MAGNETIC RESONANCE IMAGING Sh Goharpey Physicaltherapist Magnetic Resonance

MAGNETIC RESONANCE IMAGING Sh. Goharpey Physicaltherapist

Magnetic Resonance Imaging • • Rooted in a chemical technique known as nuclear magnetic resonance spectroscopy Dropped the word “nuclear” 1 st NMR image of a human in 1977 1980 Moore and Hinshaw produced first recognizable image of the brain

Magnetic Resonance Imaging • • Capable of producing axial images Multiplanar capabilities Soft tissue contrast and spatial resolution exceeds CT Capable of obtaining physiologic information

MRI Technical • • • Superconductive magnets most frequently employed Temp near absolute zero by liquid cryogens (helium and nitrogen) Magnet strength in tesla (T) or gauss (G) 1 T = 10, 000 G Magnet strength of 1. 5 T is ~ 25, 000 times > the earth’s magnetic attraction

MRI Technical • Categories of magnetic field strengths: • Ultra high field > 4 T • High field 1. 5 – 4. 0 T • Medium field 0. 5 – 1. 0 T • Low field 0. 1 - 0. 5 T • Ultra low field < 0. 1 T

MRI Technical • • The larger the field strength of the magnet the greater the signal to noise ratio. SNR also influenced by • Surface coils • Software

IMAGE PRODUCTION • Proton Properties: • • Moving electric charge magnetic field Size of charge and speed affects field strength Nucleus of hydrogen (single proton) has a small positive electric charge and is spinning small measurable magnetic field Water is biggest source of protons followed by fat

IMAGE PRODUCTION • Number of spins in excess related to magnet field strength • 0. 5 T 3 protons out of 2, 000 • 1. 0 T 6 protons out of 2, 000 • 1. 5 T 9 protons out of 2, 000

IMAGE PRODUCTION • • • Signal used to produce the MR image comes from the protons in excess SNR is higher in higher field scanners Vol of H 20 2 x 5 mm 3 1. 2 x 10(22) total protons • 6. 02 x 10 (15) in excess (6 million billion)

IMAGE PRODUCTION • Frequency of precession is directly proportional to the strength of the magnetic field • Defined by the Larmor equation • At magnet field strengths used in clinical MRI systems 0. 5 – 2. 0 T the resonance (Lamor, precessional) frequency of Hydrogen is 2. 13 MHz to 85 MHz

IMAGE PRODUCTION • RF pulse matching the Larmor frequency of the protons is applied at a 90 degree angle to the magnetic field. The protons resonate and absorb the energy of the RF pulse. • Misaligns higher energy state • Angle of misalignment controlled by RF pulse • Flip angle

IMAGE PRODUCTION • When the RF pulse is turned off, the protons realign • Energy is release, expressed as an RF signal • Processed by the computer image

IMAGE PRODUCTION • Spin echo sequences repeat the RF pulses many times first at 90 degrees followed by 180 degrees. • TR (repetition time) interval between the two 90 degree pulses • TE (echo time) time between the pulse and the detection of the re-emitted signal

THE IMAGE • Factors affecting the image • • • Motion Signal and resolution Tissue contrast

THE IMAGE • Motion • • One of the m/c causes of image degradation Cardiac motion, peristalsis, respiration Body movement related to patient discomfort Prolonged examination • Set imaging protocols are useful

THE IMAGE • Signal and Resolution: • • Signal: amount of information on an image Inc slice thickness or field of view or, alternatively decreasing the matrix will increase signal to noise ratio # of signal acquisitions affects signal Proximity of the slices • Interposed gap to reduce cross talk

THE IMAGE • Signal and Resolution: • • • Resolution: ability to distinguish small objects Changes designed to improve resolution will negatively affect signal Use of coils (surface or body) help to minimize the trade off

THE IMAGE • Tissue Contrast: • • Soft tissue contrast in MRI is related to differences in proton resonance Differences in tissue specific properties can be emphasized by changing the imaging parameters • Known as weighting the image

Pulse Sequences Sequence T 1 Proton T 2 FSE T 2 GRE T 1 GRE T 2 FSE STIR TR (Msec) < 1000 > 2000 variable >2000 TE (Msec) < 30 > 60 TI (Msec) N/A N/A N/A 120 -150 Flip Ang 90 90 ETL 2 -16 70 -110 5 -20 180 90 ETL 2 -16

Relative MR Signal Intensities Feature Calcium Cortical bone Tendon Ligament Fat Red Marrow Yellow Marrow Water Muscle T 1 T 2 VLow Low High Low Isointense VLow Low Isointense High Isointense

Relative MR Signal Intensities Feature Bone bruise Occult Fx Stress Fx Acute Infection Osteonecrosis Tumor (majority) Benign (most) Malignant (most) T 1 T 2 Low Sig void Low edem Low Isointense Homogen High Sig void High edem High Homogen Heterogen

Gadolinium (Gd-DTPA) gadolinium diethylenetriamine-pentaacid • Indications: • • • Mass: cyst vs. solid Mass: viable tumor vs. necrosis Infection: soft tissue edema vs abscess Spine: disc herniation vs scar tissue Spine: Evaluation of cord lesions (tumors, MS, nerve sheath tumors)

MRI Safety and Contraindications • Contraindications: • Pacemaker • Intracranial aneurysm clips • Cochlear implant • Intraocular metal • Pregnancy

MRI Safety and Contraindications • Items To Be Removed: • Dentures/Bridgework • Excessive eye makeup • Wigs • Hearing aids • Bobby pins/ hair clips • Watches • Keys • Wallets • Belts and other loose metal

MRI Safety and Contraindications • Items That May Degrade Image Quality: • Hip pins and Herrington rods • Bone rods, pins or screws • Braces • Most implants • Hip and knee prosthetics • Abdominal and thoracic surgical clips • Dural clips