MR imaging of the prostate in clinical practice

Magnetic resonance imaging (MRI) is the imaging tool of choice in the evaluation of prostate cancer. The main applications of MR imaging in the management of prostate cancer are: (1) to guide targeted biopsy when prostate cancer is clinically suspected and previous ultrasound-guided biopsy results are negative; (2) to localize and stage prostate cancer and provide a roadmap for treatment planning; and (3) to detect residual or locally recurrent cancer after treatment. Other MR techniques such as proton MR spectroscopic imaging (MRSI), diffusion-weighted imaging (DWI), and contrast-enhanced MRI (CE-MRI) complement conventional MR imaging by providing metabolic and functional information that can improve the accuracy of prostate cancer detection and characterization. In everyday clinical practice, and to account for patient comfort, MR imaging studies are limited to 1 h. To obtain consistently high-quality images, a well-designed protocol is necessary. Routine MR imaging can be supplemented by other MR techniques such as MRSI, DWI or CE-MRI depending on the expertise available and the clinical questions that need to be answered. This review summarizes the role of MR imaging in the management of prostate cancer and describes practical approaches to implementing anatomic, metabolic and functional MR imaging techniques in the clinic.     

Challenges of short magnet design

The desire to improve patient comfort and acceptance of high field magnetic resonance imaging (MRI) systems results in new-approaches to magnet and other critical MRI component design. One such approach, based upon an anatomical field of view analysis, is presented in this paper. The approach is utilized to define imaging volume requirements for short, high field, solenoid magnet designs that maintain whole body imaging capability. A short magnet design with excellent magnetic field homogeneity is presented accordingly. Combined with a novel flared gradient coil the overall system achieves improvements in openness.     

Acoustic noise reduction in a 4 T MRI scanner

High-field, high-speed magnetic resonance imaging (MRI) can generate high levels of noise. There is ongoing concern in the medical and imaging research communities regarding the detrimental effects of high acoustic levels on auditory function, patient anxiety, verbal communication between patients and health care workers and ultimately MR image quality. In order to effectively suppress the noise levels inside MRI scanners, the sound field needs to be accurately measured and characterized. This paper presents the results of measurements of the sound radiation from a gradient coil cylinder within a 4 T MRI scanner under a variety of conditions. These measurement results show: (1) that noise levels can be significantly reduced through the use of an appropriately designed passive acoustic liner; and (2) the true noise levels that are experienced by patients during echo planar imaging.     

Magnetic resonance imaging of rodent teeth

In this paper, the anatomical structures of rat jaws are studied using magnetic resonance imaging (MRI) with high spatial resolution. In vivo microimages of teeth from 3- and 12-week-old (young and adult) rats have been performed. A 2 T nuclear magnetic resonance microscope was used to collect, with multiple orientations, non-invasive 2D data images using the spin-echo technique. MRI appears well suited to give images of the oral area and may represent a useful tool for diagnosis of dental diseases and more particularly of pulp pathologies. This work was presented at the ESMRMB meeting in Brussels, September 1997.     

超导核磁共振成像磁屏蔽技术的进展

本文对超导核磁共振成像磁屏蔽技术的发展过程，几种屏蔽方式的比较、研究的问题和发展趋势等进行了概括性的论述。     

Rat cerebral ischemia induced with photochemical occlusion of proximal middle cerebral artery: a stroke model for MR imaging research

A stroke model in rats with photochemically induced thrombosis (PIT) of proximal cerebral middle artery (MCA) is introduced for magnetic resonance imaging (MRI) study. Thirty-seven rats subjected to surgical and optical procedures for inducing the PIT models were scanned using a 1.5-T scanner with T1-weighted imaging (T1WI), T2-weighted imaging (T2WI), diffusion-weighted imaging (DWI), and contrast-enhanced perfusion-weighted imaging (PWI) at 1 h and 24 h after MCA occlusion. The penumbra evolution and PWI-derived parameters including relative cerebral blood volume (rCBV) and relative cerebral blood flow (rCBF) were monitored; and the relative lesion size (RLS) was compared with the final RLS on the gold standard triphenyl tetrazolium chloride (TTC) staining at 24 h. The results showed that the focal cerebral ischemic lesions were detectable in all rats with different MR approaches. The lesion on PWI at 1 h and on all MR images at 24 h was matched well with that seen on TTC staining; the peri-infarct area decreased from 6.2 ± 7.2% of the brain volume at 1 h to 0.3 ± 5.6% at 24 h. Compared to that in the contralateral hemisphere, rCBV in ischemic region was 52.6 ± 21.4 and 40.0 ± 15.8% (p > 0.05), and rCBF was 64.6 ± 11.2 and 47.3 ± 11.1% (p < 0.05) at 1 h and 24 h respectively. The present PIT model in rats has been successfully adopted for MRI research, which might be feasible for certain stroke studies and should be beneficial for the evaluation on effects of potential diagnostic and therapeutic approaches.     

