The four main considerations determining image quality are:

103-104

• signal to noise ratio (SNR)
• contrast to noise ration (CNR)
• spatial resolution
• scan time

The ___________ is the ratio of the amplitude of the signal received to the average amplitude of the noise.

104

The ________ is the voltage induced in the receiver coil by the precession of the NMV in the transverse plane.

104

The ______ represents frequencies that exist randomly in space and time.

104

The noise is constant for every patient and depends on three things:

104

• the build of the patient
• the area under examination
• the inherent noise of the system

The factors that affect SNR include:

104

• magnetic field strength of the system
• proton density of the area under examination
• voxel volume
• TR, TE and flip angle
• NEX
• receive bandwidth
• coil type

The magnetic field strength plays an important part in determining __________.

104

The NMV decreases/increases in size at higher field strengths and as a result there is more available magnetization to image the patient. SNR therefore decreases/increases.

104

The number of ____________ in the area under examination determines the amplitude of signal received.

105

Any selection that decreases the size of the ________ decreases the SNR and vice versa.

106

Decreasing the size of the voxel is done in three ways:

106

• changing the slice thickness
• changing the image matrix
• changing the FOV

T2 weighted sequences that use a short/long TE usually have a lower SNR that T1 or PD weighted sequences that use a short/long TE.

110

A  short/long TR increases SNR and a short/long TR reduces SNR.

111

A  short/long TE reduces SNR and a short/long TE increases SNR.

111

The lower the flip angle, the lower/higher the SNR.

111

This is the number of times data are collected with the same amplitude of phase encoding slope.

114

Number of signal averages

(NEX, NSA, Naq)

As the receive bandwidth decreases, the SNR decreases/increases as less noise is sampled as a proportion of signal.

122

_________ placed close to the area under examination also increase the SNR.

123

To induce maximum signal, the surface coil must be positioned ___________________.

123

The _____________ is defined as the difference in the SNR between two adjacent areas.

123

The CNR is increased in the following ways:

123

• use a T2 weighted image
• use contrast agents
• use chemical pre-saturation technique
• use magnetization transfer contrast

The _________ resolution is the ability to distinguish between two points as separate and distinct, and is controlled by the voxel size.

126

In large voxels, individual signal intensities are averaged together and not represented as distinct within the voxel. This results in _____________.

126

When scanning anatomy that has a smaller dimension in the phase axis then frequency, a _______________ may be desired.

127

The spatial resolution can be maintained by:

132

• selecting as thin a slice as possible
• selecting a fine matrix
• selecting a small FOV
• selecting rectangular FOV where possible

Resolution is controlled by the size of the ________.

133

To achieve thin slices, the slice select gradient slope is shallow/steep.

133

To achieve a small FOV, the frequency encoding gradient is shallow/steep.

133

To achieve a fine matrix, a high proportion of the phase encoding gradient slopes are shallow/steep.

133

To achieve the shortest scan time:

133

• use the shortest TR possible
• select the coarsest matrix possible
• reduce the NEX to a minimum

When it comes to selecting image parameters, it is inadvisable to select:

135

• a very short TR in spin echo sequences (choose 400ms not 200ms)
• a very long TE (choose 100ms not 200ms)
• very low flip angles (choose 20º not 5º)
• very thin slices (choose 4mm not 3mm)
• a very small FOV (choose 120mm not 80mm), unless you are using a good local coil

__________ imaging is advantageous in that very small lesions can be demonstrated because the slice thickness can be drastically reduced compared with conventional imaging, and there is no slice gap.

137

To obtain equal resolution in every plane and at every angle of obliquity, each voxel should be ____________.

138

Volume imaging has many potential applications, but it is widely used for imaging of ___________, where anatomy is often confusing and not strictly in plane.

139