High-resolution imaging at 3T and 7T with multiring local volume coils

Magnetic resonance (MR) microimaging of the human is becoming increasingly common for studies of tissue microstructure and microfunction. In this study, we consider the constraints that such experiments place on the design of radio-frequency (rf) coils, and describe the advantages of multiring coils, which offer a locally uniform B₁ field. We show that these coils are particularly suitable for high-field imaging of a restricted region of larger experimental animals or humans, offering the same simplicity and efficient use of rf power as a simple surface coil but without requiring sequence modifications such as adiabatic pulses. Imaging results are shown from human brain and from the abdominal aorta of an experimental animal.Acknowledgements. We thank Dr. S. King for his helpful assistance.     

Magnetic resonance imaging at 1.5 T with immunospecific contrast agent in vitro and in vivo in a xenotransplant model

Object: Demonstrating the feasibility of magnetic resonance imaging (MRI) at 1.5 T of ultrasmall particle iron oxide (USPIO)-antibody bound to tumor cells in vitro and in a murine xenotransplant model. Methods: Human D430B cells or Raji Burkitt lymphoma cells were incubated in vitro with different amounts of commercially available USPIO-anti-CD20 antibodies and cell pellets were stratified in a test tube. For in vivo studies, D430B cells and Raji lymphoma cells were inoculated subcutaneously in immunodeficient mice. MRI at 1.5 T was performed with T1-weighted three-dimensional fast field echo sequences (17/4.6/13°) and T2-weighted three-dimensional fast-field echo sequences (50/12/7°). For in vivo studies MRI was performed before and 24 h after USPIO-anti-CD20 administration. Results: USPIO-anti-CD20-treated D430B cells, showed a dose-dependent decrease in signal intensity (SI) on T2*-weighted images and SI enhancement on T1-weighted images in vitro. Raji cells showed lower SI changes, in accordance to the fivefold lower expression of CD20 on Raji with respect to D430B cells. In vivo 24 h after USPIO-anti-CD20 administration, both tumors showed an inhomogeneous decrease of SI on T2*-weighted images and SI enhancement on T1-weighted images. Conclusions: MRI at 1.5 T is able to detect USPIO-antibody conjugates targeting a tumor-associated antigen in vitro and in vivo.     

Non-invasive pulmonary perfusion assessment in young patients with cystic fibrosis using an arterial spin labeling MR technique at 1.5 T

Object  

To assess lung perfusion in young patients with cystic fibrosis (CF) using an arterial spin labeling (ASL) technique.     

Benign prostate hyperplasia: evaluation of treatment response with DCE MRI

Benign prostate hyperplasia (BPH) is a major disease and its non-surgical therapy a major area of interest. The purpose of this study was to establish perfusion parameters in beagles with BPH using dynamic contrast-enhanced (DCE) MRI and to investigate changes due to the effects of finasteride treatment. Twelve male beagles (mean age 4.4±0.9,years) were divided into a control and treatment group that received a daily dose of 1 mg/kg finasteride. DCE MRI was carried out in a clinical scanner using a 3D spoiled gradient echo sequence prior to and during treatment. 0.2 mmol/kg contrast agent (gadoteridol) was administered with an injection rate of 0.2 ml/s followed by a 15 ml flush of saline. Contrast enhancement was evaluated by pharmacokinetic mapping of a two-compartment model with colour overlay images in addition to regional ROI analysis. Quantitative parameters were defined by the amplitude of contrast enhancement A, the exchange rate k_ep and the time to maximum signal enhancement. Dynamic contrast-enhanced MRI investigations of the prostate revealed two distinct zones, an inner, periurethral zone and an outer, parenchymal zone. The periurethral zone is highly vascularized, whereas the parenchymal zone is moderately vascularized when compared to other parenchymal organs. During treatment, in the parenchymal zone the intensity of enhancement (amplitude A) and the time to maximum signal enhancement increased, while the exchange rate k_ep decreased. Dynamic contrast-enhanced MRI of BPH reveals distinct differences between individual zones within the prostate. Moreover, changes during successful treatment suggest increased blood volume per volume of tissue and decreased vessel leakiness.     

