M -AGNETIC R -ESONANCE I -MAGING CONTENT • PHYSICS • SAFETY • APPLICATIONS • QUESTIONS PHYSICS TYPES OF ATOMIC MOTION 1. The electron orbits the nucleus 2. The electron spins on its own axis 3. ***The nucleus spins on its own axis*** MRI USES THE HYDROGEN ATOM •1 electron orbits the nucleus •The nucleus contains no neutrons but contains 1 proton THE HYDROGEN NUCLEUS HAS A NET POSITIVE CHARGE •Hydrogen nucleus is a spinning, positively charged particle LAW OF ELECTROMAGNETISM •A charged particle in motion will create a magnetic field •The postitively charged, spinning hydrogen nucleus generates a magnetic field WHY HYDROGEN? •Very abundant in the human body-H20 •Has a large magnetic moment MAGNETIC MOMENT The tendency of an MR active nuclei to align its axis of rotation to an applied magnetic field MR ACTIVE NUCLEI odd # protons or odd # neutrons or BOTH e.g. Hydrogen1, Carbon13, Nitrogen15, Oxygen17, Fluorine19, Sodium23, Phosphorus31 STABLE ATOMS # protons = # electrons IONS # protons # electrons When a body is placed into the bore of the scanner, the strong magnetic field will cause the individual hydrogen nuclei to either: A) ALIGN ANTI-PARALLEL TO THE MAIN MAGNETIC FIELD (B0) OR B) ALIGN PARALLEL TO THE MAIN MAGNETIC FIELD (B0) Anti-parallel high energy B0 NMV Parallel low energy NET MAGNETIZATION VECTOR • An excess of hydrogen nuclei will line up parallel to B0 and create the NMV of the patient N N S S size direction The magnetic vector THE NUCLEI WILL ALSO PRECESS… PRECESSION • Due to the influence of B0, the hydrogen nucleus “wobbles” or precesses (like a spinning top as it comes to rest) • The axis of the B0 nucleus forms a path around B0 known as the “precessional path” Precessional path Hydrogen nucleus PRECESSION • The speed at which hydrogen precesses depends on the strength of B0 and is termed the “precessional frequency” • The precessional frequency of hydrogen in a 1.5 Tesla magnetic field is 63.86 MHz • The precessional paths of the individual hydrogen nucleus’ is random, or “out of phase” WE NEED THEM TO BE “INPHASE” OR TO RESONATE… RESONANCE Occurs when an object is exposed to an oscillating perturbation that has a frequency close to its own natural frequency of oscillation •Ella Fitzgerald •Tacoma Narrows bridge failure RESONANCE con’t • Frequency of the hydrogen proton in a 1.5T magnetic field can be found in the RF band of energy in the electromagnetic spectrum RADIOFREQUENCY ENERGY • Follows the Law of Electromagnetism (charged particles in motion will generate a magnetic field) • Magnetic field known in MR as B1 • Applied as a “pulse” during MR sequences • The RF pulse is applied so that B1 is 90 to B0 DURING RESONANCE… 1) The hydrogen atoms begin to precess “in phase” 1) 2) The hydrogen atoms align with the RF’s magnetic field (B1) and they flip!! RF B0 B0 NMV NMV flips! PULSE B1 B1 AS THE NUCLEI PRECESS IN-PHASE IN THE B1 PLANE, A CHANGING MAGNETIC FIELD IS CREATED IF YOU PLACE A RECEIVER COIL (ANTENNA) IN THE PATH OF THE CHANGING MAGNETIC FIELD, A CURRENT WILL BE INDUCED THIS IS FARADAY’S LAW OF INDUCTION FARADAY’S LAW OF INDUCTION A changing magnetic field will induce an electrical current in any conducting medium COILS Used to: •transmit pulses of radiofrequency energy •receive induced voltage - MR SIGNAL •increase image quality by tuning in to one body part at a time RELAXATION When the RF pulse is turned “off”, the NMV “relaxes” back to B0 (away from B1) NMV B0 B1 •RF pulses are applied very quickly in succession - RF PULSE SEQUENCE •3 minute sequence (20 slices, axial brain) - 60 RF pulses may be applied MR SIGNAL • Collected by a coil • Encoded through a series of complex techniques and calculations (magic?) • Stored as data • Mapped onto an image matrix TR - REPETITION TIME Time from the application of one RF pulse to another RF pulse TE - ECHO TIME Time from the application of the RF pulse to the peak of the signal induced in the coil T1 WEIGHTING •A short TR and short TE will result in a T1 weighted image •Excellent for demonstrating anatomy T2 WEIGHTING •A long TR and long TE will result in a T2 weighted image •Excellent for demonstrating pathology MANY OTHER DIFFERENT TYPES OF IMAGES THAT COMBINE ABOVE AND INCLUDE OTHER PARAMETERS T1 WEIGHTED IMAGE T2 WEIGHTED IMAGE SAFETY THE MAGNET IS ON ALL THE TIME!!! OHM’S LAW OF RESISTANCE V = IR V = voltage I = current R = resistance R depends on the material, the length, the cross-sectional area, and the temperature of the loops of wire through which the current flows **Decreasing the temperature of the wire will decrease resistance to the flow of electricity SUPERCONDUCTING MAGNET • No resistance to flow of