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TitleMRI in Practice
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Table of Contents
                            Title Page
Preface to the Fourth Edition
Chapter1 Basic principles
	Atomic structure
	Motion in the atom
	MR active nuclei
	The hydrogen nucleus
	The Larmor equation
	The MR signal
	The free induction decay signal (FID)
	T1 recovery
	T2 decay
	Pulse timing parameters
Chapter 2 Image weighting and contrast
	Image contrast
	Contrast mechanisms
	Relaxation in different tissues
	T1 contrast
	T2 contrast
	Proton density contrast
	T2* decay
	Introduction to pulse sequences
Chapter 3 Encoding and image formation
		Slice selection
		Frequency encoding
		Phase encoding
	Data Collection and Image Formation
		K space description
		K space filling
		Fast Fourier transform (FFT)
		Important facts about K space
		K space traversal and gradients
		Options that fill K space
		Types of acquisition
Chapter 4 Parameters and trade-offs
	Signal to noise ratio (SNR)
	Contrast to noise ratio (CNR)
	Spatial resolution
	Scan time
	Decision making
	Volume imaging
Chapter 5 Pulse sequences
	Spin Echo Pulse Sequences
		Conventional spin echo
		Fast or turbo spin echo
		Inversion recovery
		Fast inversion recovery
		STIR (short tau inversion recovery)
		FLAIR (fluid attenuated inversion recovery)
		IR prep sequences
	Gradient Echo Pulse Sequences
		Conventional gradient echo
		The steady state and echo formation
		Coherent gradient echo
		Incoherent gradient echo (spoiled)
		Steady state free precession (SSFP)
		Balanced gradient echo
		Fast gradient echo
		Single shot imaging techniques
	Parallel Imaging Techniques
Chapter 6 Flow phenomena
	The Mechanisms of Flow
	Flow Phenomena
		Time of flight phenomenon
		Entry slice phenomenon
		Intra-voxel dephasing
	Flow Phenomena Compensation
		Even echo rephasing
		Gradient moment rephasing (nulling)
		Spatial pre-saturation
Chapter 7 Artefacts and their compensation
	Phase mismapping
	Aliasing or wrap around
	Chemical shift artefact
	Out of phase artefact (chemical misregistration)
	Truncation artefact
	Magnetic susceptibility artefact
	Cross=-excitation and cross talk
	Zipper artefact
	Shading artefact
	Moiré artefact
	Magic angle
Chapter 8 Vascular and cardiac imaging
	Conventional MRI vascular imaging techniques
	Magnetic resonance angiography (MRA)
	Cardiac MRI
	Cardiac gating
	Peripheral gating
	Pseudo gating
	Multiphase cardiac imaging
Chapter 9 Instrumentation and equipment
	Permanent m agnets
	Superconducting electromagnets
	Fringe fields
	Shim coils
	Gradient coils
	Radio f requency (RF)
	Patient transportation system
	MR computer systems and the user interface
Chapter10 MRI safety
	Government guidelines
	Safety terminology
	Hardware and magnetic field considerations
	Radio f requency fields
	Gradient magnetic
	The main magnetic  field
	Siting considerations
	MRI f acility zones
	Safety education
	Protecting the general public from the fringe field
	Implants and prostheses
	Devices and monitors in MRI
	Patient conditions
	Safety policy
	Safety tips
Chapter 11 Contrast a gents in MRI
	Mechanism of action of contrast agents
	Molecular tumbling
	Dipole–dipole interactions
	Magnetic susceptibility
	Gadolinium safety
	Other contrast agents
	Current applications of gadolinium contrast agents
Chapter 12 Functional imaging techniques
	Diffusion weighted imaging (DWI)
	Perfusion imaging
	Susceptibility weighting (SWI)
	Functional imaging (f MRI)
	Interventional MRI
	MR spectroscopy (MRS)
	Whole body imaging
	MR microscopy (MRM)
Document Text Contents
Page 229

Flow phenomena Chapter 6


Figure 6.14 Spatial pre - saturation.

saturated to 180 ° , it has no transverse component of magneti zati on and produces a signal void
(Figure 6.14 ).