In vivo mouse spinal cord imaging using echo-planar imaging at 11.75 T

Object To evaluate the feasibility of mouse spinal cord MR imaging using echo-planar imaging (EPI). Materials and methods Optimized multi-shot spin-echo-EPI sequences were compared to conventional spin-echo (c-SE) at 11.75 T and used for high-spatially resolved acquisitions and relaxation-time measurements. Results Good quality images were obtained, with clear delineation of gray and white matter. Acquisition-time gain factor was up to 6 (vs. c-SE) and resolution up to 74 × 94 μm² was achieved. T ₁ and T ₂ relaxation times were reliably measured. Conclusion High-temporally and spatially resolved mouse spinal cord EPI imaging is feasible. This technique should greatly benefit to long acquisition-time experiments (diffusion imaging) and imaging of rapidly-evolving pathologies. V. Callot and G. Duhamel equally contributed to this work.     

Optimization of cardiac cine in the rat on a clinical 1.5-T MR system

Object: the overall goal was to study cardiovascular function in small animals using a clinical 1.5-T MR scanner optimizing a fast gradient-echo cine sequence to obtain high spatial and temporal resolution. Materials and methods: normal rat hearts (n = 9) were imaged using a 1.5-T MR scanner with a spiral fast gradient-echo (fast field echo for Philips scanners) sequence, three Cartesian fast gradient-echo (turbo field echo for Philips scanners) sequences with different in-plane resolution, and with and without flow compensation and half-Fourier acquisition. The hearts of four rats were then excised and left-ventricle mass was weighed. Inter- and intra-observer variability analysis was performed for magnetic resonance imaging (MRI) measurements. Results: half-Fourier acquisition with flow compensation gave the best sequence in terms of image quality, spatial as well as temporal resolution, and suppression of flow artifact. Ejection fraction was 71 ± 4% with less than 5% inter- and intra-observer variability. A good correlation was found between MRI-calculated left-ventricular mass and wet weight. Conclusions: using optimized sequences on a clinical 1.5-T MR scanner can provide accurate quantification of cardiac function in small animals and can promote cardiovascular research on small animals at 1.5-T     

Basic pulse sequences for fast cardiac MR imaging

3. Summary Technical challenges of cardiac MRI include minimizing the effects of cardiac and respiratory motion and developing techniques that allow for both high spatial resolution and high SNR given the small size of small structures such as the coronary vessels. Fast imaging techniques provide considerable time savings and increased flexibility which allow to further optimize image quality.     

MR perfusion imaging: correlation with PET and quantitative angiography

4. Conclusions The presented MR approach reliably identifies patients with anatomically and hemodynamically signiticant coronary artery stenoses. This is due to the fact, that the pulse sequence used produces a substantial change in signal intensity in the perfused versus poorly perfused myocardial regions. Analysis of upslope in this setting rather than of other parameters provides a very sensitive and specific measure of myocardial ischemia. As upslope is a semiquantitative measure of absolute perfusion, even patients with triple vessel disease may be evaluated using this method. This is not the case when using conventional nuclear techniques. Furthermore, the spatial resolution of the MR images permits one to resolve the subendocardial layers of the myocardium, which thus can be evaluated separately from the entire wall. Again, this is not possible using nuclear cardiology perfusion imaging. The robustness of this MR perfusion imaging approach and the fact, that most of the heart can be covered may qualify for its clinical application in the management of coronary artery disease.     

Oxygen enhanced MR ventilation imaging of the lung

The current work is a continuation of a new MRI technique that was proposed for the non-invasive assessment of regional lung ventilation using inhaled molecular oxygen as aT ₁ contrast agent. Several improvements of this technique are described in this work. The signal-to-noise ratio in the ventilation-scan images was optimized using a centrically reordered single-shot RARE sequence with a short effective echo time and short inter-echo spacing. The contrast-to-noise ratio was improved using an optimized inversion delay time. The optimized MR-ventilation-scan was successfully performed in healthy volunteers and in an animal model with airway obstruction. The experimental results demonstrate the feasibility and clinical potential of the MR ventilation imaging technique for assessment of regional pulmonary function.     

