Coil optimization for low-field MRI: a dedicated process for small animal preclinical studies.

We demonstrate a method for the fast in vivo quantification of small volumes, down to 25 µL, using low-field magnetic resonance imaging (MRI) coils. The coils were designed so as to maximize the signal-to-noise ratio (SNR) in the images. For this we developed an analytical model for describing the variations of the SNR with coil design and with size/shape suited to the object under observation. Based on the conclusions drawn from the model, the coil parameters were chosen in order to reach an SNR close to the maximum. For the validation of the model, coils were finally characterized in terms of quality factor using saline phantoms. The coil design procedure is illustrated here with two examples: first, the quantification of about 200 µL of intradermal injected gel on rabbits with a single loop surface coil and second, the imaging of the intervertebral disks in rat tails using a small volume coil to detect possible lesions. Such studies would not have been feasible for the clinical low-field MRI system at our disposal using any of the commercially available medium-sized manufactured coils. As a result of this simple optimization procedure, a wide range of applications is accessible even at low magnetic fields, leading to new opportunities for low-cost, though efficient, preclinical studies.

An optical fiber-based gating device for prospective mouse cardiac MRI.

Prospective synchronization of MRI acquisitions on living organisms involves the monitoring of respiratory and heart motions. The electrocardiogram (ECG) signal is conventionally used to measure the cardiac cycle. However, in some circumstances, obtaining an uncorrupted ECG signal recorded on small animals with radio frequency (RF) pulses and gradient switching is challenging. To monitor respiratory motion, an air cushion associated with a pressure sensor is commonly used but the system suffers from bulkiness. For many applications, the physiological gating information can also be derived from an MR navigated signal. However, a compact device that can simultaneously provide respiratory and cardiac information, for both prospective gating and physiological monitoring, is desirable. This is particularly valid since small volume coils or dedicated cardiac RF coil arrays placed directly against the chest wall are required to maximize measurement sensitivity. An optic-based device designed to synchronize MRI acquisitions on small animal's respiratory and heart motion was developed using a transmit-receive pair of optical fibers. The suitability of the developed device was assessed on mice ( n = 10) and was based on two sets of experiments with dual cardiac and respiratory synchronization. Images acquired with prospective triggering using the optical-based signal, ECG, and the pressure sensor during the same experiment were compared between themselves in the first set. The second set compared prospective technique using optical-based device and ECG to a retrospective technique. The optical signal that was correlated to both respiratory and heart motion was totally unaffected by radiofrequency pulses or currents induced by the magnetic field gradients used for imaging. Mice heart MR images depict low-visible motion artifacts with all sensors or techniques used. No significant SNR differences were found between each series of image. Full fiber-optic-based signal derived from heart and respiratory motion was suitable for prospective triggering of heart MR imaging. The fiber optic device performed similarly to the ECG and air pressure sensors, while providing an advantage for imaging with dedicated cardiac array coils by reducing bulk. It can be an attractive alternative for small animal MRI in difficult environments such as limited space and strong gradient switching.

Assessment of radiofrequency self-heating around a metallic wire with MR T1-based thermometry.

Heat produced by a magnetic resonance (MR) imaging sequence in the vicinity of a conductive wire (pacemaker, electrodes, or catheter), is a subject of interest for the assessment of patient safety during imaging. For this purpose, the measurement of temperature rises during an MR imaging sequence using MR T1-based thermometry provides several advantages, mainly in its ability to retrieve in situ real-time thermal maps. Recent studies investigated the heat produced by an independent radiofrequency pulse, assessing MR imaging sequence heating using a specific MR thermometry sequence. This study focuses on self-heating for which the radiofrequency pulses used for measuring temperature create the heat. An experimental design was set up to evaluate T1-based thermometry self-heating using a coupled/decoupled wire and to compare it with a reference temperature gathered by an optical fiber device. For the tested experimental set up, T1-based thermometry is in fairly good agreement with optical fiber reference temperature.

Multimodal imaging for the detection and characterization of liver lesions in a mouse model of neuroendocrine tumor.

