Ch 20: MRI

  • no ionizing radiation
  • uses magnetic fields

radiofrequency pulses & a computer system to generate cross sectional slices from any angle

best for soft tissue detail

everyone working in the MRI area must undergo MRI safety training

MRI v CT

  • MRI is better for soft tissues & anatomy that may be obscured on CT
  • MRI is more costly & contraindicated for patients with certain medical devices and implants
  • MRI uses radiofrequency coil to measure transmission of x-ray beam
    • radiofrequency is transmitted into the body and coil is antenna that receives signal coming from the body
    • coil type depends on anatomy to be imaged

Physical principles of MRI

  • individual images are pulse sequences
    • pulse sequences vary by orientation and weighting
    • weighting determines how pathologies present

Nuclei interaction with magnetic field

  • MRI mostly uses hydrogen atoms to produce images
  • nuclei with an odd number of protons & neutrons generate spin angular momentum & are suitable for diagnostic imaging

Precession

  • the nucleus precesses around a strong static magnetic field
  • precession occurs when a spinning object is acted upon by an external force
  • rate of precession of a proton in a magnetic field increases as the strength of the magnetic field increases

Net magnetization

  • the sum of all the magnetic moments averaged together
  • MRI scanners gradually increase the static field strength, as field strength increases protons lower their energy
    • more low energy protons = more usable for imaging
  • if net magnetization increases more protons are used for imaging = higher signal to RF coil

Resonance

  • the exchange of energy between low & high energy states at a specific frequency, only occurs in protons with matching pecessional frequencies

Relaxation

  • before receiving the RF pulse the H protons are in alignment with the field & are precessing randomly in different phases/directions
  • the pulse is applied and it forces protons into the transverse plane away from the field, this forces protons into the precess phase
  • once phase pulse is off the nuclei return to equilibrium (relaxation) and as they relax the signal received diminishes
  • rate of relaxation relays information about normal tissue and pathologic processes within tissues - normal tissue and pathologies relax at different rates
    • T1 relaxation = time it takes for 63% of the longitudinal magnetization to recover in the tissue
    • T2 relaxation = time it takes for transverse magnetization to lose 63% the original value

Spin density

  • a stronger signal is received if the number of hydrogen nuclei present in a volume of tissue is increased, need to ensure there is enough protons to produce a signal

MRI magnets

  • provide a powerful static magnetic field the nuclei respond to, measured in tesla
  • strength in clinical setting varies from 0.2T - 7-45T
  • fringe field is the magnetic field outside the magnet’s center
  • permanent magnets (0.4T)
    • retain magnetism after being exposed to a magnetic field
    • open bore design that requires no cooling & no additional electric power
    • has a limited fringe field and lower operating costs
    • has high initial cost, very heavy, low field strength, can’t turn off field, and has a potential uneven field
  • resistive magnets (0.6T)
    • magnetic field created by passing electrical current through coil of wire
    • requires large amounts of electrical power and a cooling mechanism
    • heating of wire due to friction limits current that can be produced
  • superconducting magnets (1.5T - 7T)
    • most common
    • electromagnet that passes electric current through coils of superconductive wire embedded in copper and cooled by cryogens
    • tunnel shaped with consistent magnetic field
    • cryogens are the expensive part

Flared & short bore design

  • helps relieve anxiety due to claustrophobia

Open MRI system

  • may help claustrophobia or those unable to fit in traditional closed bore MRI
  • only able to do basic scans, has a longer scan time and lower resolution images

Patient aspects

  • contraindications
    • patients with electronic/electrically conductive metals, implants and some medicine pumps, pacemakers, some devices may be MR conditional but still need to be cleared, not preferred to scan during first trimester of pregnancy, pregnant patients not given gadolinium contrast. may be contraindicated for patients on ventilators or those who need heavy monitoring
  • patient prep & screening
    • scheduling provides patients with some info, MRI screening form, patient changes into scrubs or gown, remove all metallic devices, explain procedures and convey importance of removal of items, change the patient, explain to them to stay still and explain the noises, give ear plugs, inform of any contrast to be given and the length of exam, will be monitored the entire time
  • relieving patient anxiety
    • explain exam, play music, give relaxation tips, use cushions & pads, move patient slowly into magnet, take friend/family member into room (everyone who goes into the room must fill out an MR screening form)

