Slice Thickness, Slice Position and Separation

Nuclear Associates 76-908 & 76-907

Operators Manual

•The high contrast resolution should remain constant for repeated measurements under the same scan conditions and should be equal to the pixel size. For example, for a 25.6 cm field of view with a 256 x 256 acquisitions matrix, the resolution should be 1 mm.

2.3.4 Slice Thickness

Slice thickness is defined as the full width at half maximum (FWHM) of a slice profile. The full width at tenth maximum (FWTM) is an additional descriptor of the slice profile. The slice profile is defined as the response of the magnetic resonance imaging system to a point source as it moves through the plane of the reconstruction at that point.

Factors Affecting Slice Thickness include the following:

(1)Gradient field non-uniformity

(2)RF field non-uniformity

(3)Non-uniform static magnetic field,

(4)Non-coplanar slice selection pulses between excitation and readout gradient

(5)TR/T1 ratio

(6)RF pulse shape and stimulated echoes

Any typical multi-slice acquisition may be used provided TR is greater than 3T1 of the filler material and the highest pixel resolution is used. Slice thickness should be measured both centrally and peripherally within an image and at both central (magnet isocenter) and offset slice locations.

High signal ramps (HSR) at 90 degrees to each other are used for slice thickness measurement. The ramps have 1 mm gaps on one side and 2 mm gaps on the opposite side

Slice thickness (FWHM, FWTM):

In the resultant image, the signal level is read out across the ramp on a pixel-by-pixel basis along a line- of-interest oriented orthogonally to the ramp width dimension. As noted previously, to assure adequate S/N, it may be necessary to either use multiple excitations or several line profiles. The FWHM or FWTM parameters should be determined for each of the dual ramps. The FWHM from imaging opposed high signal ramps oriented at a 45 degree angle with respect to the image plane is equal to the square root of the product of a and b where a and b are the FWHM of the intensity profile of ramps I and 2.

Assuring adequate measurement accuracy, the measured value of slice thickness should generally agree with the indicated slice thickness within the 1 mm for slice thicknesses greater than 5 mm.

2.3.5 Slice Position and Separation

Slice position is the absolute location of the midpoint of the FWHM of the slice profile. Slice separation is the distance between any two-slice positions. Slice locations are indicated by external positioning devices or by the selected inter-slice spacing.

Factors Affecting Slice Position/Separation

(1)Misalignment of positioning devices

(2)Gradient field non-uniformity

(3)B-1 non-uniformity

(4)Non-coplanar slice selection pulses

(5)Static magnetic field

The midpoint of the FWHM of the slice profile in the image of interest is determined. The distance from the profile midpoint to a landmark that remains stationary from slice-to-slice (parallel to the slice selection

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