MRI

History:

  • discovered in the ’40s by Bloch and Purcell

  • Ramond Damandian took concepts and created the MR image- father of present-day MRI exams

  • first commercial scanners: 1982 (US), 1983 (Europe)

  • Anatomical imaging was one of the only ways the first MRI machines worked

  • MRI moved to organs and structures known as cardiac cine functional imaging

  • multi-phase leg runoff

  • MRI-guided breast biopsy

  • dynamic breast MRI (contrast)

  • DTI (diffusion tensor imaging)- function of neurons on white matter tracks used for surgical planning

  • functional MRI- PT asked to do something, image taken during (ex. finger-tapping or listening), mostly done for surgical planning

MRI vs. other modalities:

  • XRAY VS MRI: radiography uses x rays, MRI uses magnetic fields and radio waves to produce the image

  • CT VS MRI: CT uses and axial plane, can be reformatted, and uses x-ray and ionizing radiation displayed in cross sections. MRI is multiplanar, uses RF and magnetic fields, and images are displayed in cross-sections.

  • NUC MED VS MRI: NM uses radioactive material intravenously, but MRI is the body

  • ULTRA VS MRI: US uses sound waves to view the inside body, MRI does not use those.

MRI physics:

  • orthogonal imaging planes: axial, sagittal, and coronal planes

  • hydrogen nucleus has one proton and a small electrical charge. most abundant element in living tissue and because of the way it is structured, a water molecule bonds two hydrogen and one oxygen together.

  • RF coil is applied, protons absorb energy and move to transverse plane, RF is turned off and protons release absorbed energy, protons return to a relaxed state and realign.

  • The rotation of the proton in the transverse plane induces a current in the RF coil

  • Relaxation: the amount of time it takes for protons to return to 63% of its initial value after RF is turned off

  • timing parameters, resolution parameters, contrast parameters

  • T1- CSF dark

  • T2- CSF bright

MRI hardware:

  • three types of magnets: superconductive, resistive, permanent

  • superconductive: high field strength (0.2-3T), stable magnetic field, magnet must be cooled by cryogen

  • resistive: low field strength (0.2T), less expensive to purchase and more expensive to operate, magnetic field not as stable

  • permanent: low field strength (0.2T), need stable room temp, cant be shut off

  • Tesla (T)- 10,000 gauss

  • Earth’s magnetic field is 0.5 gauss**

  • Cylindrical or bore scanners: homogenous field, higher strength

  • Open scanners: larger PT capacity, interventional possibilities, better for claustrophobia

  • dedicated coils for different body parts*

  • RF coil basics: coils are antennas, placed on/near the sample which emit the signal, have a minimum RF field sensitivity, turns the signal from a sample into an analog signal that can be processed by the MRI system.

MRI safety:

  • MR safe: not affected

  • MR conditional: can go into the room under certain conditions

  • MR unsafe: poses a safety risk to MR environment

  • zone 1: safe,

  • zone 2: safe,

  • Zone 3: restrictive zone, outside of zone 4, where the operator is, PT prep area could be here

  • Zone 4: restrictive zone, actual exam room (bay)

  • bioeffects of static magnetic fields:

  • missile effect accidents:

  • screening PT for metallic foreign bodies:

  • ferromagnetic cardiac pacemakers: contraindication for MR

  • something safe for 1.5T is not the same as something the same 3.0T

  • NSF (nephrogenic systemic fibrosis): caused by gadolinium, thickens and tighten skin, can be fatal