Cardiac MRI Basics

• Cardiac MRI are usually done on 1.5 Tesla (30,000 times the Earth’s magnetic field)
• ⚠️ Safety issue: the magnet 🧲 is always on
• In the presence of a strong magnetic field (typically 0.5 – 3.0 Tesla (T) for clinical applications) atoms ⚛︎ in the body (typically hydrogen) are stimulated to emit radio waves 🔊. These radio waves 🔊 are detected by an antenna (more specifically, the radiofrequency coils) placed around, or over, the body part of interest allowing an image of the body to be reconstructed.
  • Extra magnetic fields (using gradient coils) are used to constantly change the magnetic field to allow images of the body to be reconstructed.
• The typical magnetic field strengths used in clinical MRI scanners are:
  • 1.5 Tesla
  • 3 Tesla
• Excitation: The RF pulse (radiowave) causes a proton to be excited → flips from low- to high-energy state
• Relaxation
  • T1 Recovery
    • the natural tendency is to from high-energy state back to a low-energy state
    • there is a predictable rate of regrowth/relaxation.
    • In the figure below, you are going from the high-energy state in the transverse plane ($M_{xy}$) where the flip angle is 90˚ (100% transverse magnetization and 0% longitudinal magnetization) and there is “regrowth” towards the low-energy state where the flip angle is 0˚ in the longitudinal axis ($M_z$) and the protons are aligned upright in the direction of the main magnetic field.
    • When this rate of regrowth is mapped over time, the T1 time constant is the amount of time it takes to regain 63% of the intrinsic (full) magnetization ($M_0$)
    • 
    • 📝 T1 time is tissue-specific, i.e. different tissues have different T1 times
      • Fat has short T1 times, i.e. hydrogen protons that are within fat will have very fast T1 recovery
        • ~200-250 ms
      • Water has very slow T1 recovery times
        • ~2,000 ms (10-fold greater T1 relaxation time compared to fat❗)
      • 
        • Figure source
  • T2 Decay (aka spin-spin relaxation)
    • When the RF excitation pulse is applied, the protons are all spinning in-phase. Immediately after this pulse is applied, they begin to dephase at a rate proportional to the amount of the magnetic field → eventually, protons will return to their out-of-phase initial pre-RF excitation state, as measured by the T2 relaxation time. (Source)
    • The rate of dephasing is different for each tissue, resulting in further tissue contrast.
    • 
      • 📝 this “dephasing” is occurring in the $xy$-plane
    • T2 decay time is the amount of time it takes to drop your magnetization to 37% of what it was after you first applied a 90˚ RF pulse.
      • 
    • T2 decay time is tissue-specific
      • Fat: T2 decay time is fast 🏎️ at 84 ms
      • Water: T2 decay time is longer (~1400 ms)
    • 
• The Larmor equation is a mathematical formula that describes the precession frequency of nuclei in a substance placed in a static magnetic field
  • $\omega =\gamma \cdot B$
  • used to calculate the frequency of an RF signal that causes a change in the nucleus spin energy level.
  • $\gamma$: the gyromagnetic ratio is different for each nucleus of different atoms.
  • $B$: the stronger the magnetic field ($B$), the higher the precessional frequency ($\omega$).
    • $B=B_0+G_z\cdot z$ (instead of $z$, also applies to $x$, $y$)
  • At 1.5T, the protons are spinning around 64 MHz
  • At 3T, protons are spinning around at 128 MHz
  • 

Earth's 🌎 magnetic field 🧲

The Earth’s magnetic field, which is typically around 25-65 microtesla (µT). Therefore, 1 Tesla is about 30,000 times the earth’s magnetic field.

MRI Machine

• Layers of the MRI scanner coils (listed from outer-to-inner)
  • Main magnet coils ($B_0$)
    • defines the strength of the constant magnetic field ($B_0$), which is typically measured in Tesla units.
  • Gradient coils ($G$)
    • Series of coils within main magnet
      • 3 sets: $x$, $y$, and $z$ directions
        • The $z$-axis is parallel to $B_0$
      • 
    • Make slight alterations to the local magnetic field in a graded fashion
      • e.g. consider the gradient along the $z$-axis. At some portion of the body (let’s say the middle of the body), the magnetic field strength is at isocenter (1.5 in a 1.5T scanner, 3 in a 3T scanner) and moving towards the head may be 1.51, 1.52, etc. (∴ precession frequency $\omega$ will be higher towards the head) and towards the feet may be 1.49, 1.48, etc. ($\omega$ will be lower moving towards the feet)
    • Units are measured in milliTesla/meter (mT/m)
    • Can be turned on/off
    • Allow for spatial localization given the coordinate axis reference system afforded by the $x$, $y$, and $z$ axes. ∴, allows you to register all of your images b/c you are in a fixed coordinate system.
    • Safety issues:
      • Loud noise
        • make the familiar banging sounds of an MRI scan
      • Peripheral nerve stimulation - temporary
  • Radiofrequency coils ($B_1$)
    • Innermost layer of coils
    • There are 2 sets of RF coils:
      • transmitter coils
        • send in a RF pulse → excitation of protons
        • generally located within the MRI scanner itself
      • receiver coils
        • receive radiowaves → signal is processed to generate your image
        • typically flexible coils places around the patient; best quality if they are as close as possible to the area you’re trying to image
      • 
    • Turn on/off
    • Help generate MRI image
    • Safety issues:
      • Burns
      • Device malfunction

Terminology

Term	Meaning
T1 [ms]	Time constant representing the recovery of longitudinal magnetization (spin-lattice relaxation)
Native T1	T1 in the absence of an exogenous contrast agent
T2 [ms]	Time constant representing the decay of transverse magnetization (spin-spin relaxation)
T2* [ms]	Time constant representing the decay of transverse magnetization in the presence of local field inhomogeneities
ECV [%]	Extracellular volume fraction, calculated by $ECV=\frac{\frac{1}{T{1}_{{\text{myo}}_{\text{post-Gad}}}}-\frac{1}{T{1}_{{\text{myo}}_{\text{native}}}}}{\frac{1}{T{1}_{{\text{blood}}_{\text{post-Gad}}}}-\frac{1}{T{1}_{{\text{blood}}_{\text{native}}}}}\times (100-\text{Hct})$ where myo = myocardium; blood = intracavitary blood pool; Hct = cellular volume fraction of blood [%]
Synthetic ECV [%]	ECV where hematocrit is not measured by laboratory blood sampling but derived from blood T1
Parametric mapping	A process where a secondary image is generated in which each pixel represents a specific magnetic tissue property (T1, T2, or T2*) or a derivative such as ECV) derived from the spatially corresponding voxel of a set of co-registered magnetic resonance source images

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”
• How can I learn how to do Cardiac MRI?
  • Combined Cardiac MRI/CT rotation
• When should I order a Cardiac MRI?
  • NICM
    • Amyloid, Sarcoid, Iron overload, Fabry’s, Non-compaction
  • Adult Congenital Heart Disease
  • Hypertrophic Cardiomyopathy
    • Echo better at detecting peak LVOT gradient, but MRI better at wall thickness, late gadolinium enhancement, LV apical aneurysm
  • Arrhythmia w/u: assess for ARVC, Sarcoid
  • r/o LV thrombus
  • Pulmonary/AV regurgitation