Test of spatial resolution and spatial linearity

To test the spatial resolution and spatial linearity of a scintillation camera on a weekly basis.

METHOD 1: FLOOD SOURCE METHOD

To be used if an flood source is available.

Materials

Flood phantom (see p. 147) containing about 200 MBq (5 mCi) or

5?Co flood source of similar activity.

Orthogonal-hole transmission pattern (OHTP) phantom (see p. 147) matched to camera resolution. Optimal hole diameter and minimum inter-hole spacing, s, is given by s = FWHM/1.75.

Procedure

1. Mount a low-energy high-resolution parallel hole collimator on the detector head. The same collimator must be used consistently in the test. Turn the head to face vertically upward.

2. Position the OHTP phantom on the face of the collimator with the pattern carefully aligned with the X and Y axes of the detector face.

3. Place the flood phantom or flood source on the OHTP phantom.

4. Centre the clinically-used PHA window on the photopeak of the radionuclide concerned (see test 6.4.2: Check of Energy Calibration of PHA).

5. Acquire an analogue image on the display device with hard copy, at a preset count of at least 10°.

6. Remove the flood phantom or flood source, and OHTP phantom.

Data analysis

Visually inspect the image, noting particularly whether the images of the holes are distinct and separated from each other by dark spaces over the entire field-of-view, and whether there are significant deviations from linearity in the X or Y direction over the field.

Observations

This test is intended to be performed as a reference test at the time of acceptance, and at weekly intervals.

The test may be performed with a Bureau of Radiological Health (BRH) graded-spacing-hole phantom, a quadrant-bar phantom or a parallel-line equal-spacing (PLES) phantom in place of the OHTP phantom.

The OHTP phantom has the advantage that it allows the entire field-of-view to be examined simultaneously in both the X and Y

directions. However, its hole diameter and inter-hole spacing must be matched to the spatial resolution for a critical test. Selection of the appropriate phantom thus requires a prior knowledge of the resolution

(unless a set of phantoms of differing hole sizes is available.

The BRH graded-spacing-hole phantom provides an estimate of the resolution, but must be imaged in two series of positions at 90° to each other for an examination of the entire field-of-view in the X and Y directions. The quadrant-bar phantom likewise provides an estimate of the resolution, but must be imaged eight times in all for an examination of the entire field-of-view in the X and Y directions.

If a PLES phantom is used, its bar width and inter-bar spacing must be matched to the resolution for a critical test. As with the OHTP phantom, therefore, selection of the appropriate phantom requires a prior knowledge of the resolution (unless a set of phantoms of differing bar widths is available). Moreover such a phantom must be imaged in two positions at 90° to each other for an examination of the entire field-of-view in the X and Y directions.

Interpretation of results

The image should be compared with the reference image and with recently acquired images to identify any changes and trends in either spatial resolution or spatial linearity.

If deterioration in resolution is noted in the entire image and a digital image processor is available, test 6.3.7: Test of Intrinsic Spatial Resolution, Method 2 should be performed to quantify the change.

If the deterioration is partial, the same test should be performed for the region involved. If no image processor is available, test 6.3.7:

Test of Intrinsic Spatial Resolution, Method 1 should be performed to estimate the extent of the change. Alternatively, an image should be acquired for this purpose with the OHTP phantom having the next larger hole diameter and inter-hole spacing.

Small deviations from linearity are to be expected, particularly with scintillation cameras without a uniformity correction circuit, but are difficult to quantify.

Deterioration in either spatial resolution or spatial linearity and would call for follow-up action.

Conclusion

Record whether or not the results confirm acceptable performance.

If not, indicate follow-up action taken.

METHOD 2: POINT SOURCE METHOD

To be used if an flood source is not available.

Materials

Point source (see p. 147) consisting 40-100 MBq (1-3 mCi) ⁹⁹Tc^m in solution in suitable container.

Source mounting for point source (see p. 147).

Orthogonal-hole transmission pattern (OHTP) phantom (see p. 149) matched to camera resolution. Optimal hole diameter and inter-hole spacing, s, is given by s = FWHM/1.75.

Procedure

1. Remove the collimator from the detector head. Align the head and the source mounting.

2. Position the OHTP phantom so that it is supported on the detector-head housing, and as close to the crystal housing as possible, with the rows of holes carefully aligned with the X and Y axes of the detector face.

3. Mount the source in the source mounting.

4. Centre the clinically-used PHA window on the photopeak (see Observations, test 6.4.1: Check of Energy Calibration of PHA).

5. Acquire an analogue image on the display device with hard copy, at a preset count of at least 10".

6. Remove the source and OHTP phantom. Replace the collimator.

Data analysis

As for Method 1: Flood Source Method Observations

If 99f^cm i^s used, the point source method has the advantage of requiring a lower activity than does the flood source method. Further, it does not require the filling of a phantom and thus exposes personnel to a lower radiation dose. Its disadvantage is that it requires the collimator to be removed from the detector head, with increased chance of crystal damage. Whichever method is chosen, it should be performed consistently.

Interpretation of results

As for Method 1: Flood Source Method.

Conclusion

As for Method 1: Flood Source Method.

6.3.16: TEST OF TOTAL PERFORMANCE Purpose of test

To test all components of a scintillation camera, including the display device and digital image processor under simulated clinical conditions.

Materials

Total performance phantom (see p. 147), either thyroid phantom containing about 7 MBq (200 ,uCi) 99Tcm or 0.4 MBq (10 jiCi) 131I or liver-slice phantom containing about 70 MBq (2 mCi) "Tcm or a similar activity of 113Inm.

Procedure

1. Mount the usual collimator for the clinical conditions simulated on detector head.

2. Set all controls to the routine settings for the radionuclide concerned.

3. Centre the clinically-used window for the radionuclide concerned on the photopeak (See test 6.4.2: Check of Energy Calibration of PHA).

4. Position the phantom in a reproducible way close to the face of the collimator according to the clinical condition simulated.

5. Acquire an analogue image on the hard copy device, following the usual clinical techniques of the simulated procedure. If a digital image processor is available, also acquire a digital image. For the analogue image, adjust the intensity of the display so that the most active part of the image just fails to saturate the display medium.

Data Analysis

Visually compare the images with the reference image and with those obtained on recent occasions of testing, with particular regard to the visibility or otherwise of the simulated lesions.

Observations

This test is intended to be performed as a reference test at the time of acceptance, and at weekly intervals.

The basis of the test is the visibility or otherwise of the lesions in images acquired at regular intervals under identical conditions. Any deterioration in performance is detected earlier in such a test than in clinical imaging because the constant shape of the phantom and constant position and size of the simulated lesions allow direct comparison of images. The choice of phantom and radionuclide should reflect the clinical workload.

Interpretation of results

Comparison of the images with the reference images and with those obtained on recent occasions should show no degradation in performance and should satisfy clinical requirements within the capabilities of the instrument.

Special regard should be given to the visibility of the smallest simulated lesions, since this provides the most sensitive criterion by which performance may be assessed. Should a change • be evident, more specific tests should be carried out to ascertain its cause.

Conclusion

Record whether or not the results confirm acceptable performance.

If not, indicate follow-up action taken.

6.3.17: TEST OF MULTIPLE-WINDOW SPATIAL REGISTRATION