MRI of Cardiac Function

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MRI of Cardiac Function

• Pamela K. Woodard, M.D.
• Mallinckrodt Institute of Radiology, St. Louis, MO

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Disclosures

• Discussion/presentation of pharmaceutical (MR
  contrast agents) classified by the FDA as
  investigational for the intended use.
• Research or consulting relationships with
  Siemens Medical Systems, Berlex Laboratories,
  TycoHealthcare/Mallinckrodt. GE Healthcare
  speakers bureau.

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Educational Objectives and Goals

• To teach methods of MR imaging for assessment of
  cardiac function.
• To teach methods of quantification of left
  ventricular volumes and ejection fraction.
• To teach stress protocols for functional
  assessment of myocardial ischemia and viability.

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• Image acquisition.
• Calculation of LV functional parameters at rest.
• Examples of uses of MR of cardiac function at
  rest.
• Pharmacologic stress protocols for cardiac
  ischemia and viability assessment.

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Why MR?

• Echocardiography preferred
• Widely available
• Portable
• Short exam time
• Less expensive (not really)
• Better reimbursement
• Easier to perform during exercise or
  pharmacologic stress.

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Why MR?

• MR The Gold Standard
• Very accurate 3D rather than 2D.
• Less interobserver and performance variability.
• More consistent results.

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Particularly good for reproducible follow-up
METHODS 60 subjects normal volunteers,
patients with heart failure, LV hypertrophy. 2
CMRs and 2 echocardiographic studies for
assessment of LV volumes, function, and mass.
RESULTS The interstudy reproducibility
coefficient of variability was superior for CMR
in all groups for all parameters. Statistically
significant for end-systolic volume (p lt0.001),
ejection fraction ( p lt0.001), and myocardial
mass (p lt0.001), with a trend for end-diastolic
volume ( p 0.17). The superior interstudy
reproducibility resulted in considerably lower
calculated sample sizes (reductions of 55 to
93) required by CMR compared with
echocardiography to show clinically relevant
changes in LV dimensions and function.
Grothues F, et al., Am J Cardiol, 2002
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MR Sequences

• Bright Blood Cine
• Cine GRE (TurboFLASH, fast SPGR, TFE/FFE)
• Rely on TOF effects, use inflow enhancement for
  image contrast. Blood pool to myocardial CNR can
  degrade with short TRs.
• Cine balance/completely refocused gradient GRE
  (SSFP, FIESTA, TrueFISP, balanced FFE)
• Image contrast relies on steady state effects
  (ratio of T2/T1). Exploits short TRs and TEs.
  No sacrificing SNR or CNR.
• Excellent CNR, SNR. Good for segmentation of
  blood pool.
• Short TR,TE. Short breath-holds (5-7 seconds).
• More accurate Better temporal resolution (40
  ms)

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Image Contrast GRE vs TrueFisp
GRE
TrueFisp
Endocardial border better seen on TrueFISP cine
due to high SNR CNR
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Tagged GRE Images

• Usually used for calculation of stress and
  strain, or for visual assessment of wall motion
  abnormalities. Poor blood pool to myocardial
  contrast.

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Methods of breath-hold shortening

• Parallel imaging implement SENSE/GRAPPA by a
  factor of 2.
• Will shorten breath-hold without affecting
  temporal resolution.
• Beware of shared-phases or view-sharing
  will shorten breath-hold, however any improvement
  in effective temporal resolution IS NOT an
  improvement in actual temporal resolution.

Slavin, Bluemke. Radiology, 2005 Kunz, et al.
JMRI, 2005
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Shared-Echo K-space (one-slice, breath-hold,
improved EFFECTIVE temporal resolution actual
temporal resolution does NOT improve)
ECG
Trigger
Acquisition
Acquisition Window
TR
a a a a a a a a a a a a a . . . a a a 1 2 3 4 5
3 1 2 3 4 5 3 1 . . . 3 4 5
Scan
3
1
Cardiac Phase
N
2
4
Peripheral lines of k-space are shared between
phases. Unique central lines of k-space occur in
½ the amount of time. True temporal resolution of
each cardiac phase does not change.
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EKG Triggering

• Prospective Triggering
• Measures less than entire cardiac cycle
• Very sensitive to arrhythmias and variable
  heartrates

Set acquisition to 100 ms less than R-R

• Retrospective Triggering
• Measures through entire cardiac cycle
• Less sensitive to arrhythmias and variable
  heartrates can be coupled to arrhythmia
  rejection algorithms.

Set acquisition either equal to R-R or greater,
depending on sequence type and vendor.
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Image Plane Set Up
Multiplane scout
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Vertical Long Axis (2 chamber LA)
Bisects the apex and is parallel to septum
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Horizontal Long Axis (4 Chamber LA)
Demonstrates all 4 chambers
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Short Axis
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Myocardial Function Cine Short Axis -- Cover
from base to apex
GRE cine sequence with good CNR SSFP FIESTA,
TrueFISP, balanced FFE
Base
Apex
Contiguous 8-10 mm slices.
Images courtesy of SIEMENS
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Basic Data That Can Be Acquired

• LVEDV
• LVES
• LV stroke volume
• LVEF ()
• LV mass

Also, RVEDV, RVESV, RV stroke volume, RVEF, wall
thickening
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Semiautomatic Segmentation
Courtesy of SIEMENS
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LV Volume and EF

• Simpsons rule for LV volume sum of the area of
  blood pool on short axis images from base to apex
  multiplied by slice thickness.
• LVEF LV End Diastolic Vol LV End Systolic
  Volume
• LV End Diastolic Vol

END DIASTOLE
END SYSTOLE
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Alternate LV Volume Technique Adaptation of
Echocardiographic Method

• Select single 4-chamber LA
• Image in systole and diastole.