BACKGROUND: The aim of this study was to compare in vivo magnetic resonance imaging (MRI) and ex vivo autoradiography with histopathological results for the detection and characterization of liver lesions in an experimental model of human neuroendocrine tumors. MATERIAL AND METHODS: Intestinal STC-1 endocrine tumor cells were injected into 30 nude mice to achieve hepatic dissemination. Seven to 30 days after injection, T2-weighted in vivo images covering the entire liver were acquired with a 7-T system. Autoradiographs were also obtained in 28 mice after injection of fluorodeoxyglucose (18F-FDG). The autoradiographic liver samples were then stained with an antichromogranin antibody before histological analysis. Tumor size and the hepatic tumor fraction were measured using the three imaging modalities. RESULTS: Metastatic tumors visualized on the histological liver sections ranged in size from 50 microm (day 7) to 3 mm (day 30). The hepatic tumor fraction increased with time, reaching 30% of the hepatic surface area on day 30. Visual analysis revealed variable tumor distribution and type (solid and/or cystic). On MRI, lesions were identified from day 12 (about 100 icrom in diameter) and the hepatic tumor fraction was up to 48% at day 30. The smallest lesions (350 microm in diameter) were also detected at day 12 on the autoradiographs. There was good correlation between tumor fractions determined from autoradiographic and histological data. CONCLUSION: In vivo, MRI appears to be well suited to the follow-up of liver lesions in a mouse model of neuroendocrine tumor. Preliminary results using 18F-FDG in this animal model are promising, showing differences in FDG uptake.

Measurement of knee cartilage thickness using MRI: a reproducibility study in a meniscectomized guinea pig model of osteoarthritis.

The in vivo precision (reproducibility) of quantitative MRI is of particular importance in osteoarthritis (OA) progression of small magnitude and response to therapy. In this study, three-dimensional high-resolution MRI performed at 7 T was used to assess the short-term reproducibility of measurements of mean tibial cartilage thickness in a meniscectomized guinea pig model of OA. MR image acquisition was repeated five times in nine controls (SHAM) and 10 osteoarthritic animals 3 months after meniscectomy (MNX), in vivo. The animals were then killed for histomorphometric assessment and correlation with the MRI-based measurements. Medial tibial cartilage thickness was measured on MR images using semi-automatic dedicated 3D software developed in-house. The reproducibility of measurements of cartilage thickness was assessed by five repeated MRI examinations with a short recovery delay between examinations (48 h). The computed coefficients of variation were 8.9% for the SHAM group and 8.2% for the MNX group. The coefficients of variation were compatible with expected thickness variations between normal and pathological animals. A positive agreement and significant partial correlation (Spearman r' = 0.74; P < 0.01) between the MRI and histomorphometric data was established. Three-dimensional high-resolution MRI is a promising non-invasive research tool for in vivo follow-up. This modality could be used for staging and monitoring therapy response in small-animal models of OA.

Optical cardiac and respiratory device for synchronized MRI on small animal.

Respiratory and cardiac motion must be overcome if MRI of the thorax or abdomen is to be performed satisfactorily. An optical-based device designed to synchronize MRI acquisition on small animal was developed using a pair of optical fibers. Light from a laser diode was focused into the transmit fiber and impinged upon the moving skin. The reflected light was detected by the receive fiber and then caries to a light-voltage photodiode, were the signal was amplified and filtered. The recorded optical-based signals are well correlated with both respiratory and heart motions. The signal amplitude recorded on both rats and mice were large enough to perform an easy adjustment of gating level with good differentiation between cardiac and respiratory signal. The device developed using thin fibers is simple to use even when space available around the mice is limited (narrow coils). The signal is totally unaffected by radiofrequency impulsions or magnetic field gradients used for imaging. This optical-based trigger system was used successfully for dual cardiac and respiratory synchronization of rat and mice for heart and liver examinations at 4.7T.

Comparison of two strategies of background-accommodation: influence on the metabolite concentration estimation from in vivo Magnetic Resonance Spectroscopy data.