Current status of cardiac MRI in small animals

Cardiac magnetic resonance imaging (MRI) on small animals is possible but remains challenging and not well standardized. This publication aims to provide an overview of the current techniques, applications and challenges of cardiac MRI in small animals for researchers interested in moving into this field. Solutions have been developed to obtain a reliable cardiac trigger in both the rat and the mouse. Techniques to measure ventricular function and mass have been well validated and are used by several research groups. More advanced techniques like perfusion imaging, delayed enhancement or tag imaging are emerging. Regarding cardiac applications, not only coronary ischemic disease but several other pathologies or conditions including cardiopathies in transgenic animals have already benefited from these new developments. Therefore, cardiac MRI has a bright future for research in small animals.     

Measurement of weak electric currents in copper wire phantoms using MRI: influence of susceptibility enhancement

The use of magnetic resonance imaging (MRI)-based methods for the direct detection of neuronal currents is a topic of intense investigation. Much experimental work has been carried out with the express aim of establishing detection thresholds and sensitivity to flowing currents. However, in most of these experiments, magnetic susceptibility enhancement was ignored. In this work, we present results that show the influence of a susceptibility artefact on the detection threshold and sensitivity. For this purpose, a novel phantom, consisting of a water-filled cylinder with two wires of different materials connected in series, was constructed. Magnitude MR images were acquired from a single slice using a gradient-echo echo planar imaging (EPI) sequence. The data show that the time course of the detected MR signal magnitude correlates very well with the waveform of the input current. The effect of the susceptibility artefacts arising from the two different wires was examined by comparing the magnitudes of the MR signals at different voxel locations. Our results indicate the following: (1) MR signal enhancement arising from the magnetic susceptibility effect influences the detection sensitivity of weak current; (2) the detection threshold and sensitivity are phantom-wire dependent; (3) sub-μ A electric current detection in a phantom is possible on a 1.5-T MR scanner in the presence of susceptibility enhancement.     

The role of MRI in prostate cancer: current and future directions

Magnetic Resonance Materials in Physics, Biology and Medicine - There has been an increasing role of magnetic resonance imaging (MRI) in the management of prostate cancer. MRI already plays an...     

Design of actively shielded main magnets: an improved functional method

An improved functional approach for designing MRI (magnetic resonance imaging) main magnets with active shielding is presented. By nulling one or two external moments as well as a certain series of internal moments of the magnetic field, new designs with improved shielding in combination with or without shorter magnet lengths are obtained. The improved method can be employed to design short and practical superconducting magnets at any given field strength. The resulting designs yield the desired field homogeneity inside the region of interest without using superconducting shim coils. This approach requires only a modest amount of computing power. One of the design steps, a contour plot of the continuous current solutions, can be utilized to study stretch goals for favorable design parameters.     

Monitoring the survival of islet transplants by MRI using a novel technique for their automated detection and quantification

Object  There is a clinical need to be able to assess graft loss of transplanted pancreatic islets (PI) non-invasively with clear-cut quantification of islet survival. We tracked transplanted PI in diabetic mice during the early post-transplant period by magnetic resonance imaging (MRI) and quantified the islet loss using automatic segmentation technique. Materials and methods  Magnetically labeled islet iso-, allo- and xenografts were injected into the right liver lobes. Animals underwent MRI scanning during 14 days after PI transplantation. MR images were processed using custom-made software, which automatically detects hypointense regions representing PI. It is based on morphological top-hat and bottom-hat transforms. Results  Manually and automatically detected areas, corresponding to PI, differed by 4% in phantoms. Signal loss regions due to PI decreased comparably in all groups during the first week post transplant. Throughout the second week post-transplant, the signal loss area continued in a steep decline in case of allografts and xenografts, whereas the decline in case of isografts slowed down. Conclusion  Automatic segmentation allows for the more reproducible, objective assessment of transplanted PI. Quantification confirms the assumption that a significant number of islets are destroyed in the first week following transplantation irrespective of allografts, xenografts or isografts.     

