MRI Techniques and Applications PDF

MRI INTRODUCTION MRI basic principles : Key notes: Inside the MRI scanner there is a powerful magnetic field. The magnetic field interacts with protons in the body. The scanner produces radiofrequency pulses which further interact with protons in the body. Protons give off 'signal' whi...

MRI INTRODUCTION MRI basic principles : Key notes: Inside the MRI scanner there is a powerful magnetic field. The magnetic field interacts with protons in the body. The scanner produces radiofrequency pulses which further interact with protons in the body. Protons give off 'signal' which is detected by the scannerThe patient lies on the scanner couch which slides into the bore of the scanner. Within the bore of the scanner there is a powerful magnetic field The scanner produces radiofrequency pulses to ‘excite’ protons in the body As the excited protons in the body ‘relax’ after each pulse, they give off radiofrequency ‘signal’ which is detected by the receiver. The receiver is placed around or near the body part being imaged. To produce 'signal', the MRI scanner interacts with protons in the body. →Randomly orientated protons become aligned with the powerful magnetic field in the bore of the scanner. →A rapidly repeating sequence of radiofrequency pulses – produced by the scanner – then causes 'excitation' and 'resonance' of protons→ Milliseconds after removal of each radiofrequency pulse the excited protons 'relax', giving off radiofrequency signal which is detected by the scanner Two types of relaxation occur – 1. Realignment of protons with the magnetic field and - 2. Dephasing of spinning protons (loss of resonance) 22 →as they do so, they give off radiofrequency 'signal' which is detected by the scanner and transformed into an image. Two types of signals can be detecte MRI SIGNALS Two types of signals can be detected T1 signal relates to the speed of realignment with the magnetic field – the more quickly the protons realign the greater the T1 signal T2 signal relates to the speed of proton spin dephasing – the slower the dephasing the greater the T2 signal The two basic types of MRI images are T1-weighted and T2-weighted images, often referred to as T1 and T2 images. The timing of radiofrequency pulse sequences used to make T1 images results in images which highlight fat tissue within the body. The timing of radiofrequency pulse sequences used to make T2 images results in images which highlight fat AND water within the body. T1 images – 1 tissue type is bright – FAT. T2 images – 2 tissue types are bright – FAT and WATER T1: Water is dark – better for anatomy (soft tissue structures) T2: Water is bright – better for pathology (inflammation, oedema, tumors) Contrast Gadolinium is the most common contrast agent used for MRI – it can be given intravenously or injected directly into a body part Abnormal tissue may enhance more than surrounding normal tissue following intravenous gadolinium Abnormal tissue may also retain gadolinium longer than normal tissue Pre and post-gadolinium T1-weighted images are compared in order to assess 'enhancement' of tissues. Abnormal tissue, such as inflamed or cancerous tissue, is often more vascular than surrounding tissue and so 'enhances', appearing brighter on post-gadolinium images Gadolinium major side effect: It can rarely cause nephrogenic systemic fibrosis (similar to scleroderma) in patients with renal failure. Contraindications Metallic foreign bodies, especially from previous eye trauma, can cause serious damage if they move during the scan. If unsure of their presence, exclude with an X-ray. Most modern implants (pacemakers, stents, joint replacements) are MRI safe, but they must always be checked to ensure compatibility. They may, however, cause a black void artefact on the MRI image. The MRI machine is very noisy and cramped. Some patients may not be able to tolerate it if they are claustrophobic or unable to lie still Loose ferromagnetic objects can turn into projectiles if inadvertently brought into the room. :Different MRI sequences DWI (diffusion weighted imaging): Diffusion restriction is bright o useful for ischemic strokes, abscesses, most tumors. FLAIR (fluid attenuated inversion recovery): Like T2, but Water is dark o useful for multiple sclerosis (periventricular lesions) STIR (short tau inversion recovery): Like T2, but fat is dark o useful for oedema in tissues, perianal abscesses MRA (magnetic resonance angiography): Vessels are bright o useful for AVMs, aneurysms (can be done with or without contrast!) 1. T1-Weighted Imaging Purpose: Produces images where fat appears bright, and water or fluids are darker. Use: Ideal for anatomical detail, helping to visualize structural changes in tissues like fat, muscle, and the brain. 