01-Physics of Mri

PHYSICS OF CARDIAC MRIPHYSICS OF CARDIAC MRI

By:

Azlan Che Ahmad, PhD

Department of Biomedical Imaging

University of Malaya

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Overview

• Challenges in cardiac MRI

• Cardiac gating

• Pulse sequences

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Introduction

• More difficult compared to neurologicaland musculoskeletal MRI

• Major challenges– Heart motion

– Respiratory

– Pulsatile blood flow

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Challenges in Cardiac MRI

• Motion during image acquisition willdegrade the image. Commonly result inghosting artefacts

• Arises because of changes in signalintensity or phase as a function of timeduring MR signal acquisition

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A typical ghosting artefact causes by motion during image acquisition

(Busberg et al., 2002)

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MR signal K-space MR image

Steps in typical MR image production

• Conventional MRI is relatively slow. Motionoccurring during signal acquisition will degrade theimage

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Imaging time = TR x PE no. x “slice no.” x NSA


e.g.: 3D spoiled gradient echoTR = 10 ms, PE no.= 256, “slice no.”= 50, NSA = 1Imaging time = 10 ms x 256 x 50 x 1 = 2 min 8 s

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Cardiac Gating

Image data are synchronized with the periodicity of the beating heart

• Motion artefact can be reduced by ensuringthat the signal is acquired at the same instantin the motion cycle of the object being imaged

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Cardiac Gating

Cardiac-gated MRI is performed by monitoring the patient’s ECG using MR compatiblemonitoring equipment

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Cardiac Gating

• The detected R-wave is used to trigger RF excitation

Cardiac-gated pulse sequence with one excitation per heartbeat.

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Cardiac Gating

Cardiac gated pulse sequence with multiple excitation per heartbeat for dynamicimaging.

(McRobbie et al., 2007)

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Cardiac Gating

Cardiac gated imaging with segmented k-space data acquisition

(McRobbie et al., 2007)

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Breathing Motion

• Can be minimized by:–Breath holding

–Respiratory gating

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Cardiac Gating and Breath holding

(a) Ghosting artefact causes by of poor breath holding and cardiac gating, (b) aghosting of the heart alone causes by a poor cardiac gating, and (c) No artefactsby adequate breath holding and cardiac gating .

(Saremi et al., RadioGraphics 2008)

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Pulse Sequences in Cardiac Imaging

• Rapid acquisition with relaxation enhancement SE– TSE, FSE

• Fully rewound GE– True-FISP, FIESTA, balance FFE

• Spoiled GE– FLASH, SPGR, T1-FFE

• Rapid acquisition GE– Turbo FLASH, fast-SPGR, TFE

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Dual IR Pulse Sequence

• Used to null signal from flowing blood (black blood)

A simplified pulse sequence diagram for cardiac-gated dual IR-TSE.16

(Philips application guide)



Dual IR Pulse Sequence

Blood inside the slice that received both inversion pulses is replaced by blood thatonly received the first non-selective inversion pulse.

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(Philips application guide)



Summary

• Subject motions are the main challange in cardiacMRI

• The ghosting artefact causes by motions can bereduced by breath holding and cardiac-gating

• Imaging time in cardiac MRI can be reduced byusing a segmented k-space technique

• Different types of pulse sequences can beimplemented in cardiac MRI (depending on theinformation required)

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REFERENCES

• Bushberg et al. The essential physics of medicalimaging, Philadelphia, Lippincott Williams &Wilkins

• McRobbie et al, MRI from picture to proton,Cambridge Uni. Press

• Saremi et al., optimizing cardiac MR imaging:practical remedies for artifacts, RadioGraphics2008, 28:1161-1187

• MR application guide: cardiac imaging, PhilipsHealthcare.

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