To be eff ecti ve, pre - saturati on pulses should be placed between the fl ow and the imaging stack
so that signal from fl owing nuclei entering the FOV is nullifi ed. In sagitt al and axial imaging, pre -
saturati on pulses are usually placed above and below the FOV so that arterial fl ow from above
and venous fl ow from below are saturated. Right and left pre - saturati on pulses are someti mes
useful in coronal imaging (especially in the chest), to saturate fl ow from the subclavian vessels.

Spati al pre - saturati on pulses can be brought into the FOV itself. This permits artefact - producing
areas (such as the aorta) to be pre - saturated so that phase mismapping can be reduced ( see
Chapter 7 ). Pre - saturati on pulses are only useful if they are applied to ti ssue. If they are applied
to air they are not eff ecti ve. They increase the amount of RF that is delivered to the pati ent, which
may increase heati ng eff ects ( see Chapter 10 ). The use of pre - saturati on pulses may also decrease
the number of slices available and should therefore be used appropriately.

Pre - saturati on pulses are also only eff ecti ve if the fl owing nucleus receives the 90 ° pre - saturati on
pulse. Pulses are applied around each slice just before the excitati on pulse. The TR and the number
of slices therefore govern the interval between the delivery of each pre - saturati on pulse. To opti -
mize pre - saturati on, use all the slices permitt ed for a given TR. As pre - saturati on produces a signal
void, it is usually used in T1 and proton density weighted images where fl uid (blood and CSF) is
dark anyway. Figures 6.15 and 6.16 show axial T1 weighted gradient echo images of the abdomen
with and without pre - saturati on. Ghosti ng of the aorta seen on Figure 6.15 is largely eliminated
by using spati al pre - saturati on pulses in Figure 6.16 . Note also that the signal intensity of the aorta
is reduced by using pre - saturati on.

Page 230

Chapter 6 MRI in Practice


Figure 6.15 Axial T2 * coherent gradient echo through the abdomen demonstrating fl ow
artefact in the aorta. No spatial pre - saturation was used.

Pre - saturati on nullifi es signal and can therefore be used specifi cally to eliminate certain signals.
The main uses of this are:

• chemical pre - saturati on
• spati al inversion recovery (SPIR).

Chemical p re - s aturation
Hydrogen exists in diff erent chemical environments in the body, mainly fat and water ( see Chapter
2 ). The precessional frequency of fat is slightly diff erent from that of water. As the main magneti c
fi eld strength increases, this frequency diff erence also increases. For example at 1.5 T the preces-
sional frequency between fat and water is approximately 220 Hz, so fat precesses 220 Hz lower
than water. At 1.0 T this frequency diff erence is reduced to 147 Hz. The frequency diff erence
between fat and water is called chemical shift and can be used to specifi cally null the signal from
either fat or water. This technique is important to diff erenti ate pathology (which is mainly water)
and normal ti ssue (which oft en contains fat). To saturate or null either fat or water, the preces-
sional diff erence between the two must be suffi ciently large so that they can be isolated from
each other. Fat or water saturati on is therefore most eff ecti vely achieved on high fi eld systems.

Page 458

Index MRI in Practice


white matt er (brain) conti nued
suppression, FLAIR, 160, 163
T1 and T2 relaxati on ti mes, 28, 167

White Paper on MRI Safety
American College of Radiology, 342–3, 369–70

on contrast injecti on, 380
warning signs, 361

whole body imaging, 410
windows, ‘pop-out’, 353
window setti ngs, 339, 426

see also acquisiti on windows

pacemakers, 367
see also cables

moiré artefact, 256
see also aliasing

Z-axis, 61
zipper artefact, 255–6, 259

MRI safety, 356, 358–9

Page 459

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