cardiac MRI made easy Chapter 1-1

• Cardiovascular magnetic resonance (CMR) describes the use of magnetic resonance imaging (MRI) for the anatomical and functional evaluation of the heart and vascular tree
• CMR is a contemporary and complementary technology which is rapidly gaining popularity in this dynamic field
• This book provides a more easily digestible synopsis which is readily portable and can be rapidly updated
• The Royal Brompton Hospital has been at the vanguard of CMR throughout its journey from a fledgling research idea to fully grown clinical ideal
• The text is targeted predominantly at the needs of cardiologists and cardiothoracic surgeons wishing to acquaint themselves with CMR — what it can do and what it cannot
• The chapters concentrate on the cardiac side of CMR but also address its more established vascular uses in a later section
• Before both of these, the book outlines some of the basic principles of MRI
• The statements made in this book refer to CMR performed at a magnet field strength of 1.5 Tesla
• Cardiovascular magnetic resonance (CMR) describes the use of magnetic resonance imaging (MRI) for the anatomical and functional evaluation of the heart and vascular tree
• CMR is a contemporary and complementary technology which is rapidly gaining popularity in this dynamic field
• This book provides a more easily digestible synopsis which is readily portable and can be rapidly updated
• The Royal Brompton Hospital has been at the vanguard of CMR throughout its journey from a fledgling research idea to fully grown clinical ideal
• The text is targeted predominantly at the needs of cardiologists and cardiothoracic surgeons wishing to acquaint themselves with CMR — what it can do and what it cannot
• The chapters concentrate on the cardiac side of CMR but also address its more established vascular uses in a later section
• Before both of these, the book outlines some of the basic principles of MRI
• The statements made in this book refer to CMR performed at a magnet field strength of 1.5 Tesla
• MRI


  Based on nuclear magnetic resonance, the phenomenon of the resonance of atomic nuclei in response to radiofrequency (RF) waves
• Hydrogen atom


  The simplest and most abundant element in the body, consisting of one proton nucleus orbited by one electron
• Proton


  The hydrogen nucleus, which has a magnetic axis that normally is randomly orientated
• Proton alignment in magnetic field

  1. Protons align in synchrony and spin around an axis in line with the main magnetic field - this spinning is termed precession

  2. Precession frequency changes linearly with increasing magnetic field strengths

  3. At equilibrium, overall proton alignment is in the direction of the main magnetic field and they have net longitudinal magnetization
• Proton excitation


  Degree of proton excitation is proportional to the amplitude and duration of the RF pulse
• Proton relaxation


  • After excitation, proton relaxation occurs as the energy is dissipated, defined by two parameters T1 and T2
  • T1 relaxation measures the time to recover the longitudinal magnetization
  • Transverse magnetization decays at a rate measured by T2, which is faster than the rate of T1 recovery
• T1 and T2 relaxation

  • T1 and T2 values vary according to the environment of the hydrogen atom within tissues
  • T1 and T2 values tend to parallel each other when proton motion is relatively random
  • Tissues with a more organized structure contain abundant bound water, leading to shorter T2 values than T1
• Localization of anatomical position

  1. Done with the application of frequency- and phase-encoding gradients

  2. The corresponding direction of application of these gradients is known as the frequency encode or phase encode direction

  3. Modifications of the phase-encoding gradients can differentiate flowing blood from stationary anatomy via alterations in the phase of the MR signal

  4. Velocity of material is proportional to the phase change or phase shift caused by its movement during gradient application
• Transmission and reception of RF energy

  Via special aerials known as coils, with subsequent conversion of raw data into images using ultrafast computers and Fourier transformation
• Main sequences


  • Gradient echo (GE)
  • Spin echo (SE)
• Gradient echo (GE) sequences

  Blood and fat appear white, also known as white-blood imaging
• Spin echo (SE) sequences

  Blood is usually black but fat is white, giving rise to the term black-blood imaging
• Cine imaging using SSFP

  • Allows visualization of areas of focal myocardial dysfunction and abnormal flow patterns
  • Obtained by rapid repetition of a variant of the basic GE sequence to obtain a series of cardiac images at progressively advancing points of the cardiac cycle
• SSFP sequence


  Weighting depends on the ratio of T2/T1, so most fluids and fat have a high signal and appear white, while muscle and other solid tissues have a long T1 and short T2 so appear in shades of grey
• Velocity mapping


  1. Determines the average velocity within a single imaging voxel

  2. Operator selects the required plane and sets a maximal encoding velocity (Venc)

  3. Venc represents the practical upper limit of velocities that can be depicted unambiguously and should be set just greater than the true velocity

  4. Velocity aliasing occurs if Venc is set much higher or lower than the true velocity
• Spin echo (SE) sequences


  • More robust to system imperfections like magnetic field inhomogeneities
  • Structures need to be stationary for the delivery of two RF pulses
  • Flowing blood moves out of the selected slice before receiving the second pulse and so gives no signal and appears black
• Variations of SE sequences

  • Fast (or turbo) spin echo (FSE or TSE)
• Velocity mapping CMR


  Used to confirm abnormal chamber communication and the ratio of pulmonary to systemic flow in shunts such as septal defects
• SE pulse sequences


  • More robust to system imperfections, such as magnetic field inhomogeneities
  • Structures need to be stationary for the delivery of two RF pulses
  • T2 value of stationary fluid is long and gives high signal
  • Flowing blood moves out of the selected slice before receiving the second pulse and so gives no signal and appears black
  • Slower flowing blood can give persistent signal of varying signal intensity
• Fast (or turbo) spin echo (FSE or TSE)


  Allows faster imaging than standard SE by acquiring more lines of data for every RF pulse delivered and allows acquisition of an entire image in a single heartbeat
• Inversion recovery technique


  Uses a prepulse to create high T1 tissue contrast which is important for infarct imaging
• Contrast-enhanced magnetic resonance angiography (CE-MRA)


  Requires use of a contrast agent
• MRI contrast agents


  • Commonly based on chelates of gadolinium which are paramagnetic, one example being gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA)
  • All gadolinium chelates currently approved for clinical use are extravascular and therefore become distributed within the interstitium following initial intravenous delivery
• CMR is performed by applying these main sequences and their variants to evaluate cardiovascular physiology and anatomy, characterize tissue, and perform vascular angiography