Chemical-shift imaging

The echo-planar shift mapping (EPSM) technique, a variant of echo-planar imaging (EPI), is evaluated with theoretical studies using computer simulations and with experimental studies using fluorine resonances in phantoms. In situations where the signal-to-noise ratio permits, it is shown that EPSM can produce in a few seconds chemical-shift spectra at all points in a specified spin-density image corresponding to a selected slice. Applications of this technique are likely to be for proton and fluorine chemical-shift measurements and rapid studies of magnetic field inhomogeneities produced by magnet non-alignment and field shifts around magnetic materials.     

Chemical-shift imaging using filtered back-projection algorithms

A method of performing chemical-shift imaging is described that avoids some of the disadvantages inherent in the techniques which use spin echos and phase encoding. A series of FIDs are recorded during the application of a series of static gradients and the spectral image is reconstructed using a conventional filtered back-projection algorithm.     

Chemical-shift imaging with large magnetic field inhomogeneity

A new echo-time-encoded chemical-shift imaging technique applicable to dual-peak spectroscopic imaging with large magnetic field inhomogeneity is proposed and studied. The basic concept and its applications to modest field homogeneity (approximately 3.0 ppm) as well as to relatively large field inhomogeneity (approximately 10.5 ppm) are discussed. Actual pulse sequences are given and some experimental results on human volunteers obtained with a 2.0-T KAIS NMR system are also presented.     

Chemical-shift magnetic resonance imaging of two-line spectra by gradient reversal

A method of chemical-shift imaging is described using the invariance of chemical shifts to changes in magnetic field gradients used for frequency encoding of position in imaging. This enables separation of the effects on the observed signal of chemical shift from the effects of different positions along the imaging gradient when the signal is observed with different gradients. A simple implementation for a two-line spectrum is presented using signals observed with normal and reversed imaging gradients. This is used to create "fat" and "water" images of the thigh.     

Chemical-shift MR imaging of acetic acid during percutaneous chemical ablation therapy: preliminary work

The purpose of this study was to test the hypothesis that chemical-shift magnetic resonance (MR) imaging may be used to map the distribution of acetic acid during percutaneous chemical ablation procedures. Chemical-shift MR imaging was performed with use of standard methods on a 1.5-T scanner. Phantom and ex-vivo data demonstrated focal increases in the observed signal in chemical-shift MR imaging that correlate well with the site of injection. Preliminary study in a patient with hepatoma revealed focal signal at the injection site. These preliminary results suggest that chemical-shift MR imaging may be used to visualize acetic acid distribution during percutaneous chemical ablation procedures.     

Cardiac hybrid imaging: state-of-the-art

The field of noninvasive cardiac imaging has experienced enormous advances including computerized tomography coronary angiography (CTCA). Invasive angiography remains the anatomic standard of reference but it is associated with a non-negligible peri-procedural morbidity and mortality which suggests confining its use to patients who will benefit from a revascularization procedure. Many factors that are beyond the simple quantification of diameter narrowing and therefore cannot be fully assessed with luminology will eventually determine whether or not a given lesion produces stress-induced ischemia. Myocardial perfusion scintigraphy by single photon emission computerized tomography (SPECT) is one of the most widely used and well established noninvasive tools for the diagnosis of ischemic heart disease. Although positron emission tomography (PET) offers a higher accuracy than SPECT its use is often limited to large centers. This article explains the great potential of cardiac hybrid imaging which allows a comprehensive evaluation of coronary artery disease as it combines both morphological and functional information by fusing either SPECT or PET with CTCA. SPECT/CT and PET/CT hybrid imaging can provide entirely noninvasively unique information which helps improving diagnostic assessment and risk stratification and also impacts decision making with regard to revascularization in patients with coronary artery disease. The University Hospital Zurich holds a research contract with GE Healthcare (Milwaukee, WI, USA).     

