Diffusion and perfusion imaging of the liver

MRI of the liver is an important tool for the detection and characterization of focal liver lesions, for assessment of tumor response to treatment, and for the evaluation of diffuse liver disease. With recent advances in technology, functional MRI methods such as diffusion-weighted (DW) and perfusion-weighted (PW)-MRI are increasingly used in the abdomen with promising results, particularly in the evaluation of diffuse and focal liver diseases. In this review, we will discuss background, technical considerations, acquisition, applications, limitations and future applications of DW-MRI and PW-MRI applied in evaluation of diffuse and focal liver diseases.     

Diffusion and perfusion of the kidney

MRI of the kidney currently makes the transition from depiction of morphology to assessment of function. Functional renal imaging methods provide information on diffusion and perfusion on a microstructural level. This review article presents the current status of functional renal imaging with focus on DWI (diffusion-weighted imaging) and DCE-MRI (dynamic contrast-enhanced MRI), as well as BOLD (blood-oxygenation level dependent) MRI, DTI (diffusion tensor imaging) and arterial spin labeling (ASL). Technical background of these techniques is explained and clinical assessment of renal function, parenchymal disease, transplant function and solid masses is discussed.     

Diffusion and perfusion of the breast

This article is a review of the current published clinical applications of DWI and perfusion of breast MR explaining possibilities and limits of both techniques.DWI in a fast time acquisition and without contrast medium gives information as regards cellularity of breast lesions. The technique can be used for distinguishing between benign and malignant breast lesions and monitoring therapies in locally advanced breast cancer.Perfusion can give additional information as regards vascularization of breast lesions, useful in the characterization of breast lesions doubt at DCE-MRI and also in monitoring chemotherapic effect.     

Improved visualization of delayed perfusion in lung MRI

Introduction

The investigation of pulmonary perfusion by three-dimensional (3D) dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) was proposed recently. Subtraction images are generated for clinical evaluation, but temporal information is lost and perfusion defects might therefore be masked in this process. The aim of this study is to demonstrate a simple analysis strategy and classification for 3D-DCE-MRI perfusion datasets in the lung without omitting the temporal information.

Materials and methods

Pulmonary perfusion measurements were performed in patients with different lung diseases using a 1.5 T MR-scanner with a time-resolved 3D-GRE pulse sequence. 25 3D-volumes were acquired after iv-injection of 0.1 mmol/kg KG Gadolinium-DTPA. Three parameters were determined for each pixel: (1) peak enhancement S_n,max normalized to the arterial input function to detect regions of reduced perfusion; (2) time between arterial peak enhancement in the large pulmonary artery and tissue peak enhancement τ to visualize regions with delayed bolus onset; and (3) ratio R = S_n,max/τ was calculated to visualize impaired perfusion, irrespectively of whether related to reduced or delayed perfusion.

Results

A manual selection of peak perfusion images is not required. Five different types of perfusion can be found: (1) normal perfusion; (2) delayed non-reduced perfusion; (3) reduced non-delayed perfusion; (4) reduced and delayed perfusion; and (5) no perfusion. Types II and IV could not be seen in subtraction images since the temporal information is necessary for this purpose.

Conclusions

The analysis strategy in this study allows for a simple and observer-independent visualization and classification of impaired perfusion in dynamic contrast-enhanced pulmonary perfusion MRI by using the temporal information of the datasets.     

大鼠超急性脑梗死弥散-灌注磁共振成像的实验研究

目的：应用弥散－灌注磁共振成像技术对改良线栓法制作的鼠大脑中动脉阻塞超急性脑梗死进行实验研究。材料与方法：34只SD大鼠，随机分成5组，其中2只为假手术A组，其余按栓塞时间30分钟，13，6小时分成B，C，D，E4组，A组于2，24小时后，B，C，D＜E组分别于栓塞后30分钟，1，3，6小时以及线栓拔出后2，24小时行弥散和灌注成像扫描。结果：假手术A组弥散，灌注成像无异常信号，实验B，C，D，E组的均见灌注异常信号且范围无变化，弥散高信号区随时间延长而范围增大，6小时后与灌注异常信号区一致。栓塞2小时后T2WI显示高信号。结论：弥散磁共振成像可信号超急性脑梗死灶大小及其演变规律，灌注成像显示脑梗死缺血范围。     

3He diffusion MRI of the lung

RATIONALE AND OBJECTIVES: MR imaging of the restricted diffusion of laser-polarized 3He gas provides unique insights into the changes in lung microstructure in emphysema. RESULTS: We discuss measurements of ventilation (spin density), mean diffusivity, and the anisotropy of diffusion, which yields the mean acinar airway radius. In addition, the use of spatially modulated longitudinal magnetization allows diffusion to be measured over longer distances and times, with sensitivity to collateral ventilation paths. Early results are also presented for spin density and diffusivity maps made with a perfluorinated inert gas, C3F8. METHODS: Techniques for purging and imaging excised lungs are discussed.     

