RAD 417 LEC.4.pdf

DIFFUSION AND PERFUSION CASE AND CLINICAL STUDIES By: Haya Alshegri Diffusion DWI : Diffusion Weighted Imaging Target: Study of the microscopic motions of extracellular water molecules. Basic Principles: Diffusion MRI measures the mobility of the water protons and thus pro...

DIFFUSION AND PERFUSION CASE AND CLINICAL STUDIES By: Haya Alshegri Diffusion DWI : Diffusion Weighted Imaging Target: Study of the microscopic motions of extracellular water molecules. Basic Principles: Diffusion MRI measures the mobility of the water protons and thus provides a window to tissue microstructure. Diffusion relies on the incoherent thermal motion of water molecules called Brownain motion. B-value bValue = is a factor that reflects the strength and timing of the gradients used to generate diffusion-weighted images, expressed in sec/mm2. The b-value depends on the strength, duration, and spacing of these pulsed gradients. A larger b-value is achieved with increasing the gradient amplitude and duration and by widening the interval between gradient pulses.  B-value General rule : an increase of the b value increases the diffusion weighted ( the signal attenuation) b 0 = immediate b 1000 = more b 3000 = very high imaging, will result in diffusion weighted diffusion weighted T2 imaging. images. images, lower SNR ADC map An apparent diffusion coefficient (ACD) map can be calculated by utilizing at least two diffusion weighted images that are differently sensitized to diffusion but remain identical with respect to the other parameters. ADC map : show the strength of diffusion Have reversed contrast Eliminates the T2 contrast in DWI imaging Eliminate common artifacts ADC map High ADC Value Areas: In regions with high water diffusion, such as fluids or areas with fewer cellular structures, ADC values are high. On the ADC map, these areas typically appear brighter or lighter in color. For example, cerebrospinal fluid- filled spaces in the brain or cystic structures might exhibit high ADC values, appearing bright on the map. Low ADC Value Areas: Tissues with restricted water diffusion, often due to increased cellularity or compactness, have low ADC values. On the ADC map, these areas appear darker. Strokes, tumors, areas of inflammation, and regions with cellular proliferation might have lower ADC values, reflecting reduced water mobility.  T2 Shine through Refers to high signal in DWI that is not due to restricted diffusion, but rather to high T2 signal which is ‘Shine Though’ to the DWI images. T2 Shine Through occur because of long T2 decay time in some normal tissue.  advanced MRI techniques (DWI) RESOLVE DWI : is an advanced MRI technique developed to obtain high- resolution diffusion-weighted images with reduced susceptibility-related artifacts. (offer clearer images, especially in anatomical regions that are prone to artifacts.) Susceptibility artifact is a common finding in DWI  advanced MRI techniques (DWI) ZOOMit DWI: is an advanced MRI technique designed to provide high-resolution, diffusion- weighted images of specific regions of interest. Unlike traditional DWI, which captures larger anatomical areas and can suffer from susceptibility artifacts, ZOOMit DWI narrows the field of view (FOV) to target and magnify particular regions, ensuring clearer images with enhanced detail and reduced artifacts. DWI Clinical application Very useful for : whole-body cancer screening brain imaging prostate imaging gynecology imaging breast imaging anterior neck imaging small bowel imaging liver imaging spine imaging sacroiliac joint imaging DWI Cases : Differentiation Between Chronic, Acute and subacute Infarcts DWI and brain tumours DWI Liver low ADC indicates active lesion Prostate DWI Bone metastasis Whole body DWI Lymphoma screening Metastases screening Treatment monitoring and follow-up used in- replacement to PET. MR PERFUSION WEIGHTED IMAGING (PWI) PWI Perfusion normally refers to the delivery of blood at the level of the capillaries, and measures in units of milliliters per 100 gram per minute. Perfusion is closely related to the delivery of oxygen and other nutrients to the tissue. Clinical applications of PWI Stroke Brain tumors Dementia Psychiatric illnesses Trauma Epilepsy Multiple sclerosis Approaches to PWI Dynamic susceptibility contrast (DSC) Dynamic contrast