MR/PET: a new challenge in hybrid imaging

OBJECTIVE: This article provides a short overview of hybrid imaging and the potential advantages of combined MR/PET. We will address some of the challenges that had to be met before MR/PET could become clinically available as well as further scientific work that has to be done to increase the potential benefit of this emerging hybrid modality. CONCLUSION: Hybrid imaging, the combination of two imaging modalities into one, promises the compensation of specific deficits of the modalities involved. PET/CT has gained wide acceptance for oncologic imaging in recent years; however, MRI has certain advantages that could make combined MR/PET more tempting in various clinical applications. The development of new clinical whole-body MR/PET systems offers new insights in metabolic and functional processes in oncology as well as cardiovascular and neurologic diseases.     

Chemical-shift MR imaging of the femoral head: an in vitro study of normal hips and hips with avascular necrosis

To study the conventional and chemical-shift MR images of fatty, hemopoietic, and ischemic bone marrow, four normal femurs from autopsies and six femoral heads from total hip replacement done for avascular necrosis (AVN) were imaged with a small-bore 1.4-T MR unit. T1- and T2-weighted spin-echo images, fat- and water-selective spin-echo images, and out-of-phase images were obtained. The specimens were then sectioned, radiographed, and studied histologically. The capital epiphysis and greater trochanter in normal femurs had the highest signal on T1-weighted, fat-selective, and out-of-phase images owing to their high fat content. Fat-selective and water-selective images accurately depicted regions of fat and water, whereas T1-weighted, T2-weighted, and out-of-phase images were misleading in some cases. On T2-weighted images, a double line consisting of a rim of low signal with a high-signal inner border was noted surrounding AVN lesions. Histologic correlation showed that the low-signal outer rim was due to sclerotic bone, while the high-intensity inner border was due to granulation tissue and/or chondroid metaplasia. Fractures within the six AVN lesions were best seen on T2-weighted images, but these could not be distinguished from granulation tissue. On T2-weighted images, the weight-bearing cartilage had lower signal in five of six femoral heads with AVN. We conclude that the MR appearance of AVN reflects its complex histopathology. Direct spin-echo chemical-shift imaging can help correlate MR images with histology by providing information not available from conventional or out-of-phase images.     

Multiplanar imaging of the hip: a systematic approach

Computed tomography has become an invaluable adjunct for the evaluation of musculoskeletal pathology. Although the transaxial image presentation has distinct advantages over conventional radiography, the ability to reformat the acquired data into coronal, sagittal, or oblique planes contributes to a more thorough understanding of complex anatomy such as that of the hip joint. This article reviews our experience with a series of over 500 hip cases using the multiplanar reformatting and display [MPR/D] package developed for use on the Siemens Somatom DR-3 scanner.     

MR imaging of the pancreas: a practical approach

This article reviews the current practical MRI techniques in assessment of the pancreas. With the comprehensive "one-stop-shopping" approach, the great majority of pancreatic diseases can be detected and characterized by the use of a combination of T1, T2-weighted, MRCP, and fat-suppressed T1-weighted dynamic post-gadolinium SGE sequences. This approach may provide the clinician with information regarding the site, nature, and staging of pancreatic tumor in a single setting. In many institutions worldwide, however, including our own, CT remains the main imaging method for the assessment of acute pancreatic diseases, due largely to its wide availability. MR imaging is reserved for the indications listed above, most importantly, the detection of small and non-organ-deforming pancreatic ductal adenocarcinoma, islet cell tumors, choledocholithiasis and pancreatic duct calculi, cholangiocarcinomas, and in cases of pancreatic head enlargement with no mass discernable on CT.     

