🫀 Mirza's Cardiology Notes

Cardiac MRI

5 min read

  • When there is acute infarct/inflammation →
    • Rupture of cellular membrane
    • Cellular swelling and edema
    • Gadolinium enters the extracellular space and clears slowly, i.e. delayed washout
      • Gadolinium loves collagen and globs onto it
  • Gadolinium-based contrast agents (GBCAs) shorten the T1 relaxation time of the surrounding tissues proportional to the local gadolinium concentration.
    • Normal: gadolinium, which is a large, high-density element made metabolically inert by chelation, enters the extracellular space readily after intravascular injection, but it is unable to cross the cell membrane of a normal myocyte.
    • If myocyte cell membrane is damaged (eg, acute MI) or if there is an ↑ in the extracellular space between myocytes (eg, acute interstitial edema or chronic fibrosis or MI), GBCA accumulates in the extracellular space, and its washout is delayed after the injection.
      • ∴, areas of abnormal myocardium (eg, infarcted, fibrotic, inflamed, or infiltrated myocardium) will have elevated per-voxel GBCA concentration and therefore will have a bright signal on T1-weighted images relative to the surrounding normal myocardium.

Layers of the MRI Scanner

  • Main magnet ()
  • Gradient coils ()
    • Series of coils within main magnet
    • Can be turned on/off
    • Allow for spatial localization
    • Safety issues:
      • Loud noise
      • Peripheral nerve stimulation - temporary
  • Radiofrequency coils ()
    • Innermost layer of coils
    • Turn on/off
    • Help generate MRI image
    • Safety issues:
      • Bums
      • Device malfunction

When should I order a cardiac MRI?

MRI instead of Echo for EF follow-up assessment

Orme says replace your 3 month follow-up Echo with a CMR (more accurate assessment of EF and assess for other cardiomyopathies).

T1- and T2-weight imaging

Understanding T1 and T2-weighted MRI images can be a bit tricky, but I’ve got a simple explanation and a helpful mnemonic to make it easier for you.

What do T1 and T2 mean?

T1 and T2 are types of magnetic resonance imaging (MRI) sequences that use different techniques to generate images. The “T” stands for “time,” and the numbers refer to the time constants used in the imaging process.

  • T1: longitudinal relaxation
  • T2: vertical relaxation

T1-weighted images:

T1-weighted images are created by using a short repetition time (TR) and a short echo time (TE). This means that the MRI machine sends a pulse of energy to the body, and then quickly measures the signal that bounces back. T1-weighted images are sensitive to the T1 relaxation time of tissues, which is the time it takes for protons (hydrogen atoms) to realign with the magnetic field after being disturbed.

T2-weighted images:

T2-weighted images use a longer TR and TE. The MRI machine sends a pulse, waits a bit longer, and then measures the signal. T2-weighted images are sensitive to the T2 relaxation time of tissues, which is the time it takes for protons to lose their coherence and stop emitting a signal.

Key differences:

  • T1-weighted images:
    • Show anatomy well (e.g., bone, muscle, and fat)
    • Make fat appear bright (hyperintense)
    • Make water appear dark (hypointense)
  • T2-weighted images:
    • Show pathology well (e.g., edema, tumors, and inflammation)
    • Make water appear bright (hyperintense)
    • Make fat appear dark (hypointense)

Mnemonics

  • WW2 (Water is White in T2)
    • Fat is bright in T1
  • “T1: Tissues, T2: Tears”
    • T1-weighted images are good for looking at Tissues (anatomy)
    • T2-weighted images are good for looking at Tears (pathology, like edema or inflammation, which can cause tissues to appear “tearful” or bright)
  • “T1: Fat is Fancy, T2: Water is Wonderful”
    • T1-weighted images make Fat appear Fancy (bright)
    • T2-weighted images make Water appear Wonderful (bright)

FAQs

  • Is my patient too big for a Cardiac MRI?
    • Table weight limit is 400 lbs, but distribution of weight is the more important feature.
    • Wide bore MRI scanners can help

Note

There is no “open MRI” for Cardiac MRI.

  • Is my patient’s kidney function too poor for a Cardiac MRI?
    • Lot of Cardiac MRI scans don’t require contrast
    • We do need contrast for late-gadolinium enhancement
    • Nephrogenic Systemic Fibrosis
      • first described in HD patients in the late 90s; NSF d/t systemic manifestations
    • Avoid gadolinium in patients with poor renal function: severe renal disease (eGFR <30 mL/min/1.73 m.2)
      • Some exceptions are made with shared decision making, patient consent
    • Newer agents (macrocyclic agents) should be fine w/ poor renal function(?)
      • Compared to older (linear) agents
  • Can I order a Cardiac MRI if my patient has a device (e.g. pacemaker, ICD)?
    • If pt is pacemaker dependent, MRI may suppress the function of the pacemaker. Thus, some adjustments may be needed prior to the MRI.
    • If ICD, Cardiac MRI could make device think pt in VT → inadvertent shock!
  • Can I order Cardiac MRI on a pregnant 🤰 or lactating woman 🤱?
    • Gadolinium is potentially teratogenic
    • Gadolinium does go into the breastmilk for 24-48 hrs, so can “pump and dump”

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