2. T2-Weighted Imaging Purpose: In T2-weighted images, water and fluids appear bright, and fat appears darker. Use: Helpful for identifying pathology, such as tumors, inflammation, or edema, since these conditions often cause increased water content in tissues. 3. Diffusion-Weighted Imaging (DWI) Purpose: Detects the movement of water molecules in tissue. Use: Commonly used to detect acute strokes, as it can show early changes in the brain caused by restricted water diffusion..4 Functional MRI (fMRI) Purpose: Measures brain activity by detecting changes in blood oxygenation and flow. Use: Typically used in neuroscience research and pre-surgical mapping to study brain function, particularly during tasks like movement or speech. 5. Magnetic Resonance Angiography (MRA) Purpose: Visualizes blood vessels without the use of catheters or contrast injections (though contrast can be used to enhance images). Use: Useful for studying arteries and veins, detecting aneurysms, blockages, and vascular malformations. 6. Magnetic Resonance Spectroscopy (MRS) Purpose: Measures the concentration of biochemical compounds in tissues. Use: Often used to study metabolic disorders, brain tumors, and certain other neurological conditions by analyzing the chemical makeup of tissues. Brain MRI. T1-Weighted Imaging: Provides high-resolution anatomical detail, useful for identifying normal brain structures and certain pathologies like brain tumors. T2-Weighted Imaging: Helpful in detecting lesions with high water content, such as edema, inflammation, or tumors. Fluid-Attenuated Inversion Recovery (FLAIR): Suppresses fluid signals to highlight lesions like multiple sclerosis plaques or areas of stroke. Diffusion-Weighted Imaging (DWI): Used to detect acute ischemic strokes within minutes of onset by highlighting areas of restricted water diffusion. Functional MRI (fMRI): Measures brain activity by detecting changes in blood oxygenation, often used in pre-surgical planning or brain function studies. Magnetic Resonance Spectroscopy (MRS): Analyzes brain metabolites, aiding in the diagnosis of tumors, epilepsy, or metabolic disorders MRI BRAIN Spine MRI.2 T1-Weighted Imaging: Provides detailed anatomical views of the vertebrae, intervertebral discs, and the spinal cord. T2-Weighted Imaging: Excellent for evaluating disc herniations, spinal cord lesions, and degenerative diseases because it highlights water content in tissues. Short Tau Inversion Recovery (STIR): Suppresses fat signals, enhancing visualization of spinal cord edema or inflammation, useful for detecting soft tissue abnormalities or trauma. Diffusion-Weighted Imaging (DWI): Can be used to detect spinal cord infarction or certain spinal cord tumors. CS MRI LUMBER MRI Musculoskeletal MRI (Joints, Bones, and Soft Tissues ) T1-Weighted Imaging: Useful for assessing bone marrow, ligaments, and soft tissues around joints. It provides good anatomical detail. T2-Weighted Imaging: Excellent for evaluating joint effusions, inflammation, and soft tissue abnormalities because it highlights fluid and edema. Short Tau Inversion Recovery (STIR): Fat suppression technique useful in detecting bone marrow edema, soft tissue injuries, and inflammatory conditions like arthritis or tendonitis. Dynamic Contrast-Enhanced MRI (DCE-MRI): Occasionally used to evaluate vascularity and perfusion in soft tissue tumors. Cartilage Imaging: Special sequences (like T2 mapping or DCE-MRI) can evaluate the cartilage in joints, especially useful in early osteoarthritis detection. SHOULDER MRI KNEE MRI Hand mri Foot mri Abdomen and Pelvis MRI. T1-Weighted Imaging: Provides detailed images of abdominal organs (e.g., liver, spleen, pancreas) and helps detect abnormalities like tumors, cysts, or organ structure changes. T2-Weighted Imaging: Useful for detecting fluid-filled structures or pathology such as ascites, cysts, or inflammatory conditions in organs like the liver, pancreas, or bowel. Diffusion-Weighted Imaging (DWI): Helps identify malignant lesions in organs like the liver, pancreas, or lymph nodes. Dynamic Contrast-Enhanced MRI (DCE-MRI): Used with contrast agents to evaluate organ perfusion and vascular characteristics of tumors, especially in the liver, pancreas, or kidneys. Pelvic MRI (T1 and T2): Helps evaluate pelvic organs like the uterus, ovaries, prostate, and bladder, useful for conditions such as fibroids, tumors, and endometriosis. Blood Vessels and Vascular System Magnetic Resonance Angiography (MRA): Used to visualize arteries and veins throughout the body, often used in the brain, neck, abdomen, and legs to detect aneurysms, stenosis, or vascular malformations. Time-of-Flight (TOF) MRA: Useful for imaging arteries, especially in the brain and neck, without the need for contrast agents. Phase-Contrast MRA: Used to assess blood flow dynamics in vessels, particularly in cases of cardiovascular disease