Characterization of necrotic meningioma using diffusion MRI,perfusion MRI,and MR spectroscopy: case report and review of the literature

Necrotic meningiomas are relatively rare, accounting for 1.3–3.9% of primary intracranial tumours and for 10–15% of meningiomas, but are of special clinical importance as they may resemble metastases or malignant gliomas. We report the magnetic resonance findings of diffusion-weighted imaging, perfusion-weighted imaging, and spectroscopy in a patient with a necrotic meningioma, in whom clinical symptoms and signs suggested a central nervous system infection.     

Liver MRI: From basic protocol to advanced techniques

Liver MR is a well-established modality with multiparametric capabilities. However, to take advantage of its full capacity, it is mandatory to master the technique and optimize imaging protocols, apply advanced imaging concepts and understand the use of different contrast media. Physiologic artefacts although inherent to upper abdominal studies can be minimized using triggering techniques and new strategies for motion control. For standardization, the liver MR protocol should include motion-resistant T2-w sequences, in-op phase GRE T1 and T2-w fast spin echo sequences with fat suppression. Diffusion-weighted imaging (DWI) is mandatory, especially for detection of sub-centimetre metastases. Contrast-enhanced MR is the cornerstone of liver MR, especially for lesion characterization. Although extracellular agents are the most extensively used contrast agents, hepatobiliary contrast media can provide an extra-layer of functional diagnostic information adding to the diagnostic value of liver MR. The use of high field strength (3T) increases SNR but is more challenging especially concerning artefact control. Quantitative MR belongs to the new and evolving field of radiomics where the use of emerging biomarkers such as perfusion or DWI can derive new information regarding disease detection, prognostication and evaluation of tumour response. This information can overcome some of the limitations of current tests, especially when using vascular disruptive agents for oncologic treatment assessment. MR is, today, a robust, mature, multiparametric imaging modality where clinical applications have greatly expanded from morphology to advanced imaging. This new concept should be acknowledged by all those involved in producing high quality, high-end liver MR studies.     

磁共振扩散及灌注成像技术在新生儿急性缺氧缺血性脑病中的应用

缺氧缺血性脑病在新生儿中发病率较高,其致残及致死率居高不下,MRI已成为诊断HIE及评估预后的主要影像检查方法。近年来,一些新型MRI技术,例如扩散加权成像、扩散张量成像、扩散峰度成像、动脉自旋标记成像、体素内不相干运动等在扩散及灌注成像方面有了显著发展,能够更好地反映HIE的病理改变。就MRI扩散及灌注成像新技术在HIE中的成像特点及应用进行综述。     

Magnetic resonance: perfusion and diffusion imaging

Summary The use of magnetic resonance imaging to detect normal and pathological problems of perfusion and diffusion is reviewed. Motion sensitised spin-echo images can be used to detect changes in slow flow velocity within a voxel (intravoxel coherent motion (IVCM) as well as intravoxel incoherent motion (IVIM) effects attributable to both diffusion and perfusion. Changes have been identified in a variety of brain diseases in the absence of changes in conventional images but the techniques are very vulnerable to motion artefact of all types. More rapid and more sensitive approaches using steady state free precision and echo-planer imaging are being investigated. Anisotropic diffusion imaging enables white matter tracts to be demonstrated within the brain and spinal cord as a function of their direction because diffusion of water across axons is much more restricted than it is along them. This technique provides a unique method for localisation of lesions and displays obvious changes in disease in which diffusion becomes less restricted.     

Thorax perfusion CT in non-small cell lung cancer

OBJECTIVES: We aimed to determine the perfusion differences according to the histological type, stage, volume and prognoses in the non-small cell carcinoma by thorax perfusion CT. MATERIALS AND METHODS: Twenty-four non-small cell carcinoma patients were included in the study. Thorax perfusion CT was done to evaluate the tumors in terms of perfusion parameters: blood flow (BF) and time to peak (TTP) values. RESULTS: The total blood flow of the tumor in squamous cell carcinoma was significantly higher than adenocarcinoma (p=0.031). There was no statistical difference between the perfusion parameters and other parameters. CONCLUSIONS: Perfusion CT may help us in evaluating non-small cell carcinomas.     

