 100 microm). Using a single blinded operator, excellent agreement was obtained between volume coil (acquisition time: 88 min) and the fourfold accelerated (<3 min) data sets (e.g., LV mass 436 +/- 21 mg vs 433 +/- 19 mg; ejection fraction 74 +/- 5% vs 75 +/- 4%). This finding demonstrates that it is possible to complete a rat cine-MRI study under 3 min with low variability and without losing temporal or spatial resolution, making high throughput screening programs feasible.     

In vivo multiple-mouse MRI at 7 Tesla.

We developed a live high-field multiple-mouse magnetic resonance imaging method to increase the throughput of imaging studies involving large numbers of mice. Phantom experiments were performed in 7 shielded radiofrequency (RF) coils for concurrent imaging on a 7 Tesla MRI scanner outfitted with multiple transmit and receive channels to confirm uniform signal-to-noise ratio and minimal ghost artifacts across images from the different RF coils. Grid phantoms were used to measure image distortion in different positions in the coils. The brains of 7 live mice were imaged in 3D in the RF coil array, and a second array of 16 RF coils was used to 3D image the whole bodies of 16 fixed, contrast agent-perfused mice. The images of the 7 live mouse brains at 156 microm isotropic resolution and the 16 whole fixed mice at 100 microm isotropic resolution were of high quality and free of artifacts. We have thus shown that multiple-mouse MRI increases throughput for live and fixed mouse experiments by a factor equaling the number of RF coils in the scanner.     

Gene expression patterns and tumor uptake of 18F-FDG, 18F-FLT, and 18F-FEC in PET/MRI of an orthotopic mouse xenotransplantation model of pancreatic cancer

Our aim was to use PET/MRI to evaluate and compare the uptake of 18F-FDG, 3-deoxy-3-18F-fluorothymidine (18F-FLT), and 18F-fluorethylcholine (18F-FEC) in human pancreatic tumor cell lines after xenotransplantation into SCID mice and to correlate tumor uptake with gene expression of membrane transporters and rate-limiting enzymes for tracer uptake and tracer retention. METHODS: Four weeks after orthotopic inoculation of human pancreatic carcinoma cells (PancTuI, Colo357, and BxPC3) into SCID mice, combined imaging was performed with a small-animal PET scanner and a 3-T MRI scanner using a dedicated mouse coil. Tumor-to-liver uptake ratios (TLRs) of the tracers were compared with gene expression profiles of the tumor cell lines and both normal pancreatic tissue and pancreatic tumor tissue based on gene microarray analysis and quantitative polymerase chain reaction. RESULTS: 18F-FLT showed the highest tumor uptake, with a mean TLR of 2.3, allowing correct visualization of all 12 pancreatic tumors. 18F-FDG detected only 4 of 8 tumors and had low uptake in tumors, with a mean TLR of 1.1 in visible tumors. 18F-FEC did not show any tumor uptake. Gene array analysis revealed that both hexokinase 1 as the rate-limiting enzyme for 18F-FDG trapping and pancreas-specific glucose transporter 2 were significantly downregulated whereas thymidine kinase 1, responsible for 18F-FLT trapping, was significantly upregulated in the tumor cell lines, compared with normal pancreatic duct cells and pancreatic tumor tissue. Relevant genes involved in the uptake of 18F-FEC were predominantly unaffected or downregulated in the tumor cell lines. CONCLUSION: In comparison to 18F-FDG and 18F-FEC, 18F-FLT was the PET tracer with the highest and most consistent uptake in various human pancreatic tumor cell lines in SCID mice. The imaging results could be explained by gene expression patterns of membrane transporters and enzymes for tracer uptake and retention as measured by gene array analysis and quantitative polymerase chain reaction in the respective cell lines. Thus, standard molecular techniques provided the basis to help explain model-specific tracer uptake patterns in xenotransplanted human tumor cell lines in mice as observed by PET.