Test of slice thickness at the centre of the field of view

4.3.7. Test of slice thickness at the centre of the field of view

Purpose of test To test the thickness of a tomographic slice at the centre of the field of

view. To ensure that the spatial resolution along the tomographic Z axis is within acceptable limits.

Materials

A small point source of 99m Tc, as used for the test of tomographic resolution in air (Section 4.3.4) and the test of centre of rotation offset (Section 4.3.6).

Procedure The data collected for this test are the same as those acquired for the test

of tomographic resolution in air (Section 4.3.4). Only the analysis of those data differs here and the same raw data may be used for this test.

(1) Place the point source in air within 1 cm of the centre of rotation, near the centre of the field of view. (2) Set the radius of rotation to be approximately 15 cm, or, if this cannot be

achieved, to be as small as possible. Use a circular orbit of rotation. (3) Perform a tomographic acquisition using the normal matrix size and number of angles used clinically, collecting about 10 000 counts per angle. (4) Reconstruct the data with filtered backprojection, using either a ramp filter or the sharpest filter that the system will permit. Do not sum transaxial slices together; a slice thickness of a single pixel must be used.

Data analysis (1) Locate the slice in which the point source is most clearly seen (and is a

maximum amplitude) and locate the pixel in which the maximum number of counts is observed. Note the (X, Y) coordinates of this pixel and note this maximum value.

(2) Record the number of counts at this same (X, Y) pixel position for all slices adjacent to and including the slice in which the maximum was found, such that all slices containing counts of more than 5% of the maximum are included. Generate a profile of the point source along the Z axis using these values.

(3) Calculate the FWHM of this profile (Section 2.3.8) and convert into millimetres using the pixel size as determined in Section 4.3.2.

Observations This test is intended to be performed as a reference test and at half-yearly

intervals.

It is particularly important that the point source be small and that only a single point source be employed. It is common when filling a syringe that activity remains in the needle used to fill the syringe, which will result in the presence of a second (albeit small) point source. While this does not affect the results obtained for the resolution test given in Section 4.3.4, it is likely to affect the results given here. The presence of such a secondary point source may be observed by inspection of any of the original (raw) projection images. If this is observed, change the needle for a clean one, or a blind cap. Reconstructions close to, but not including, the point source may contain significant streak artefacts, which should be ignored.

Interpretation of results The FWHM for the slice thickness (here, as measured in air) should be

the same as the normal transverse tomographic resolution in air for the distance corresponding to the radius of rotation.

Limits of acceptability The slice thickness at the centre should be within 10% of the tomographic

resolution as determined in Section 4.3.4. Conclusion

Record whether or not the results confirm acceptable performance. If not, indicate the follow-up action taken.