BASIC FUNDAMENTALS OF MRI

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 MAGNETIC Presence of a strong magnetic field This leads to alignment of protons in the tissue acc to the magnetic field- LEVEL 1

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

IMAGE PROCESSING & SEQUENCES TESLA FACTOR !! FIELD SRENGTH GRADIENTS TYPE OF MAGNETIC FIELD VERTICAL/ HORIZONTAL IMAGE QUALITY S/N Ratio Contrast TYPE OF COILS CAMERA TYPE OF COMPUTER IMAGE PROCESSING & SEQUENCES

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

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 T2 W FLAIR- WATER SUPP

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 DIFFUSION FLAIR DIFFUSION

ADVANCED APPLICATION DIFFUSION FLAIR 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 MRCP T2 W Axial 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 : 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 : 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 MRAngio AVM PCA ANEURYSM

ADVANCED APPLICATION KINEMATIC STUDY EXTENSION NEUTRAL FLEXION

Hypintense on T1 & Hyperintense in T2 Pathology inflammation, infection, infarct, tumor etc. have high water content Hypintense on T1 & Hyperintense in T2 SOLID FLUID T1W - +++ T2W - +++ FLAIR - SUPP T1W - + T2W - + HIGH PROTEIN FLUID FAT – T1W +++ T2W ++ CALCIFICATION – T1W+++ T2W+++ FLOW – T1W +++ T2W +++ T1W - ++ T2W - ++ BLOOD- METH Hb T1W - ++ T2W - ++

LESION CHARACTERISATION BRAIN EDEMA VASOGENIC CYTOTOXIC Due to tumor, infection, venous infarct Involves only white matter. NOT along specific arterial distribution Not bright on diffusion. CONTRAST MRI INDICATED Due to arterial infarct. Extends thru grey-white matter junction (involves grey matter). Along specific arterial distribution. Bright on diffusion.

LESION CHARACTERISATION BRAIN EDEMA CYTOTOXIC 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 +++ FLAIR AXIAL +++ H/O headache following vehicular accident.

LESION CHARACTERISATION T1W T2W T1W T2W POST CONT T1 T 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 FAT FLUID T1W WHITE BLACK T2W NOT SO WHITE T2 FAT SUPP STIR T2 FLU SUPP FLAIR CSF/CLEAR FLUID - BLACK PATH FLUID- WHITE

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