Tumor perfusion assessed by dynamic contrast-enhanced MRI correlates to the grading of renal cell carcinoma: Initial results

In this study, we investigated whether assessment of the tumor perfusion by dynamic contrast-enhanced magnetic resonance imaging (DCE MRI) enables to estimate the morphologic grading of renal cell carcinomas.A total of 21 patients with suspected renal cell cancer were examined using a Gadobutrol-enhanced, dynamic saturation-recovery, turbo-fast, low-angle shot sequence. Tumor perfusion and the tissue-blood ratio within the entire tumor and the most highly vascularized part of the tumor were calculated according to the model of Miles. Immediately after examination, patients underwent surgery, and the results from imaging were compared with the morphological analysis of the histologic grading.Fourteen patients had G2 tumors, and seven patients had G3 tumors. Significantly higher perfusion values (p < 0.05) were obtained in G3 tumors than in G2 tumors when the entire tumor area was considered (1.59 ± 0.44 (ml/g/min) vs. 1.08 ± 0.38 (ml/g/min)) or its most highly vascularized part (2.14 ± 0.89 (ml/g/min) vs. 1.40 ± 0.49 (ml/g/min)). By contrast, the tissue-blood ratios did not differ significantly between the two groups.In conclusion, unlike tissue-blood ratio, surrogate parameters of the tumor perfusion determined by DCE MRI seem to allow an estimation of the grading of renal cell carcinoma. However, further studies with high case numbers and including patients with G1 tumors are required to evaluate the full potential and clinical impact.     

The role of dynamic, contrast-enhanced MRI in differentiating lung tumor subtypes

The aim was to correlate dynamic magnetic resonance imaging perfusion parameters of pulmonary tumors with histological tumor classification. Eighty-six patients with lung cancer were examined. A differentiation of non-small cell lung cancer vs. small cell lung cancer was possible with the parameters tumor necrosis, maximum contrast upslope, and the time until the maximum contrast upslope was reached. The beginning of a relevant contrast uptake, the mean time to peak and the time until the maximum contrast upslope was reached allowed a differentiation between squamous cell carcinoma and adenocarcinoma.     

Alterations in diffusion and perfusion in the pathogenesis of peritumoral brain edema in meningiomas

Magnetic resonance perfusion and diffusion studies were undertaken to clarify the significance of ischemia in the pathogenesis of peritumoral brain edema in patients with meningiomas. Included in this study were 26 patients with 27 meningiomas and 5 gliomas. Perfusion-weighted imaging (PWI) was performed using a gradient-echo, echo-planar-imaging (EPI) sequence for calculation of the relative regional cerebral blood volume (rrCBV) and the relative regional cerebral blood flow index (rrCBFi). Furthermore, multi-slice spin-echo EPI sequences were applied in order to obtain anisotropic and isotropic diffusion-weighted imaging (DWI). Apparent diffusion coefficient (ADC) values were then calculated for peritumoral brain parenchyma from tumors, with and without edema, using various diffusion sensitivities. Meningiomas without edema demonstrated a minimal increase of perfusion parameters in the peritumoral brain tissue. In contrast, cases with brain edema had highly significant ( p<0.0005) lower rrCBV and rrCBFi. The edema index (EI) correlated strongly with the rrCBV. A fitting procedure resulted into the following mathematical relation: EI=0.1/rrCBV(2). The DWI showed a significantly larger ADC value within areas of brain edema, compared with the normal white matter (0.74 x 10(-3) vs 1.55 x 10(-3) mm(2)/s; p<0.0001). Increases in EI correlated with increases in ADC values. In 31% of the meningiomas associated with edema, areas with increased signal, probable ischemia, demonstrated significantly lower ADC values, in comparison with the rest of the edematous areas. These areas were confined to tissue immediately adjacent to the tumor. In general, the decrease in rrCBV in brain edema represents a consequence from, rather than a cause of, vasogenic edema. Ischemic alterations can be regarded as secondary, facultative phenomena in the pathogenesis of meningioma-related brain edema.     

Visualizing water clearance in the lung with MRI

Current indirect measurements of alveolar fluid clearance (AFC) suggest that the rate of fluid clearance correlates with morbidity and mortality in patients with pulmonary edema. In a traditional AFC-measurement, fluid laced with a tracer macromolecule is instilled into the lung and thereafter repeated samples of the instilled fluid are extracted from the lung's fluid-filled airspaces. The change in concentration of the tracer molecule indicates the AFC-rate. In this work, a new MRI technique was developed to image lung water clearance by adding Gadolinium-DTPA to the instilled fluid. As fluid is absorbed by the animal, the concentration of gadolinium will increase, reducing the T(1) relaxation time. By repeatedly measuring the T(1) relaxation time, the AFC can be tracked over time with high spatial resolution. The new technique was tested both in phantoms and 10 Yorkshire piglets.     

