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M -AGNETIC R -ESONANCE I -MAGING. CONTENT PHYSICS SAFETY APPLICATIONS QUESTIONS.

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Presentation on theme: "M -AGNETIC R -ESONANCE I -MAGING. CONTENT PHYSICS SAFETY APPLICATIONS QUESTIONS."— Presentation transcript:

1 M -AGNETIC R -ESONANCE I -MAGING

2 CONTENT PHYSICS SAFETY APPLICATIONS QUESTIONS

3 PHYSICS

4 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***

5 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

6 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-H 2 0 Has a large magnetic moment

7 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. Hydrogen 1, Carbon 13, Nitrogen 15, Oxygen 17, Fluorine 19, Sodium 23, Phosphorus 31 STABLE ATOMS # protons = # electrons IONS # protons  # electrons

8 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 (B 0 ) OR B) ALIGN PARALLEL TO THE MAIN MAGNETIC FIELD (B 0 ) B0B0 NMV Anti-parallel high energy Parallel low energy

9 NET MAGNETIZATION VECTOR An excess of hydrogen nuclei will line up parallel to B 0 and create the NMV of the patient

10 N S N S direction size The magnetic vector

11 THE NUCLEI WILL ALSO PRECESS…

12 PRECESSION Due to the influence of B 0, the hydrogen nucleus “wobbles” or precesses (like a spinning top as it comes to rest) The axis of the nucleus forms a path around B 0 known as the “precessional path” Hydrogen nucleus B0B0 Precessional path

13 PRECESSION The speed at which hydrogen precesses depends on the strength of B 0 and is termed the “precessional frequency” The precessional frequency of hydrogen in a 1.5 Tesla magnetic field is MHz The precessional paths of the individual hydrogen nucleus’ is random, or “out of phase”

14 WE NEED THEM TO BE “IN- PHASE” OR TO RESONATE…

15 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

16

17 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

18 RADIOFREQUENCY ENERGY Follows the Law of Electromagnetism (charged particles in motion will generate a magnetic field) Magnetic field known in MR as B 1 Applied as a “pulse” during MR sequences The RF pulse is applied so that B 1 is 90  to B 0

19 DURING RESONANCE… 1) The hydrogen atoms begin to precess “in phase” 1) 2) The hydrogen atoms align with the RF’s magnetic field (B 1 ) and they flip!! B0B0 B0B0 B1B1 B1B1 NMV NMV flips! RF PULSE

20 AS THE NUCLEI PRECESS IN-PHASE IN THE B 1 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

21 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

22 RELAXATION When the RF pulse is turned “off”, the NMV “relaxes” back to B 0 (away from B 1 ) B0B0 B1B1 NMV RF pulses are applied very quickly in succession - RF PULSE SEQUENCE 3 minute sequence (20 slices, axial brain) - 60 RF pulses may be applied

23 MR SIGNAL Collected by a coil Encoded through a series of complex techniques and calculations (magic?) Stored as data Mapped onto an image matrix

24 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

25 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

26 T1 WEIGHTED IMAGE

27 T2 WEIGHTED IMAGE

28 SAFETY

29 THE MAGNET IS ON ALL THE TIME!!!

30 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

31 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!!!

32 Gauss - measure of magnetic field strength refrigerator magnet G 10,000 Gauss = 1T MRI - 0.2T - 1.5T  100x stronger that fridge magnet

33 THE STRONG MAGNETIC FIELD OF THE MAGNET CAN TURN THE FOLLOWING INTO DANGEROUS PROJECTILES: coins scissors trauma boards sandbags safety pins wheelchairs oxygen tanks I.V. poles I.D. tags keys

34 Monitoring equipment Infusion pumps Credit cards Cellular telephones Any electronic device THE CHANGING MAGNETIC FIELDS CAN DO DAMAGE TO:

35 Gold Silver Digital watches Eyeglass frames Snaps/zippers fastened to clothing Dental work THE FOLLOWING ARE (USUALLY*) OKAY:

36 APPLICATIONS

37 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

38 KNEE

39 ANGIOGRAPHIC TECHNIQUES Circle of Willis angiograms without any contrast

40 ANGIOGRAPHIC TECHNIQUES Studies using contrast can also be performed

41 RENAL MRA

42 GADOLINIUM USEFUL FOR DETECTION OF: Tumours pre- and post-operative Infection Inflammation Post-traumatic lesions Post-operative changes MRA’s

43 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

44 BRAIN Hemorrhage (stages of) Demyelinating disorders (M.S.) Infectious processes (encephalitis, meningitis) Abscesses Neoplasms Neurofibromatosis Trauma Vascular disorders (AVM’s, infarcts, aneurysms)

45 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)

46 AXIAL T2 BRAIN

47 SPINE Radiculopathy Tumours Trauma/contusion Syringomyelia Metastasis Vascular disorders Cord edema M.S. plaques

48 SPINE (cont’d) Cauda equina syndrome Tethered cord Arachnoiditis Marrow-replacing processes Degenerative disc disease Discitis Congenital anomalies

49 SPINE

50 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

51 SHOULDER

52

53 ABDOMINAL/PELVIC Liver pathology Kidney pathology Renal artery MRA Fetal abnormalities

54 ABDOMINAL IMAGING Breath-hold scans to overcome motion artifact problem MRCP’s - images of the biliary and pancreatic ductal systems performed non- invasively (no contrast or endoscope!) within seconds Fetal imaging very diagnostic

55 MRCP

56 FETAL BREATH-HOLD IMAGE

57 FETAL ENCEPHALOCELE

58 CARDIAC Co-arctation RV dysplasia Cinematic studies Measure cardiac output, stroke volume, ejection fraction

59 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

60 Spectrum

61 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

62 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

63 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

64 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

65 MANY OTHER WORKS IN PROGRESS…

66 QUESTIONS?????


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