• Trace area (A) of LV
• endocardial border.

A

• Measure length from apex
• to mitral valve annulus.

Volume 0.85 x A2/L
LV ASSUMED TO BE AN ELLIPSE
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Myocardial Mass

• Sum of the area of myocardium on short axis
  images from base to apex multiplied by slice
  thickness to get myocardial volume (V).

myocardial density (D) 1.05 g/mL
Mass Vmyocardium (D) or Mass Vmyocardium
(1.05 g/mL)
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Nomenclature
anterior
anteroseptal
anterolateral
L
Lx
L
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Lx
R
R
posterolateral
posteroseptal
L LAD Lx L. Circumflex R RCA
posterior
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Wall Motion Analyzed Using a 17 Segment Model
1
7
L
13
L
2
6
L
8
12
Lx
L
Lx
L
16
L
17
Lx
14
Lx
Lx
R
R
5
11
R
9
R
3
R
15
10
4
Apex
Base
Mid
Based on the standards suggested by the American
Society of Echocardiography and the American
Heart Association Pina,et al., Circulation,
1995.
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Segment Wall Motion Evaluation

• Normokinetic (1 point)
• Hypokinetic (2 points)
• Akinetic (3 points)
• Dyskinetic (4 points)
• Sum of points is divided by the number of
  segments to yield a wall motion score.
• Normal contraction total score 1

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EXAMPLES
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HYPOKINETIC POSTEROSEPTAL WALL
SUBENDOCARDIAL ENHANCEMENT, POSTEROSEPTAL WALL
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THINNED, AKINETIC POSTEROLATERAL WALL
TRANSMURAL ENHANCEMENT
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DYSKINETIC ANTERIOR WALL and APEX (LV ANEURYSM)
AKINETIC ANTEROSEPTAL WALL
Two-chamber long axis
Four-chamber long axis
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SARCOID THINNED and MILDLY HYPOKINETIC
POSTEROSEPTAL REGION
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Reporting

• LVEDV
• LVES
• LV stroke volume
• LVEF ()

Describe focal regions of wall motion
abnormality at rest using correct nomenclature.
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Stress Protocols

• Ischemia high dose dobutamine. Look for
  development of focal regions of wall abnormality.
  These regions are ischemic.
• Viability low dose dobutamine. Look for
  segmental improvement in wall thickening or
  recovery in regions of dysfunctional myocardium
  at rest.

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Stress Protocol -- Ischemia
At rest and then, dobutamine IV, 5, 10, 20, 30,
40 µg/kg per minute for three minutes at each
dose level. At 40 µg/kg may add atropine (up
to 1 mg) to reach submaximal heart rate
(220-age) x 0.85. Stop at any dose level when
wall motion abnormality is observed. At each
dose level, acquire 4-chamber LA, 2 chamber LA
and 3-5 SA images.
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Stress Protocol -- Viability

• LOW DOSE Dobutamine
• Same SA and LA images at rest, 5µg and
  10µg/kg/min of IV dobutamine.
• 3 min infusion at each dose level.
• Viable dysfunctional regions show increase in
  systolic function, improvement in wall
  thickening.
• Analysis visually or measure myocardial
  thickness.

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Monitoring

• 12 lead EKG before and after study.
• Run lead II tracing during study, vital signs,
  SaO2.
• Fully stocked crash cart and defibrillator should
  be available.
• ACLS-certified personnel.

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Monitoring

• Ability to communicate with patient.
• Must watch images as they appear AND be able to
  compare with images at the lower Dobutamine
  levels to assess for wall motion abnormalities
  before going on to next step. Once abnormal wall
  motion develops STOP.
• If necessary, effects of dobutamine can be
  reversed using 0.5 mg/kg of esmolol injected IV
  as a slow bolus.

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For High-Dose Dobutamine Administration Need
Satellite Workstation

• Should be adjacent and in addition to
  technologists console ideally would like to be
  able to see images from all 4 levels
  simultaneously.

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Contraindications to Dobutamine

• Unstable angina.
• Severe aortic stenosis.
• Hypertension of 220/120 mmHg or greater.
• Complex arrhythmias.
• Hypertrophic cardiomyopathies.
• Myocarditis.

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Ischemia posterolateral hypokinesis worsens w/
dobutamine
Rest
10 µg/Kg/min Dobutamine chest pain
M. Pasque, Wash. U.
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Nonviable posterior and posterolateral walls
akinesis/wall thinned at rest, no augmentation
with dobutamine
10 µg/Kg/min Dobutamine
Rest
M. Pasque, Wash. U.
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Viable myocardium --posteroseptal akinesis,
augmented w/ low dose dobutamine
10 µg/Kg/min Dobutamine
Rest
M. Pasque, Wash. U.
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Thanks

• Michael Pasque, M.D., Cardiothoracic Surgery,
  Washington University
• Gary McNeal, M.S., Siemens Medical Systems