Test to determine the absolute size of a pixel

4.3.2. Test to determine the absolute size of a pixel

Purpose of test To determine the absolute pixel size in the matrix used for tomographic

reconstruction.

Materials One or two point sources and an accurate ruler. This test should be

performed for all the matrix sizes and tomographic zoom conditions used in clinical practice.

4.3.2.1. Method 1: Using one point source Procedure (1) Place the point source on the camera face along the X axis, about 5 cm

from the edge of the field of view. (2) Set up the system to perform a conventional static acquisition of about

50 000 counts using the finest possible matrix size, for example, 256  256 or 512 × 512. Ensure that no zoom is used. (3) Acquire one planar image. (4) Now move the point source horizontally to a position about 5 cm away

from the other edge of the field of view, by a distance known to within

1 mm. (5) Repeat the acquisition.

4.3.2.2. Method 2: Using two point sources (1) Place two point sources as indicated in steps (2) and (4) above.

(2) Acquire one planar image using the finest matrix size available. For both methods (1) Repeat the whole procedure by placing the point sources along the

Y axis. (2) Repeat for all tomographic zoom conditions used in clinical practice.

Data analysis The analysis requires the calculation of the centre of gravity of a point

source along either the X or Y axis directions for each raw projection image. The centre of gravity in X (COGX) is estimated from:

  ¥ MATRIX (,)

ij

ii = 1 jj = COGX 1 =

   MATRIX (,)

ij

ii = 1 jj = 1

along a profile of thickness j 1 to j 2 and width i 1 to i 2 bounding the point source, where i is the index of the matrix along the X axis and j corresponds to Y. The centre of gravity along the Y axis (COGY) is similarly obtained from:

j ¥ (,)  ij  MATRIX jj = 1 ii = COGY 1 =

   MATRIX (,)

ij

jj = 1 ii = 1

where j is the index in the Y direction. The values COGX and COGY should be estimated to a fraction of a pixel.

(1) Calculate the centre of gravity of the point source(s) for each image. Four centre of gravity values must be obtained. These are: (X1, Y1) and (X2, Y2) for the two positions along the X axis; and (X3, Y3) and (X4, Y4) for the two positions along the Y axis.

(2) From each pair of observations, calculate the distances between the position of each of the pairs of point sources using:

DISTANCE X = ( XX 1 - 2 ) 2 + ( YY 1 - 2 ) 2 (18)

2 DISTANCE Y 2 = ( X3 X4 - ) + ( Y3 Y4 - ) (19) For the horizontal displacement, Y1–Y2 should be small, as should

X3-X4 for the vertical displacement. (3) Calculate the pixel size by dividing the distance between the point sources in millimetres by the corresponding distance in pixels. This gives the size in millimetres of the pixel used in order to perform this measurement.

(4) Multiply the value found in step (3) by an appropriate factor so that it corresponds to the matrix size and the pixel size as used in tomography, for example, by a factor of four if the data were collected in a 256 × 256 matrix (a matrix of 64 × 64 is used in tomography).

Observations This test is intended to be performed as an acceptance and reference test

and at half-yearly intervals. The pixel size is that required for tomographic reconstruction, for example, for the attenuation correction algorithm. The positioning of the point sources must be performed carefully and the distance between the first and second positions of the point source (or the positions of the two point sources) must be accurate to within 1 mm. The pixel size in the X direction and the Y direction may not be the same.

Interpretation of results In order to measure resolution, in order to apply attenuation correction

and in order to perform quantitative estimates of the size of organs, it is necessary to know the absolute size of a pixel in millimetres. Compare the pixel size in X and Y.

Limits of acceptability

The difference between the values in X and Y should be less than 5%. Conclusion Record whether or not the results confirm acceptable performance. If

not, indicate the follow-up action taken.