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Basic Fundamentals of MRI Acquisition of MRI images - PHYSICS Basic Sequences & Advanced applications Characterisation of Lesions. MRI Physics MRI Physics Magnetic Resonance Imaging MRI Physics MAGNETIC Tissues are made of Protons (H+) Positive electrical charge spinning around its own axis. Moving Electrical charge produces its own magnetic field. Protons are analogous to small BAR MAGNETS. MRI Physics Presence of a strong magnetic field This leads to alignment of protons in the tissue acc to the magnetic field- LEVEL 1 MAGNETIC MRI Physics RESONANCE Apart from spinning motion these protons are also vibrating at a frequency. A specific radiofrequency wave passed in a specific manner & specific plane. Radiofrequency waves are NOT Radiation. MRI Physics RESONANCE MRI Physics RESONANCE Raises further raise the energy level of protons – Level 2 Level of excitation is different for different tissues hence signal is different – CONTRAST IMAGING Radiofrequncy switched off & excited protons come back to Level1 Energy / Signal is released.. Signal picked up by the Coil & processed by a computer. MRI Physics IN NUTSHELL Patient is placed in a strong magnetic field. Radiowave is sent in for a short time & switched off. Patient emits signal. Recived & used for reconstruction of image. TESLA FACTOR !! Tesla is the unit of magnetic field strength. TYPES OF MAGNETS : • Resistive Magnets – Low & Ultralow field strenghts. • Permanent Magnets – Low & Med field strength (upto 0.3 T). • Superconducting Magnets 0.5 - 14T TESLA FACTOR !! FIELD SRENGTH GRADIENTS TYPE OF MAGNETIC FIELD VERTICAL/ HORIZONTAL IMAGE QUALITY S/N Ratio Contrast TYPE OF COILS TYPE OF COMPUTER IMAGE PROCESSING & SEQUENCES CAMERA TESLA FACTOR !! As Magnetic field strength increases SNR increases – true for analyitical NMR But NOT for whole-body MRI. In higher field strengths body produces an inherent noise. Most important factor in Medical imaging is tissue contrast – best in medium & low field strngths. TESLA FACTOR !! SEQUENCES BASIC SEQUENCES : T1W. T2W. FAT SUPP T2W/ STIR. FLUID SUPP T2W/ FLAIR. ADVANCED APPLICATIONS : MR ANGIOGRAPHY. MR CHOLANGIOPANCREATOGRAPHY & MR UROGRAPHY. DIFFUSION. JOINT MOTION STUDY – KINEMATIC MRI. BASIC SEQUENCES T1 Weighted – ANATOMY. FAT- WHITE FLUID- BLACK BASIC SEQUENCES T2 Weighted – PATHOLOGY. FLUID- WHITE FAT- NOT SO WHITE BASIC SEQUENCES T2 Weighted FAT SUPP/ STIR. - PATHOLOGY FOR MOST EXTRACRANIAL REGIONS. FLUID- WHITE FAT- BLACK BASIC SEQUENCES T2 W T2 W FAT SUPP BASIC SEQUENCES T2 Weighted WATER SUPP/ FLAIR. - BRAIN PATHOLOGY CSF/ CLEAR FLUID BLACK PATHOLOGY/ PATHOLOGICAL FLUID- WHITE BASIC SEQUENCES FLAIR- WATER SUPP T2 W ADVANCED APPLICATION DIFFUSION Very sensitive to motion - picks up microscopic motion of protons in edema. Can differentiate between Cytotoxic Edema (infarct) & Vasogenic Edema (tumor/infection etc). Special sequence, picks up infarct within hours (2-3 hrs). To diffrentiate between Old & Fresh infarct. ADVANCED APPLICATION FLAIR DIFFUSION DIFFUSION ADVANCED APPLICATION FLAIR DIFFUSION DIFFUSION H/O dense Left Hemiplegia – 3 hrs back. FLAIR DIFFUSION ADVANCED APPLICATION MRCP Heavily T2W sequence – Only Fluid appears bright. 