Test of tomographic uniformity of the system

4.3.3. Test of tomographic uniformity of the system

Purpose of test To test the tomographic uniformity of a rotating scintillation camera

SPECT system.

Materials The tomographic uniformity phantom should be filled with about

200-400 MBq (5–10 mCi) of 99m Tc, making sure that the activity is well mixed. Procedure

This test should be performed after the test for planar uniformity has been performed.

(1) Ensure that all the camera uniformity correction calibration procedures have been correctly performed. (2) Place the phantom with its centre at least within 2 cm of the axis of rotation, as close as possible to the centre of rotation. (3) Ensure that the central axis of the phantom is parallel to the axis of rotation. (4) Set up a tomographic acquisition using a normal matrix size (e.g. 64 × 64 or 128 × 128) and the number of angles used clinically, using a circular orbit.

(5) Perform a standard tomographic acquisition, collecting a total of about one million counts per slice. This typically corresponds to 15 million total counts for a phantom 10 cm in length, or about 240 000 counts per angular position for a 64 angle acquisition.

(6) Perform uniformity correction as recommended by the manufacturer. (7) Reconstruct the data with a ramp (or sharp) filter. (8) Where possible, perform attenuation and scatter correction using the

method prescribed by the manufacturer. The attenuation correction is essential unless special purpose software is used.

Data analysis (1) Inspect images of the phantom at various transaxial positions.

(2) Place a profile about 5 pixels thick through the centre of the image (normally through the point corresponding to the centre of rotation). Estimate the depth or height of any artefacts corresponding to circular (ring) artefacts by measuring their contrast with respect to the surrounding activity, as defined in Section 4.3.3.

(3) Identify the minimum or maximum value corresponding to the location of a ring artefact as seen in the reconstructed image. Record this value, terming it C min/max .

(4) Record the two values along the profile of the uniform source just beyond the edges of the artefact identified in step (3), terming them C1 and C2. (5) Calculate C ave = (C1 + C2)/2. (6) Estimate the contrast as (C min/max –C ave )/(C min/max +C ave ). (7) Repeat for all the other transaxial sections within the phantom and

determine the maximum absolute value of contrast. (8) For a central slice, determine the central value by averaging over 5 pixels (about 3 cm for a 64 × 64 matrix) on the profile corresponding to the centre of the phantom, or use a 5 × 5 pixel region of interest to give this value.

(9) Determine the edge value by averaging over 3 cm on the profile centred

2 cm from the observed edge (50% value). (10) Where possible, measure the size of the body contour in the horizontal and vertical directions in pixels and convert into distances in millimetres.

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

intervals and whenever a uniformity problem is suspected. The uniformity of a rotating scintillation camera SPECT system must be as good as possible, because any non-uniformity is amplified by the tomographic reconstruction process. The planar uniformity of a scintillation camera when used in SPECT should be better than 4%. This is very difficult to achieve. However, if the NEMA integral uniformity index is worse than 6% after uniformity correction, it is clear that the camera needs attention and should be tuned. Compare the measured values with those obtained for conventional planar uniformity at the time of acceptance. In particular, those variations in planar uniformity lying along or close to the vertical (Y) axis are very important and the limits of acceptability should be much stricter.

All the reconstructed transaxial slices passing through the phantom should be inspected for circular (ring) artefacts, except those within 2 pixels of the edge of the phantom. It is helpful to mark the central point of the image, for example, by marking a horizontal and vertical profile through the centre of the tomographic slice. Artefacts are always circles centred about this point (for a circular orbit). All visible artefacts are significant. The measured values of contrast as calculated above are rather variable and it is advised that more than one estimate of the amplitude of a ring artefact be made. Do not sum together

a number of transaxial slices or smooth the data since this may cause ring artefacts to disappear.

If the system has not been accurately centred (see Section 4.3.6), circular artefacts may not be visible because their effects have been ‘smeared out’. When performing the attenuation correction, a typical value for the attenuation correction factor for 99m Tc is 0.12 cm –1 when no scatter correction has been performed.

Interpretation of results Thick rings, widely spaced, are usually indicative of variations in the

uniformity of the camera itself. Narrow (thin) rings, often close together, are usually an indication of errors in the camera–computer interface, or of the digital part of any uniformity correction hardware. A sharp cold or hot spot may be seen exactly at the point corresponding to the centre of rotation and represents a problem in uniformity along the projection of the axis of rotation.

A 1% non-uniformity at the centre of rotation will be amplified into about 20% non-uniformity in the reconstruction.

Limits of acceptability The contrast measured between any ring artefacts and the uniform

background, as measured using a profile, should not exceed 10%. The difference between the central value and the edge value should not exceed 10%. If this is not the case, the body contour should be checked first, followed by the attenuation correction coefficient (see Fig. 47) and pixel size used (see Section 4.3.2).

The horizontal and vertical directions measured on the body contour should be within 1 cm of the corresponding real dimensions of the phantom, if attenuation correction using this body contour is to be used.

Conclusion Record whether or not the results confirm acceptable performance. If

not, indicate the follow-up action taken.

FIG. 47. Profiles to check the accuracy of attenuation correction. A cylindrical phantom,

20 cm in diameter, containing a homogeneous solution of 99m Tc was used to acquire a high count data set. Acquisition: 128 × 128 matrix, 360º total angle of rotation, 128 projections, 800 000 counts in the first projection, radius of rotation 19 cm, circular orbit, pixel size 3.2 mm. Reconstruction: Filtered backprojection with a Butterworth filter, transverse slices. Attenuation correction was applied using different linear attenuation coefficients (Chang method). A profile was drawn across one of the transverse slices. A: No attenuation correction. B: Attenuation correction with m = 0.08 cm –1 . C: Attenuation correction with m = 0.11 cm –1 . D: Attenuation correction with m = 0.14 cm –1 . Only for the attenuation correction using m = 0.11 cm –1 is the profile through the slice essentially flat, apart from statistical fluctuations in the profile, indicating that the attenuation correction software is correct. For the image with a profile that is lower in the centre (B), the attenuation coefficient was too small. For the image with a profile that is higher in the centre (D), the attenuation coefficient was too large (see Ref. [3]).