Localized proton Magnetic Resonance Spectroscopy brain signals acquired at short echo-time contain contributions from metabolites, water and a ;background' which mainly originates from macromolecules and lipids. The purpose of the present study was to compare the influence of the background-accommodation strategy on the metabolite concentration estimates. Two strategies were investigated to accommodate the background, 1) the measured background signal was incorporated in the metabolite basis-set; and 2) the background signal was estimated and subtracted from the in vivo signal using Subtract-QUEST. The influence of the background-accommodation strategy was addressed with the aid of Monte Carlo and in vivo studies. For the considered signals of this study, the concentration estimates obtained using the first approach were below those obtained with Subtract-QUEST. Indeed, the presence of residual contribution of metabolite signals with short longitudinal relaxation times (T1) in the measured background led to an underestimation of metabolite concentration estimates. Conversely, the observed underestimation of the background contribution using Subtract-QUEST led to an overestimation of the metabolite estimates.

A magnetic resonance elastography (MRE) approach for liver investigation.

Due to its key role in carbohydrate metabolisation, blood detoxification, filtering and substance storing, liver is prone to a wide variety of diseases: hepatitis, cirrhosis, cancer. As such, liver investigation is a major field of interest. We describe here a non invasive way to obtain important information on liver elasticity properties. In vitro Magnetic Resonance Elastography (MRE) experiments are realized using a normalized substance (Agar gel in different concentrations) and this technique is also demonstrated in the context of an in vivo investigation of the elastic properties of a normal control liver.

Characterization of neuro-endocrine tumors in an athymic nude mouse model using dedicated synchronization strategies for T2-weigted MR imaging at 7T.

Endocrine tumours, with digestive localization, are tumours with variable forecast which are independent of their invasive and metastatic extensions. The experimental model of endocrine tumors with liver dissemination is available for evaluation of new medical therapeutics such as antiangiogenic therapy. MRI is a non invasive modality allowing in vivo examinations and is suitable to follow liver lesion evolution during longitudinal study on animal models. The goal of this study was to assess the detection level and to characterize the liver lesions in an athymic nude mouse model, using a dedicated MRI protocol and an optimized synchronization strategy at high magnetic field strength. The experiments were performed at 7T. To detect liver lesions, respiratory-triggered T2-weighted MR images is the sequence of choice. With conventional acquisition strategies used on small animal MR systems, trigger signal is performed at each respiratory cycle and thus, the T2 contrast is not freely controlled. Additionally, the slice number is limited by the expiration delay. To overcome these drawbacks, we proposed an original strategy enabling true T2-weighted imaging with minimal movement artifacts, regardless of the respiratory period and the number of slices. This protocol was used to carry out a longitudinal follow-up of hepatic lesions in 8 nude mice at stages D7, D12, D17 and D24. The fraction of lesion over the total liver volume was quantified. Moreover, the characterization of cystic or non-cystic type of lesions was achieved using various TE leading to T2 maps. In conclusion, the level of lesion detection and characterization of liver lesions was performed using a devoted protocol with original synchronization strategy dedicated to high field MRI. MR imaging could be used with relevance in the evaluation of new therapeutics protocol for treatment of liver lesions in neuroendocrine tumors using small animal model.

A dedicated two-element phased array receiver coil for high resolution MRI of rat knee cartilage at 7T.

Despite that on clinical systems phased array technology is now widely used, the high field MRI experimental systems with multiple receiver channels just became available few years ago. For this reason and due to the large range of magnetic field (frequencies between 200 and 500 MHz for proton resonance), commercial phased arrays implemented in narrow bore for high field applications are rare and relatively expensive. Array coil imaging is an advanced method for acquiring high resolution images with enhanced Signal-to-Noise Ratio (SNR) and/or enlarged Field Of View (FOV) compared for example to single loop surface coil. The volume of interest is then covered by several coil elements and images reconstructed for every single channel are combined afterwards. The goal of this work was to develop a dedicated two-element array coil operating at 300 MHz (7T) for high-resolution imaging of rat knee joint in order to quantify cartilage thickness and volume. A dedicated two-element array coil with two square elements encompassing knee joint was designed and built. Decoupling between elements was achieved with a capacitor inserted on the common leg of the two elements. The average gain in SNR compared to a 15 mm reference single loop coil was 2.2. This SNR gain was used to improve spatial resolution of 3D acquisition by decreasing the voxel size from 59 x 59 x 156 microm(3) to 51 x 51 x 94 microm(3) without time penalty.