2 types – (1) Single shot breath hold (3 secs) (2) 3D MRCP. No contrast / injection. Non-invasive modality for deliniation of biliary anotomy. All non-theraputic indications of ERCP. ADVANCED APPLICATION MRCP T2 W FSE 3D MRCP ADVANCED APPLICATION T2 W Axial MRCP Single Shot MRCP - 3 sec ADVANCED APPLICATION MRAngio Flow sensitive sequences – picks up flow. No contrast / injection. Arterial study – Head & Neck vessels(2D/3D TOF) Venous study – Intracranial venous sinuses (2D TOF & Phase Contrast) ADVANCED APPLICATION MRAngio INDICATIONS : 1. 2. 3. 4. 5. 6. Stroke – infarct (Single vessel / Multiple vessel). TIA. Secondary Hemorrhage – AVM, Aneurysm. VBI. Trigeminal Neuralgia & Hemifacial spasm – aberrent loops. Venography - Venous sinus thrombosis . Contrast Enhanced MRA : 1. 2. Renal Vessels – RAS. Limb vessels – Femoral, Popliteal , Subclavian & Brachial. ADVANCED APPLICATION MRAngio MRA – NECK VESSELS MRA – CIRCLE OF WILLIS ADVANCED APPLICATION MRAngio CONTRAST MRA – RENAL VESSELS. ADVANCED APPLICATION PCA ANEURYSM MRAngio AVM ADVANCED APPLICATION KINEMATIC STUDY EXTENSION NEUTRAL FLEXION Pathology inflammation, infection, infarct, tumor etc. have high water content Hypintense on T1 & Hyperintense in T2 SOLID + T2W - + T1W - T2W - +++ HIGH PROTEIN FLUID FLAIR - SUPP FAT – T1W +++ T1W - ++ T2W ++ T2W - ++ CALCIFICATION – T1W+++ BLOOD- METH Hb T2W+++ T1W - ++ FLOW – T1W +++ T2W - ++ T2W +++ FLUID T1W - +++ LESION CHARACTERISATION BRAIN EDEMA CYTOTOXIC VASOGENIC Due to arterial infarct. Due to tumor, infection, venous infarct Extends thru grey-white matter junction (involves grey matter). Involves only white matter. Along specific arterial distribution. NOT along specific arterial distribution Bright on diffusion. Not bright on diffusion. CONTRAST MRI INDICATED LESION CHARACTERISATION CYTOTOXIC EDEMA BRAIN EDEMA VASOGENIC EDEMA T1W AXIAL FLAIR COR T2W AXIAL T2W SAG T1W +++ T2W +++ FLAIR NOT SUPP POST CONT ENHANCEMENT LESION CHARACTERISATION T1 W AXIAL ++ T2 W AXIAL +++ H/O headache following vehicular accident. FLAIR AXIAL +++ LESION CHARACTERISATION T1W T2W T1W T T2W POST CONT T1 POST CONT T1 LESION CHARACTERISATION T2W AXIAL. T1W AXIAL T2W SAG T1W SAG LESION CHARACTERISATION T2W AXIAL. T1W AXIAL. T2W SAG T1W SAG LESION CHARACTERISATION T1W T2W POST CONT T1 LESION CHARACTERISATION T2W AXIAL. T1W AXIAL. T2W CORONAL T1W CORONAL T2 FAT SUPP LESION CHARACTERISATION T2W AXIAL. T1W AXIAL. T2W SAG FAT SUPP COR LESION CHARACTERISATION T2W AXIAL T1W AXIAL T2 SAG T1 SAG MRI Basics SEQUENCES SEQUENCE T1W FAT WHITE FLUID BLACK T2W NOT SO WHITE WHITE T2 FAT SUPP STIR T2 FLU SUPP FLAIR BLACK WHITE CSF/CLEAR FLUID BLACK PATH FLUID- WHITE I've got so much to learn!