Differential diagnostics of portal hypertension studied by MR imaging

This study has examined the ability of magnetic resonance imaging (MRI) to reveal the features of intrahepatic and extrahepatic portal circulation in portal hypertension syndrome (PHS). A retrospective analysis of the abdominal MRI examinations of 96 patients was performed. The spin-echo imaging technique was used to obtain coronal, transaxial, and sagittal images for all patients. The main distinctive features of revealed extrahepatic PHS in nine patients were the thrombosed lumen of the portal vein, the widespread net of collateral vessels around the portal vein, and the widened hepatic artery. The smooth liver surface without evident disturbances of the architectonics of intrahepatic vasculature was depicted in all patients and the caudal lobe hypertrophy was observed in five patients. The widening of the azygos vein was displayed in six of nine patients and hemiazygos vein was delineated in one case. Typical morphologic features of liver cirrhosis and different degrees of the stenosis of the intrahepatic segment of the inferior vena cava (IVC) (10–30%) were found in 28 patients. More than 30% stenosis of the IVC was displayed in 7 of 18 patients (average 28%) with the preascitic liver cirrhosis and in all patients with ascites (average 47%). In two patients the Budd-Chiari syndrome was characterized by the reduced caliber or by the absence of hepatic veins, the comma-shaped intrahepatic collateral vessels, and the constriction of the intrahepatic IVC. The primary ascending thrombosis of the IVC (1 patient) and the occlusion or direct extensions of tumor (16 patients) were usually well seen on MR images.     

A volume adjustable four-coil phased array for high resolution MR imaging of the hip

A four-coil phased array was specifically designed and built for MR imaging of the hip at 1.5 T. Its RF and imaging properties were evaluated using phantom and in-vivo studies and the results were compared to those of three different commercial coils commonly used for hip imaging. Our coil gave a significantly higher S/N at anatomic locations commonly evaluated for hip diagnosis. The increased S/N supports higher image spatial resolution and improves the visualization of fractures and lateral injuries.     

Linear microstrip surface coil for MR imaging of the rat spinal cord at 4.7 T

A new design of RF coil based on a quasi-transverse electromagnetic field is described. The coil was developed for the acquisition of MR images of the rat spinal cord at 4.7 T. Different materials for the construction of the coil were tested, and the best results were obtained with Teflon. The design of the microstrip coil enables the investigator to change the length of the coil in a longitudinal direction and yields a relatively high signal-to-noise ratio due to the restricted field of view. Low RF field penetration depth also helps in suppressing motion artifacts generated by, e.g., breathing or heartbeats.     

A method of RF inhomogeneity correction in MR imaging

A direct postprocessing method for correcting RF inhomogeneity in MR imaging is proposed. First, two images with different flip-angles of ψ and 2ψ are obtained. Next, the spatial distribution maps of the sensitivity of the surface coil and theB ₁ field intensity are produced by employing those images. Finally, the correction of the MR image is achieved, dividing the original image by distribution maps of the coil sensitivity and theB ₁ field intensity. The method was applied to image obtained by a gradient echo sequence and the corrected image is presented.     

Detection of colorectal liver metastases: prospective comparison of unenhanced and ferumoxides-enhanced magnetic resonance imaging at 1.5 T, dual-phase spiral CT, and spiral CT during arterial portography

The purpose of this study was to compare the efficacy of unenhanced and ferumoxides-enhanced magnetic resonance imaging with that of dual-phase spiral CT and spiral CT during arterial portography (CTAP) for the detection of colorectal liver metastases. Fourteen patients with liver metastases candidates for partial hepatectomy were examined with dual-phase spiral CT, unenhanced and ferumoxides-enhanced MR imaging at 1.5 T, and spiral CTAP. Imaging tests were read blinded, prospectively, quantitating number of lesions excepting CTAP which used US to exclude cysts. Subsequent intraoperative US and pathologic findings were correlated with preoperative imaging results. At surgery, 36 lesions 0.5–13 cm in diameter (mean ±standard deviation, 2.9±2.1 cm) were identified. Dual-phase spiral CT depicted 21/36 (58%); precontrast MR imaging, 19/36 (53%); ferumoxides-enhanced MR imaging, 30/36 (83%); and spiral CTAP, 34/36 (94%) lesions. Ferumoxides-enhanced MR imaging was significantly more sensitive than spiral CT and unenhanced MR imaging (P<0.01). The difference in sensitivity between ferumoxides-enhanced MR imaging and spiral CTAP was not statistically significant (P>0.1). Spiral CTAP, however, depicted nine false-positive lesions (2 hemangiomas, 7 perfusion defects). The positive predictive value was 79% for spiral CTAP and 100% for combined pre- and postcontrast MR imaging. We conclude that ferumoxides-enhanced MR imaging is superior to unenhanced MR imaging and biphasic spiral CT for depiction of colorectal liver metastases. Further investigation is needed to clarify whether MR imaging with use of ferumoxides might replace spiral CTAP for preoperative evaluation of liver resection candidates. Recipient of a Cum Laude award for a scientific exhibit at the 1997 ESMRMB annual meeting.     