Knee cartilage thickness measurements using MRI: a 4(1/2)-month longitudinal study in the meniscectomized guinea pig model of OA.

OBJECTIVE: The aim of this study was to follow, over a 4(1/2)-month period, the medial tibia cartilage thickness on a meniscectomy (MNX) guinea pig osteoarthritis (OA) model and to compare with control animals, using three-dimensional high-resolution magnetic resonance imaging (3D HR-MRI). METHODS: MRI experimentations were performed in vivo at 7 T on guinea pig knee joints. 3D HR-MR images were acquired in 60 controls (SHAM) and 45 osteoarthritic animals (MNX) at four time-points (15, 45, 90 and 135 days) after surgery. Medial tibial cartilage thickness was measured from MRI images using in-house dedicated 3D software followed by a statistical analysis. At each time-point 15 SHAM and 15 MNX animals were sacrificed for histomorphometric assessments. RESULTS: No significant difference of mean cartilage thickness between the groups was found at early stage (D45) using MRI; however, significant differences were found between the groups at D90 (P<0.001) and D135 (P<0.001). Histomorphometry data confirmed the pathological status of the animals and was well correlated with MRI at D15 (r=0.79, P<0.01), D45 (r=0.67, P<0.01), and D135 (r=0.39, P<0.05) for SHAM, and at D45 (r=0.63, P<0.01), and D135 (r=0.81, P<0.01) for MNX. CONCLUSION: Medial tibial cartilage measurement based on HR-MR images enables the monitoring of longitudinal cartilage thickness changes. This technique showed significant differences between SHAM and MNX as from D90 after surgery. It could be used as a noninvasive and reproducible tool to monitor therapeutic response in this OA model.

Synchronisation strategies in T2-weighted MR imaging for detection of liver lesions: Application on a nude mouse model.

AIM: The objective of this work was to propose original synchronisation strategies based on T2-weighted sequence performed on a small animal MRI spectrometer in order to improve the image contrast and detect mouse liver lesions at high magnetic field. MATERIALS AND METHODS: The experiments were performed in vivo at 7T using a 32 mm inner diameter cylindrical volumetric coil for both RF emission and reception. A sensitive pressure sensor was used to detect external movements due to both respiration and heart beats. The pressure sensor was interfaced with a commercial ECG Trigger Unit to use dedicated functionalities (trigger levels, delays and window). To enable T2-weighted imaging with minimised T1 effects, an acquisition strategy with controlled TR spanning over several respiratory cycles was developed. With this strategy, the slices were acquired over several respiratory periods. RESULTS: The acquisition, performed over several respiratory periods, enables a longer TR than the typical mouse respiratory period. The image contrast is controllable and independent of the respiratory period. The heavily T2-weighted images obtained with the developed strategy allow better visualisation of lesions at high magnetic field. Moreover, double respiratory and cardiac synchronisation, based on a unique sensitive pressure sensor, improves image quality with less motion artifacts, especially in the ventral liver region. The total slice number is independent of respiratory period and thin slices can be acquired to cover the whole liver. CONCLUSION: The developed strategy enables high quality pure T2-weighted imaging with minimal motion artifacts. This strategy improves T2-weighted image contrast and quality, especially at high magnetic field, on animals with short respiratory periods. The strategy was demonstrated using a mouse model of liver lesions at 7T. This protocol could be used to carry out a longitudinal follow-up.

[Magnetic resonance angiography of splanchnic artery aneurysms and pseudoaneurysms]. / Angiographie par résonance magnétique dans les anévrismes et pseudoanévrismes des artères splanchniques.