High-resolution magnetic resonance imaging of iron-labeled myoblasts using a standard 1.5-T clinical scanner

Myoblast transplantation is a promising means of restoring cardiac function in infarcted areas. For optimization of transplant protocols, tracking the location and fate of the injected cells is necessary. An attractive imaging modality for this is magnetic resonance imaging (MRI) as it is noninvasive and as iron-labeled myoblasts provide a signal attenuation in T2*-weighted protocols. The aim of this study was to develop an efficient iron-labeling protocol for myoblasts and to visualize single-labeled cells using a clinical 1.5-T scanner. Pig myoblasts were labeled with a superparamagnetic iron oxide (SPIO) agent using a liposome transfection agent. Labeling efficiency, toxicity, cell viability, and proliferative capacity were measured for 10 days. Magnetic resonance (MR) of myoblast cultures used a T2*-weighted three-dimensional protocol with a maximum in-plane resolution of 19.5 × 26.0 m² and 50 m slices. Use of liposomes improved SPIO labeling efficiency. Labeling did not induce toxicity or affect cell viability or proliferation. The cell distribution as observed with light and fluorescence microscopy matched the signal voids observed in the MRI datasets. Liposomes promote fast, nontoxic and efficient SPIO labeling of myoblasts that can be tracked by MRI microscopy in clinical scanners using susceptibility-weighted protocols.     

Human rapid acquisition with relaxation enhancement imaging at 8 T without specific absorption rate violation

A standard fast imaging sequence, rapid acquisition with relaxation enhancement (RARE), has been applied to human magnetic resonance at 8 T. RARE is known for its speed, good contrast and high RF power content. HighlyT ₂ weighted images, the hallmark of RARE imaging, were acquired from the human brain. It is demonstrated that whileT ₂ values may be reduced at 8 T, high quality RARE images could still be acquired at this field strength. Most importantly however, it is demonstrated that RARE images could be acquired without violating specific absorption rate (SAR) guidelines. Since it is well known thatT ₂ weighted images are of significant value in clinical diagnosis, the implementation of RARE at this field strength will provide ultra high field MRI (UHFMRI) with a valuable imaging protocol at this field strength without exceeding SAR limitations.     

The role of endorectal coil MRI in preoperative staging and decision-making for the treatment of clinically localized prostate cancer

Introduction  The optimal management of newly diagnosed prostate cancer requires individualization of the treatment plan based upon the most accurate clinical characterization of tumor location and extent of disease. The role of imaging in prostate cancer staging continues to evolve. In this review, we address the utility of endorectal coil magnetic resonance imaging (eMRI) in both local staging and its ability to facilitate the decision in choosing one treatment strategy over another after the initial diagnosis of localized prostate cancer. Materials and methods  Using the PubMed database and reference lists of key articles, we identified studies addressing the use of eMRI in tumor characterization and risk stratification in patients undergoing treatment for clinically localized prostate cancer. Results  The findings identified within 54 selected studies were incorporated into a summary discussing the current limitations in cancer staging and the role eMRI plays in both the preoperative assessment and clinical decision-making in an attempt to improve our ability to individualize management approaches and tailor treatment. Conclusion  eMRI allows for more accurate local staging by complementing the existing clinical variables through improvements in spatial characterization of the prostatic zonal anatomy and molecular changes. These improvements in tumor staging enhance our ability to individualize treatment selection and tailor the approach to maximize cancer control while minimizing treatment related morbidity.     

Quantification in magnetic resonance spectroscopy based on semi-parametric approaches

Magnetic resonance spectroscopy (MRS) is a value-added modality to magnetic resonance imaging (MRI) that is often used in diagnosis, treatment and progression monitoring, as well as in non-destructive, non-invasive studies of disease states in humans and model systems in animals. The availability of high magnetic field strengths and use of hyperpolarized nuclei, combined with the possibility of acquiring spectra at very short echo-time, have dramatically increased the potential of MRS. For the last two decades, a challenge has been to quantify short echo-time proton spectra that exhibit many metabolites, and to estimate their concentrations. Quantification of such spectra is challenging. Because the model function describing the acquired MRS signal is incomplete, semi-parametric techniques are required for estimation of the wanted metabolite concentrations. The semi-parametric approaches, QUEST, AQSES, TARQUIN, LCModel and SiToolsFITT, are reviewed and discussed according to handling of macromolecule signal and unknown decay of the metabolite signal (lineshape). Estimation of noise-related errors on model parameters and compromise used in real-world applications are detailed, with emphasis on the bias-variance trade-off. Applications of the semi-parametric methods QUEST and AQSES to quantification of MRS, HRMAS and MRSI data are also provided.     