electricity • Coils of wire surrounded by cryogen bath (Helium) at -273 C • No external source of energy required • Magnetic field present ALL THE TIME!!! Gauss - measure of magnetic field strength refrigerator magnet - 150-250 G 10,000 Gauss = 1T MRI - 0.2T - 1.5T 100x stronger that fridge magnet THE STRONG MAGNETIC FIELD OF THE MAGNET CAN TURN THE FOLLOWING INTO DANGEROUS PROJECTILES: • • • • • wheelchairs oxygen tanks I.V. poles I.D. tags keys • • • • • coins scissors trauma boards sandbags safety pins THE CHANGING MAGNETIC FIELDS CAN DO DAMAGE TO: •Monitoring equipment •Infusion pumps •Credit cards •Cellular telephones •Any electronic device THE FOLLOWING ARE (USUALLY*) OKAY: •Gold •Silver •Digital watches •Eyeglass frames •Snaps/zippers fastened to clothing •Dental work APPLICATIONS ADVANTAGES • Superior soft tissue contrast resolution excellent pathological discrimination • No ionizing radiation • Direct multi-planar imaging (transverse, coronal, sagittal, any oblique) • Non-invasive - vascular studies can be performed without contrast KNEE ANGIOGRAPHIC TECHNIQUES • Circle of Willis angiograms without any contrast ANGIOGRAPHIC TECHNIQUES •Studies using contrast can also be performed RENAL MRA GADOLINIUM USEFUL FOR DETECTION OF: • Tumours pre- and post-operative • Infection • Inflammation • Post-traumatic lesions • Post-operative changes • MRA’s DISADVANTAGES OF MRI • • • • Expensive Long scan times Audible noise (65-115dB) Isolation of patient (claustrophobia, monitoring of ill patients) • Exclusion of patients with pacemakers and certain implants BRAIN • Hemorrhage (stages of) • Demyelinating disorders (M.S.) • Infectious processes (encephalitis, meningitis) • Abscesses • Neoplasms • Neurofibromatosis • Trauma • Vascular disorders (AVM’s, infarcts, aneurysms) BRAIN (cont’d) • • • • • • • Metastasis Internal auditory canal pathology Pituitary pathology Hydrocephalus Child abuse Cranial nerve pathology Congenital anomalies (for anatomical review) • Epilepsy (seizures in general) AXIAL T2 BRAIN SPINE • • • • • • • • Radiculopathy Tumours Trauma/contusion Syringomyelia Metastasis Vascular disorders Cord edema M.S. plaques SPINE (cont’d) • • • • • • • Cauda equina syndrome Tethered cord Arachnoiditis Marrow-replacing processes Degenerative disc disease Discitis Congenital anomalies SPINE MUSCULOSKELTAL (shoulder, knee, ankle, wrist, elbow, TMJ) • • • • • • • • • Meniscal pathology Ligament/tendon injury Muscle/nerve impingement Avascular necrosis Labral tears (shoulder, hip) Chondromalacia Inflammation (osteomyelitis) Primary bone tumours Soft tissue tumours SHOULDER ABDOMINAL/PELVIC • • • • Liver pathology Kidney pathology Renal artery MRA Fetal abnormalities ABDOMINAL IMAGING • Breath-hold scans to overcome motion artifact problem • MRCP’s - images of the biliary and pancreatic ductal systems performed noninvasively (no contrast or endoscope!) within seconds • Fetal imaging very diagnostic MRCP FETAL BREATH-HOLD IMAGE FETAL ENCEPHALOCELE CARDIAC • • • • Co-arctation RV dysplasia Cinematic studies Measure cardiac output, stroke volume, ejection fraction MR Spectroscopy (MRS) • Information obtained is in the form of a spectrum which provides the biochemical information contained within a selected voxel of tissue • Used to detect the absence or presence of a certain compound • Assists in differential diagnosis when standard clinical radiological tests fail or are too invasive Spectrum MRS Current Applications • • • • • • • • Multiple Sclerosis Leigh’s Huntington’s Parkinson’s Alzheimer’s Epilepsy other dementias metabolic disorders • Stroke • Asphyxiation or ischemic injury • Tumours and intracranial lesions • Prostate cancer • Encephalopathies • Leukodystrophies Functional MRI (fMRI) • research topic • Detects changes in blood flow or metabolism associated with specific motor or sensory functions or stimuli • Performed by scanning specific areas of the brain/spine while: a) the subject performs a certain motor task or b) exposing the subject to certain external/internal stimuli fMRI cont’d • Subjects are scanned at rest and then during exercise or exposure to various stimuli • The two conditions are subtracted to reveal areas of brain activation • Areas of activation will have increased levels of blood flow and are therefore detectable fMRI cont’d • Mapping of the brain’s motor and sensory areas • Delineating primary cortical areas prior to surgery on patients with tumours (to avoid paralysis when operating on tumours in dangerous locations) • Assessment of brain function following injury MANY OTHER WORKS IN PROGRESS… QUESTIONS?????
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