PURPOSE: To correlate Magnetic resonance angiography (MRA) with computed tomography for the diagnosis of splanchnic artery aneurysms. MATERIALS-METHODS: We retrospectively reviewed the MRA findings of splanchnic arteries performed in 16 patients with known splanchnic aneurysms. Sixteen patients underwent computed tomography (CT), and MRA. Conventional angiogram was performed to confirm the diagnosis in six cases. This study was conducted on a 1.5T MR Symphony system (Siemens, Erlangen, Germany). CE-MRA was performed by using a 3D RF-spoiled FLASH sequence. Acquisition of source images was performed in the coronal plane and reconstructed by means of maximum-intensity projection (MIP) post-processing. RESULTS: Patient population included 7 men and 9 women aged from 31 to 85 years old (mean age 59.5 years old). Splanchnic aneurysms were located as follows: splenic (n=5), hepatic (n=6), celiac trunk (n=4) and gastroduodenal (n=1). In 9 cases, MRA provided results similar to CT. In 3 cases, MRA provided more accurate information on the location of the aneurysm or detected additional aneurysms. In 4 patients, a diagnosis of aneurysm was made first or only made on MRA. CONCLUSION: This small series showed a good correlation between CT and MRA for detection and characterization of splanchnic aneurysms.

Metabolite concentrations of healthy mouse brain by Magnetic Resonance Spectroscopy at 7 Tesla.

In vivo1H short echo-time Magnetic Resonance spectra are made up of overlapping spectral components from many metabolites. Typically, they exibit low signal-to-noise ratio. Metabolite concentrations are obtained by quantitating such spectra. Quantitation is difficult due to the superposition of metabolite resonances, macromolecules, lipids and water residue contributions. A fitting algorithm invoking extensive prior knowledge is needed. We quantitated1H in vivo mouse brain spectra obtained at 7 Tesla using the time-domain QUEST method combined with in vitro metabolite basis set signals. Brain metabolite concentrations estimated from eight mouse brain signals are compared to previously reported results.

Diagnosis of splanchnic artery aneurysms and pseudoaneurysms, with special reference to contrast enhanced 3D magnetic resonance angiography: a review.

Splanchnic artery aneurysms are rare. In the past, conventional angiography was the only way to detect them, but today non-invasive techniques are available. Breath-hold contrast-enhanced 3D magnetic resonance angiography has become a routine examination for evaluation of the aorta and its visceral branches. In this article, we briefly discuss the technical aspects of 3D contrast-enhanced magnetic resonance angiography and illustrate various splanchnic artery aneurysms-pseudoaneurysms with their main characteristics.

In vivo colon wall imaging using endoluminal coils: feasibility study on rabbits.

PURPOSE: To assess in vivo distal colon wall magnetic resonance imaging (MRI) feasibility on rabbits using an endoluminal radio frequency (RF) coil on a 1.5-T clinical scanner. MATERIALS AND METHODS: The endoluminal coil signal-to-noise ratio (SNR) was compared to a clinical four-element phased-array body coil. High-resolution (HR) MRI of rabbit colon walls was performed on six rabbits. The imaging protocol combined T1-weighted fast low-angle-shot (FLASH) sequences with and without fat saturation (FS), T2-weighted True-Fast imaging with steady state precession (Fisp), turbo spin-echo (TSE), and T1-weighted FLASH FS after contrast media injection. Images were compared to histological sections. Catheter tracking using an endoluminal coil in addition to external coils was also evaluated on two rabbits. RESULTS: HR images allow visualization and identification of rabbit colon wall layers. Real-time tracking allows a clear visualization and a good positioning of the endoluminal coil within the rabbit. CONCLUSION: Compared to a clinical multielement array coil, a dedicated endoluminal RF coil provides an important SNR increase at the region of interest (ROI). Very HR images of in vivo rabbit colon walls were achieved providing detailed information on the different wall layers. This technique could be considered on humans for accurate tumoral and inflammatory bowel disease diagnosis.

[MRI of the GI tract: technical considerations and future applications]. / IRM et exploration du tube digestif--aspects techniques et perspectives d'évolution.

Barium enema remains the reference method for the detection of morphological intraluminal alterations of the bowel. Optimal filling of intestinal loops allows high diagnostic sensitivity and specificity. US, CT and MRI are useful diagnostic procedures in the evaluation of mural and extramural alterations. In recent years, MR-enteroclysis and MR colonography have been developed, both enable the evaluation of luminal, extraluminal and mural alterations of the bowel. While these modalities provide good imaging evaluation of the bowel, visualization of the different layers, as seen on US, is still not available. Use of high resolution endoluminal coil on MR could improve mural evaluation of bowel to differentiate inflammatory diseases and provide accurate TNM classification of tumoral lesion with minimally invasive procedure.