多层螺旋CT灌注成像在不典型周围型肺癌诊断中的应用

目的评价多层螺旋CT灌注成像在不典型周围型肺癌诊断中的应用。方法回顾性分析12例不典型周围型肺癌利用多层螺旋CT灌注成像的影像表现。结果12例周围型肺癌表现不典型,主要有3种情况:a)周围型肺癌合并肺结核见于4例;b)直径小于3cm非实质性结节灶见于6例;c)单发小片状病灶见于2例。结论周围型肺癌可出现多种不典型征象,认识这些征象对临床诊断及治疗都有重要意义。     

联合MRI常规与弥散成像判断多发性硬化斑块的病理基础

目的：研究MRI常规与弥散成像（DWI，DTI）联合应用，在判定多发性硬化斑块病理基础中的价值。材料和方法：对14例脑部多发性硬化病例进行联合MRI常规及弥散加权（DWI）、弥散张量成像（DTI）。其中1例为继发进展型（SP型），13例为好转－复发型（RR型）。首次发作期（急性斑块）成像者5例，缓解期（慢性斑块）成像者9例。分析常规MRI T1WI、T2WI及DWI、DTI成像后所获得的ADC图、FA图上的信号改变，量化分析DTI成像中的平均D值及各向异性指数AI值。结果：急性与慢性MS斑块MRI表现明显不同。急性MS斑块较大，斑块内信号不均匀，大致可以分为两部分：中心为数毫米至2cm不等的圆形或卵圆形异常信号影，呈T1WI低、T2WI高信号，推测病理为脱髓鞘或轴索丢失。周围为片状不规则形T1WI略低、T2WI略高信号影，考虑为水肿。发作MS斑块DWI可呈现高信号，e指数ADC图亦可呈现高信号，但范围后者明显小于前者。斑块中心平均D值升高、AI值明显下降，周围ADC值亦升高，但不如前者明显，AI值无明显改变，支持常规MRI关于斑块病理的推测。慢性MS斑块较小，多呈小片状或宽条状分布于侧脑室旁，T1WI呈略低信号、T2WI高信号。DWI呈近等信号、e指数ADC图呈等或略低信号。斑块平均D值不同程度升高，AI值下降，提示为脱髓鞘或轴索丢失，而无水肿存在。结论：联合MRI常规与弥散成像可以判断多发性硬化斑块的病理，从而进一步加深了对MS斑块病理的认识。     

Non‐contrast‐enhanced perfusion and ventilation assessment of the human lung by means of fourier decomposition in proton MRI

Assessment of regional lung perfusion and ventilation has significant clinical value for the diagnosis and follow‐up of pulmonary diseases. In this work a new method of non‐contrast‐enhanced functional lung MRI (not dependent on intravenous or inhalative contrast agents) is proposed. A two‐dimensional (2D) true fast imaging with steady precession (TrueFISP) pulse sequence (TR/TE = 1.9 ms/0.8 ms, acquisition time [TA] = 112 ms/image) was implemented on a 1.5T whole‐body MR scanner. The imaging protocol comprised sets of 198 lung images acquired with an imaging rate of 3.33 images/s in coronal and sagittal view. No electrocardiogram (ECG) or respiratory triggering was used. A nonrigid image registration algorithm was applied to compensate for respiratory motion. Rapid data acquisition allowed observing intensity changes in corresponding lung areas with respect to the cardiac and respiratory frequencies. After a Fourier analysis along the time domain, two spectral lines corresponding to both frequencies were used to calculate the perfusion‐ and ventilation‐weighted images. The described method was applied in preliminary studies on volunteers and patients showing clinical relevance to obtain non‐contrast‐enhanced perfusion and ventilation data. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Probing lung microstructure with hyperpolarized noble gas diffusion MRI: theoretical models and experimental results

The introduction of hyperpolarized gases (³He and ¹²⁹Xe) has opened the door to applications for which gaseous agents are uniquely suited—lung MRI. One of the pulmonary applications, diffusion MRI, relies on measuring Brownian motion of inhaled hyperpolarized gas atoms diffusing in lung airspaces. In this article we provide an overview of the theoretical ideas behind hyperpolarized gas diffusion MRI and the results obtained over the decade‐long research. We describe a simple technique based on measuring gas apparent diffusion coefficient (ADC) and an advanced technique, in vivo lung morphometry, that quantifies lung microstructure both in terms of Weibel parameters (acinar airways radii and alveolar depth) and standard metrics (mean linear intercept, surface‐to‐volume ratio, and alveolar density) that are widely used by lung researchers but were previously available only from invasive lung biopsy. This technique has the ability to provide unique three‐dimensional tomographic information on lung microstructure from a less than 15 s MRI scan with results that are in good agreement with direct histological measurements. These safe and sensitive diffusion measurements improve our understanding of lung structure and functioning in health and disease, providing a platform for monitoring the efficacy of therapeutic interventions in clinical trials. Magn Reson Med 71:486–505, 2014. © 2013 Wiley Periodicals, Inc.