Mri diffusion
- 1. BY MAAJID MOHI UD DIN MALIK LECTURER COPMS, ADESH UNIVERSITY BATHINDA, PUNJAB
- 2. Diffusion-weighted imaging relies on the detection of the random microscopic motion of free water molecules known asBrownian movement. With the development of new magnetic resonance (MR) imaging technologies and stronger diffusion gradients, recent applications of diffusion-weighted imaging in whole- body imaging have attracted considerable attention, especially in the field of oncology. Diffusion-weighted imaging is being established asan important aspect of MR imaging in the evaluation of specific organs, including the breast, liver, kidney, and those in the pelvis.
- 3. When used in conjunction with apparent diffusion coefficient mapping, diffusion-weighted imaging provides information about the functional environment of water in tissues, thereby augmenting the morphologic information provided by conventional MR imaging Detected changes include shifts of water from extracellular to intra- cellular spaces, restriction of cellular membrane permeability, increased cellular density, and disruption of cellular membrane depolarization. These findings are commonly associated with malignancies; therefore, diffusion-weighted imaging has many applications in oncologic imaging and can aid in tumor detection and characterization and in the prediction and assessment of response to therapy.
- 4. Diffusion-weighted imaging has been used to help detect early stroke and other neurologic diseases since the 1990s. Since that time, a growing number of studies have demonstrated the useful- ness of this method in both the detection and characterization of lesions more specifically, in the field of oncologic imaging. The application of a diffusion-weighted sequence in whole-body imaging has gained more popularity with new technical developments in magnetic resonance (MR) imaging, including multichannel coils, echo-planar imaging, and stronger gradients,
- 5. Leading to a reduction in the amount of time required for diffusion-weighted imaging to less than 1 minute. Therefore, these sequences can be added to the imaging protocol without significantly increasing overall acquisition time. Another benefit of diffusion-weighted imaging is its use of inherent tissue contrast; hence, no exogenous contrast material is required.
- 6. The aforementioned improvements and a growing body of research have led to the ever- increasing utilization of diffusion-weighted imaging for specific applications, including oncologic imaging of the liver prostate gland and breast as well as whole-body imaging.
- 7. To understand the concept of diffusion- weighted imaging, one must understand the principles of free versus restricted diffusion in the cellular microenvironment. Free water molecules are in constant random motion, known as Brownian motion, which is related to thermal kinetic energy. In contrast, the motion of water molecules within the cellular microenvironment is impeded or restricted by their interaction with cellular compartments, including the cell wall and intracellular organelles.
- 8. In other words, restriction in the diffusion of water molecules is directly proportional to the degree of cellularity of the tissue. This restricted diffusion is observed primarily in malignancies, hyper cellular metastases, and fibrosis, which contain a greater number of cells with intact cell walls than does healthy tissue . In contrast, in a microenvironment with fewer cells and a defective cell membrane (e g, the necrotic center of a large mass), water molecules are able to move freely (i.e, diffusion is less restricted)
- 9. Diffusion weighted imaging (DWI) is a method of signal contrast generation based on the differences in Brownian motion. DWI is a method to evaluate the molecular function and micro-architecture of the human body. DWI signal contrast can be quantified by apparent diffusion coefficient maps and it acts as a tool for treatment response evaluation and assessment of disease progression. Ability to detect and quantify the anisotropy of diffusion leads to a new pattern called diffusion tensor imaging (DTI).
- 10. DTI is a tool for assessment of the organs with highly organized fiber structure. DWI forms an integral part of modern state-of-art magnetic resonance imaging and is indispensable in neuroimaging and oncology. DWI is a field that has been undergoing rapid technical evolution and its applications are increasing every day. This review article provides insights in to the evolution of DWI as a new imaging pattern and provides a summary of current role of DWI in various disease processes.
- 11. b value. For a diffusion-weighted image, we can alter the amount of DWI weighting we want, i.e. what our diffusion restriction 'threshold' is. ... The degree of DWI weighting is referred to as the b-value (quantitatively, b ∝ q2 * Δ , where q is the gradient strength and Δ is the time between the two gradients).
- 12. The goal of all imaging procedures is generation of an image contrast with a good spatial resolution. Initial evolution of diagnostic imaging focused on tissue density function for signal contrast generation. In 1970s, the work of Lauterbur PC, Mansfield P and Ernst R, modern clinical MRI came into the field of medicine. MRI provided an excellent contrast resolution not only from tissue (proton) density, but also from tissue relaxation properties. After initial focus on T1 and T2 relaxation properties researchers explored other methods to generate contrast exploiting other properties of water molecules. Diffusion weighted imaging (DWI) was a result of such efforts by researchers like Stejskal, Tanner and Le Bihan.
- 13. In 1984, before MRI contrast became available, Denis Le Bihan, tried to differentiate liver tumors from angiomass. He hypothesized that a molecular diffusion measurement would result in low values for solid tumors, because of restriction of molecular movement. Based on the pioneering work of Stejskal and Tanner in the 1960s, he thought that diffusion encoding could be accomplished using specific magnetic gradient pulses. It was a challenging task to integrate the diffusion encoding gradients in to the conventional sequences and initial experience in the liver with a 0.5T scanner was very disappointing. Firstly diffusion MRI was a very slow method and it was very sensitive to motion artifacts due to respiration.
- 14. It was not until the availability of Echo-Planar Imaging (EPI) in the early 1990s, that DWI could become a reality in the field of clinical imaging. EPI based diffusion sequences were fast and solved the problems of motion artifacts. Early work by Moseley et al and Warach et al established DWI as a cornerstone for early detection of acute stroke. In a DWI sequence diffusion sensitization gradients are applied on either side of the 180° refocusing pulse. The parameter “b value” decides the diffusion weighting and is expressed in s/mm2. It is proportional to the square of the amplitude and duration of the gradient applied. Diffusion is qualitatively evaluated on trace images and quantitatively by the parameter called apparent diffusion coefficient (ADC). Tissues with restricted diffusion are bright on the trace image and hypointense on the ADC map.
- 15. Acute brain ischemia: Ever since its inception acute brain ischemia has been the most successful application of DWI . Diffusion MRI today is the imaging modality of choice for stroke patients. The use of DWI in combination with perfusion MRI, which outlines salvageable areas of ischemia and MR angiography, provides a useful guide for stroke management. The b-values up-to 1000 are used for standard neuroimaging application. Despite the historical success the interpretation of the molecular basis behind the diffusion restriction has been poorly understood. The relationship of diffusion restriction with the severity of the ischemia and the clinical outcome remains unresolved.
- 16. Acute infarct. Axial FLAIR image (A) shows geographic hyper intensity involving right parieto-occipital region and basal ganglia. Diffusion weighted imaging shows restricted diffusion with high signal image (B) and low signal intensity on apparent diffusion coefficient map (C).
- 17. Brain tumors White matter diseases Pediatric brain development and aging Oncological applications Head and neck malignancies Thoracic malignancies Breast cancer Hepatobiliary pancreatic cancers Bowel disorders Genito-urinary applications Peripheral nerve imaging
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