MRI safety zones

  • zone 1- all public areas
  • zone 2- interface area between zones 1&3, waiting area, changing room
  • zone 3- area near magnet room, area where tech escorts patient to
  • zone 4- magnet area, all ferromagnetic items must be removed before entering

Level 1&2 personnel

  • 1: pass minimum MR safety education, can go up to zone 3
  • 2: personnel with more extensive training, identified by MR medical director

Safety considerations

  • projectiles - ferrous objects in MRI room may be strongly attracted to magnet and become projectile, new door sensors may alert when an MRI scanner door detects metal
  • missile effect - small object pulled toward magnet may be small object pulled toward magnet may be lethal due to speed, if patient has an emergency they should be removed from the scan room
  • electromechanical implants & devices - adversely affected due to magnetic field, some devices may be MR conditional, conditions are set by manufacturer
  • translational force & torque - patient may have metallic objects in/on body, magnetic field may cause objects to torque/twist, all surgical implants & devices should be evaluated for safety, if patient has shrapnel or metallic foreign objects - especially in eye they must be evaluated using CT or xray first

Specific absorption rate

  • the rate that energy is absorbed into tissues
  • measured in watts/kg
  • specific to body part and tissue type
  • techs need to be aware of SAR limits & how to adjust the factors based on this

MRI has 3 operating modes

  • normal operating mode - safe for all patients regardless of conditions or states, routine patient monitoring
  • first level controlled mode - requires medical supervision & assessment by qualified medical practitioner of risk v benefit
  • second level controlled mode - used only for research

Specific energy dose: total accumulated amount of energy deposited into body, Joules/kg, specific to part/tissue, monitored by tech

Peripheral nerve stimulation

  • effect on body caused by gradient magnetic fields
  • MRI has 3 pairs of gradient coils for spatial localization of MRI signal
  • biologic effects may include: temporary stimulation of muscles and nerves, visual flashes of light caused by current stimulating the retina, and noise caused by gradient switching

Patient burns

  • uncommon but may occur due to direct skin contact with bore
  • loops and cables that come into direct contact with skin may cause burns, techs place pads around patient anatomy

Pregnancy

  • may be done instead of other diagnostic studies due to lack of radiation, preferred not to scan during first trimester but no known risks
  • will not receive gadolinium contrast

Occupational hazards

  • no long term known hazards
  • sensory effects may only occur while in magnet room

Contrast agent

  • gadolinium based contrast agents only given if indicated
  • most shorten the T1 relaxation time
  • gadolinium is very rare and very magnetic
  • given using manual injector or power injector
  • contraindicated for those who are pregnant, have kidney disease, or previous allergic reactions

Physiologic gating/triggering

  • to reduce artifacts due to cardiac motion, respiratory motion, or blood flow
  • triggering - acquires images between cardiac cycles

Spin echo, turbo spin echo/fast field echo, gradient echo, inversion recovery, echo planar imaging

MRI exams

  • brain
    • indications: vascular issues, tumor, infection, white matter diseases, neoplasm, infectious disease, trauma, congenital malformations, hydrocephalus
  • spine
    • indications: chiari malformation, congenital anomalies, fractures, abscesses, tumors, RA, pre & post-op evaluations, infectious diseases, myelopathy, spinal cord infarct, white matter diseases, evaluation of vertebral bodies, severe scoliosis
  • MRA
    • indications: aneurysms, occlusions, stenosis, obstructions, AV malformations, artery dissection, sinus thrombosis, intracranial venous occlusive disease, cerebral venous thrombosis, aortic dissection, intracranial hypertension, vascular malformations & anastomoses, peripheral artery disease
  • MRV
    • indications: vein occlusion & stenosis, DVT, pulmonary emboli, peripheral vascular disease, tumors, vascular abnormalities, venous thrombosis, post-op evaluation of bypass sites and grafts
  • musculoskeletal imaging
    • indications: joint disorders, fractures, necrosis, marrow disorders, tumors, inflammation, edema, tears
  • abdomen & pelvis
    • indications: tumors, liver disease, inflammatory bowel disease, pelvic floor disease, inflammation of vessels, adrenal lesions, polycystic kidney disease, renal masses
  • breast imaging
    • dynamic MR breast imaging, MR breast biopsy, MR implant imaging

cardiac imaging, diffusion-weighted imaging on different parts, perfusion imaging, functional MRI, MR spectroscopy