Measurements of left ventricular dimensions using real-time acquisition in cardiac magnetic resonance imaging: comparison with conventional gradient echo imaging

This study investigates the use of real-time acquisition in cardiac magnetic resonance imaging (MRI) for measurements of left ventricular dimensions in comparison with conventional gradient echo acquisition. Thirty-one subjects with a variety of left ventricular morphologies to represent a typical clinical population were studied. Short-axis data sets of the left ventricle (LV) were acquired using a conventional turbo-gradient echo and an ultrafast hybrid gradient echo/echo planar sequence with acquisition in real-time. End-diastolic volume (EDV), end-systolic volume (ESV), ejection fraction (EF) and left ventricular mass (LV mass) were measured. The agreement between the two acquisitions and interobserver, intraobserver and interstudy variabilities were determined. The bias between the two methods was 5.86 ml for EDV, 0.23 ml for ESV and 0.94% for EF. LV mass measurements were significantly lower with the real-time method (mean bias 14.38 g). This is likely to be the result of lower spatial resolution and chemical shift artefacts with the real-time method. Interobserver, intraobserver and interstudy variabilities were low for all parameters. In conclusion, real time acquisition in MRI can provide accurate and reproducible measurements of LV dimensions in subjects with normal as well as abnormal LV morphologies, but LV mass measurements were lower than with conventional gradient echo imaging. Presented in abstract form at the International Society of Magnetic Resonance in Medicine meeting in Denver, Colorado in April 2000.     

Imaging birds in a bird cage: in-vivo FSE 3D MRI of bird brain

An in-vivo magnetic resonance imaging (MRI) procedure is described that allows one to obtain three-dimensional high quality images of the entire brain of small birds such as the canary (20 g) and the starling (75 g) with an image resolution of 0.1 mm (58-113 μm, dependent on the size of the imaged bird). The entire imaging procedure took about 2 h after which the birds recovered from anaesthesia uneventfully and could be reused for subsequent additional imaging. This non invasive MRI technique enables to correlate brain measures with behavioural or physiological data that are dynamic in nature and could permit significant progress for bird neurological research. © 1998 Elsevier Science B.V. All rights reserved.     

Long-term follow-up of patients with acute myocarditis by magnetic resonance imaging

Magnetic resonance imaging (MRI) reveals cardiac signal intensity changes in patients with acute myocarditis; however, the natural history of these changes and their relationship to individual outcomes are unknown. The relationship of MRI findings to long-term outcome was studied by serial MRI studies in 16 patients with acute myocarditis who were followed for 30±4 (SE) months. Myocardial contrast enhancement was monitored using contrast-enhanced T1-weighted fast spin-echo images. Left ventricular ejection fraction was measured with gradient-echo sequences. Clinical symptoms were scored. The results were compared to a control group of 26 age-matched, healthy volunteers. Myocardial contrast enhancement, which was markedly increased in the early course of the disease, decreased at 4 weeks and remained within the normal range in most patients after 30 months. Contrast enhancement 4 weeks after onset of symptoms was predictive for the functional and clinical long-term outcome. Contrast-enhanced MRI may be a useful, noninvasive tool for long-term follow-up of patients with acute myocarditis. Furthermore, relatively early MRI findings may predict longer-term outcomes. Electronic Publication     

A low‐power,area‐efficient multichannel receiver for micro MRI

A new integrated, low‐noise, low‐power, and area‐efficient multichannel receiver for magnetic resonance imaging (MRI) is described. The proposed receiver presents an alternative technique to overcome the use of multiple receiver front‐ends in parallel MRI. The receiver consists of three main stages: low‐noise pre‐amplifier, quadrature down‐converter, and a band pass filter (BPF). These components are used to receive the nuclear magnetic resonance signals from a 3 × 3 array of micro coils. These signals are combined using frequency domain multiplexing (FDM) method in the pre‐amplifier and BPF stages, then amplified and filtered to remove any out‐of‐band noise before providing it to an analog‐to‐digital converter at the low intermediate frequency stage. The receiver is designed using a 90 nm CMOS technology to operate at the main B₀ magnetic field of 9.4 T, which corresponds to 400 MHz. The receiver has an input referred noise voltage of 1.1 nV/√Hz, a total voltage gain of 87 dB, a power consumption of 69 mA from a 1 V supply voltage, and an area of 305 µm × 530 µm including the reference current and bias voltage circuits. Copyright © 2013 John Wiley & Sons, Ltd.     

Long-term sequestration of fluorinated compounds in tissues after fluvoxamine or fluoxetine treatment: a fluorine magnetic resonance spectroscopy study in vivo

Fluorine magnetic resonance spectroscopy (¹⁹F MRS), spectroscopic imaging (MRSI) and proton anatomical magnetic resonance imaging (¹H MRI) were performed on brains and lower extremities of six subjects in vivo concurrently with HPLC of serum to investigate tissue and plasma drug localization and withdrawal kinetics in humans treated with fluvoxamine or fluoxetine. ¹⁹F MRS signal was unexpectedly detected in the lower extremities months after complete disappearance of signal from plasma and brain. MRSI suggested that the lower extremity fluvoxamine signal originated mainly from bone marrow. Results suggest long-term sequestration of these drugs or their metabolites mainly in bone marrow and possibly in surrounding tissue and demonstrate the usefulness of MRS to reveal drug-trapping compartments in the body.