Utilization of MR angiography in perfusion imaging for identifying arterial input function

Magnetic Resonance Materials in Physics, Biology and Medicine - This research utilizes magnetic resonance angiography (MRA) to identify arterial locations during the parametric evaluation of...     

High-resolution MR imaging appearance of colonic tissue in rabbits using an endoluminal coil

Purpose: This study assessed the value of high-resolution magnetic resonance imaging (MRI) of the distal colon by means of a dedicated endoluminal magnetic resonance receiver coil on a 1.5-T clinical scanner. Materials and Methods: To this end, single-loop, receive-only radio-frequency coils, housed in 18 F sheaths, were built. A 1.5-T clinical imager was used. A 18 French diameter internal MRI receiver coil was inserted into the distal colon in 15 New Zealand rabbits to obtain high-resolution magnetic resonance images by using T1-weighted Flash sequences with and without Fat Saturation (FS), T2-weighted True-Fisp, turbo spin-echo, and T1-weighted Flash FS after contrast media injection. Images were compared to histological sections. Results: An adequate image quality was obtained in all specimens without significant artefacts. Based on histological reports, a five-layer structure of the wall was considered normal. On different MR sequences, only two layers were identified on the images of all rabbits specimens. The nearest layer to the mucosal surface was usually seen as a hyper intense layer and likely corresponds to the mucosa. The highest difference of signal value between internal and external layers was performed on 2D Fat saturation T1 weighted gradient echo. Comparison of mean signal value between the internal and external layers was statistically different in for each sequence used in our protocol (P< 0.05). Conclusion: Dedicated endoluminal RF coil provides good spatial resolution at the region of interest. On this prospective study of in vivo rabbit, evaluation of colon walls allowed to provide detailed information.     

Endoluminal high-resolution MR imaging protocol for colon walls analysis in a mouse model of colitis

Objective

An endoluminal magnetic resonance (MR) imaging protocol including the design of an endoluminal coil (EC) was defined for high-spatial-resolution MR imaging of mice gastrointestinal walls at 4.7 T.

Materials and methods

A receive-only radiofrequency single-loop coil was developed for mice colon wall imaging. Combined with a specific protocol, the prototype was first characterized in vitro on phantoms and on vegetables. Signal-to-noise ratio (SNR) profiles were compared with a quadrature volume birdcage coil (QVBC). Endoluminal MR imaging protocol combined with the EC was assessed in vivo on mice.

Results

The SNR measured close to the coil is significantly higher (10 times and up to 3 mm of the EC center) than the SNR measured with the QVBC. The gain in SNR can be used to reduce the in-plane pixel size up to 39 × 39 µm² (234 µm slice thickness) without time penalty. The different colon wall layers can only be distinguished on images acquired with the EC.

Conclusion

Dedicated EC provides suitable images for the assessment of mice colon wall layers. This proof of concept provides gains in spatial resolution and leads to adequate protocols for the assessment of human colorectal cancer, and can now be used as a new imaging tool for a better understanding of the pathology.

     

一种高精度定时器的设计及其应用

随着计算机软硬件技术的发展,计算机控制系统可以达到的精度越来越高。本文介绍了一种基于PentiumCPU的高精度定时方法,在不同主频的CPU下,可获得不同的定时精度,最高可达到10微秒量级。同时,介绍了这种定时方法在某种步进电机控制中的应用。     

High-resolution echo-planar imaging at 3.0 T

Echo-planar imaging (EPI) is a snapshot technique, which is useful in a wide range of clinical applications, including the study of physiological function. Over recent years, EPI has found a major new use in functional imaging of the brain. Many EPI experiments can benefit from the increased signal-to-noise ratio (S/N) which results from imaging at high magnetic field. Recently, we have built a 3.0-T EPI scanner at Nottingham University. The low-level radiofrequency and control electronics have been constructed in-house. This, coupled with software written specifically for the system, results in a performance and flexibility exceeding that of a commercial system. A quiet head gradient set produces gradients of up to 30 mT m^–1. It is driven using a series multiresonant filter circuit, which allows the production of high-strength, trapezoidal- or sinusoidal-switched gradients.Using this scanner it has been possible to obtain images comprising 256×256 pixels, with a 2.5-mm slice and 0.75 mm in-plane resolution, in 140 ms. Multislicing allows a volume set of 16,128×128 images to be obtained in 1.6 s. A comparison of tests performed at fields of 0.5 T and 3.0 T on the same phantom indicates a better than linear increase in S/N with field strength. EPI images obtained at 3.0 T have been used in studies of brain activation during visual stimulation and execution of a simple motor task.