enhanced imaging(DCE-MRI) Arterial spin labelling (ASL). Dynamic susceptibility contrast (DSC) Dynamic susceptibility contrast (DSC) MR perfusion is one of the most frequently used techniques for MRI perfusion and relies on the susceptibility induced signal loss on T2*-weighted sequences which results from a bolus of gadolinium-based contrast passing through a capillary bed. Technique : ✓ A dose of 0.2mm/kg of Gd injected IV ✓ Using power injector at rate of 5ml/s ✓ Fast T2*-w EPI seq is run to catch first pass of the contrast through microcirculation ✓ About 60 such run acquired before, during and after dynamic injection of the contrast media  Dynamic susceptibility contrast (DSC) Raw data images converted into various color-coded maps using software CBV=Cerebral Blood Volume in tissue, Darker areas in rCBV map represent regions of lower blood volume. CBF= Cerebral blood flow in tissue, Require measurement of SI within an artery supplying the tissue of interest. TTP=Time to Peak, Measures the delay between the arrival of Contrast bolus & the peak of the concentration curve. MTT =Mean Transit Time, Depicts time required for fresh blood to completely replace the blood within the volume of interest, Darker areas on the MTT map represent tissues having faster enhancement rates. DCE-MRI perfusion DCE-MRI uses rapid and repeated T1-weighted images to measure the signal changes induced by the paramagnetic tracer in the tissue as a function of time. In this method, the contrast agent is also intravenously injected to generate bolus. Extracellular contrast media diffuse from the blood into the extravascular of extracellular space of tissue at a rate determined by tissue perfusion and permeability of the capillaries and their surface area. Arterial spin labeling (ASL) MR perfusion ASL does not require intravenous administration of contrast (unlike DSC perfusion and DCE perfusion). Instead, it exploits the ability of MRI to magnetically label arterial blood below the imaging slab. The parameter most derived is cerebral blood flow (CBF). It is a non-invasive and non-ionizing MRI technique that measures tissue perfusion (blood flow), by using magnetically-labeled arterial blood water protons as an endogenous tracer. ASL is a very suitable technique to use in pediatrics, in which the use of radioactive tracers may be restricted. It is also safe to use in patients with impaired renal function and those who may need serial follow-up. CLINICAL CASES OF PWI PWI in ACUTE ISCHAEMIC STROKE Ischaemic stroke – leads to a perfusion deficits. Depiction of these deficits useful in diagnosis & treatment planning. PWI used in stroke patients to predict which tissue is irreversible & which tissue is viable but at risk of infarction. Stroke pt. The baseline T2 image DWI(b0) (A) illustrates anatomic features. The DWI (b1000) hyperintensity (B, arrow) shows the acute infarct. Quantitative perfusion maps of cerebral blood flow (CBF, C) mean transit time (MTT, D), cerebral blood volume (CBV, E) show perfusion deficits. MTT is increased in the right hemisphere (outlined) while CBV and CBF are reduced. PWI of Brain Tumours To predict brain tumour type & grade, patient survival & optimal therapeutic options. To differentiate post treatment radiation necrosis from recurrent tumor. PWI exploits tumor vasculature characteristics. Blood vessels present in higher numbers in tumors than in normal brain tissue. High-grade tumours have higher blood volume. High CBV & CBF in tumour compared to normal brain parenchyma. Hight in MTT. PWI can differentiate tumor from Radiation necrosis : Hypo-perfused, with low values of CBV, CBF & MTT. Images of patient with new onset of seizure and history of colon carcinoma. -T2 ,and T1-weighted contrast-enhanced MR, perfusion-weighted MR image (rCBV), and ADC image. -T2-WI shows cortical involvement of tumor in anterior medial part -ADC values in more solid anteromedial parts are lower than in normal brain tissue, indicating higher cellularity. -Perfusion-weighted image demonstrates rCBV elevation indicative of high-grade glioma.  There is ring enhancing mass surrounding the posterior horn of the left lateral ventricle. This is matched with an area of low relative cerebral blood volume indicating that this is more likely to be radiation necrosis than tumor recurrence. THANK YOU