Truly hybrid X-ray/MR imaging: toward a streamlined clinical system

RATIONALE AND OBJECTIVE: We have installed an improved X-ray/MR (XMR) truly hybrid system with higher imaging signal-to-noise ratio (SNR) and versatility than our first prototype. In our XMR design, a fixed anode X-ray fluoroscopy system is positioned between the two donut-shaped magnetic poles of a 0.5T GE Signa-SP magnet (SP-XMR). This paper describes the methods for increased compatibility between the upgraded x-ray and MR systems that have helped improve patient management. MATERIALS AND METHODS: A GE OEC 9800 system (GE OEC Salt Lake City, UT) was specially reconfigured for permitting X-ray fluoroscopy inside the interventional magnet. A higher power X-ray tube, a new permanent tube mounting system, automatic exposure control (AEC), remote controlled collimators, choice of multiple frame rates, DICOM image compatibility, magnetically shimmed X-ray detector, X-ray compatible MR coil, and better RF shielding are the highlights of the new system. A total of 23 clinical procedures have been conducted with SP-XMR guidance of which five were performed using the new system. RESULTS: The 70% increased power for fluoroscopy, and a new 6 times higher power single frame imaging mode, has improved imaging capability. The choice of multiple imaging frame rates, AEC, and collimator control allow reduction in X-ray exposure to the patient. The DICOM formatting has permitted easy transfer of clinical images over the hospital PACS network. The increased MR compatibility of the detector and the X-ray transparent MR coil has enabled faster switching between X-ray and MR imaging modes. CONCLUSION: The improvements introduced in our SP-XMR system have further streamlined X-ray/MR hybrid imaging. Additional clinical procedures could benefit from the new SP-XMR imaging.     

Urinary tract infections in childhood: a current imaging approach

The authors present their approach to the evaluation of urinary tract infection in childhood based on real time sonography, 99m-Tc-DTPA scintigraphy and voiding cystourethography.     

Chemical-shift MRI of pulmonary hamartomas: initial experience using a modified technique to assess nodule fat

OBJECTIVE: The aim of this study was to show the usefulness of chemical-shift MRI in the diagnosis of intranodular fat in seven patients with pulmonary hamartomas and indeterminate CT findings. CONCLUSION: In the setting of chemical-shift MRI, the average nodule signal intensity index of pulmonary hamartomas was 45.3% (SD = 25.5%). The correlation between average nodule signal intensity and CT attenuation in Hounsfield units was -0.94. Chemical-shift MRI could be an important tool for the detection of fat in pulmonary hamartomas with inconclusive CT findings.     

MRI化学位移同、反相位成像的体外实验模型研究

目的 通过建立同、反相位的体外实验模型，探讨与同相位相比，反相位上信号强度变化与脂肪含量的相关性，并从中确定反相位上信号强度明显下降时的确切脂肪含量的标准。材料与方法模型建立方法：将1％琼脂糖水溶液与色拉油先后倒入一长方形水槽内，在水-脂交界处采用层厚10mm倾斜面进行同、反相位检查。测出同、反相位图像各部感兴趣区(ROI)的信号强度(SI)，计算信号强度下降指数(SI指数)值，并且绘制脂肪含量与SI指数关系的曲线图。结果 最初脂肪比例较低时，随脂肪含量增加，反相位的信号强度呈逐渐下降表现；而当脂肪比例较高时，其信号强度随着脂肪含量的增加而升高，其反转点的脂肪含量为23％。结论 脂肪含量在23％左右时，反相位上的信号强度有较为明显的下降，而且脂肪含量愈接近该值，信号强度下降的程度就愈明显。     

Crohn's disease: current imaging approach

Purpose: This paper discusses the role of different imaging modalities in the diagnostic work-up of Crohn's disease (CD). Methods: We present a concept, which emphasizes different diagnostic aspects with regard to primary diagnosis, follow-up and assessment of complications of CD. The most effective imaging approach to various diagnostic problems of CD is discussed in detail. Discussion: With regard to the primary diagnosis barium studies should contribute to differentiate between CD and ulcerative colitis. Beyond that, these studies should evaluate location and extent of disease. During the follow-up bowel sonography provides staging of disease and enables the detection of complications at an early stage. CT is a valuable tool in the preoperative assessment of complications, such as fistulae and abscesses.