Physical and mechanical inspection of the SPECT system

4.3.1. Physical and mechanical inspection of the SPECT system

Purpose of test To check the mechanical performance of the system and its capability to

rotate the scintillation camera.

TABLE 5. LIST OF RECOMMENDED QUALITY CONTROL TESTS FOR SPECT

Frequency of routine testing Section

Test Acceptance Daily Weekly Monthly Half-yearly (test) no.

and reference

Acceptance and reference tests

4.3.1 Physical and mechanical inspection

X (reference) X

of the SPECT system

4.3.2 To determine the

X X absolute size of a pixel

4.3.3 Tomographic uniformity

X of the camera

X (reference)

4.3.4 Tomographic

X X resolution in air

4.3.5 Tomographic

X X resolution with scatter

4.3.6 Centre of rotation

X X a offset and alignment

4.3.7 Slice thickness at

X centre of slice

X (reference)

4.3.8 Variations of sensitivity and uniformity with

X (reference)

rotation of the system

4.3.9 Total performance

X Operational checks

X (reference)

4.4.1 Check of routine function and centre

X a X a of rotation offset

a In a well-functioning stable system, the check or test of centre of rotation offset should not be required at the frequency indicated and could be performed at the frequency

indicated in the next column (i.e. weekly instead of daily and monthly instead of weekly). If the stability of a SPECT system is such that the centre of rotation offset changes daily, then this system should not be used for tomographic studies until the problem of instability has been resolved.

Materials

A spirit level, a set of accurate rulers of various lengths and a stopwatch. Some manufacturers now provide suitable test equipment, for example, a laser, by means of which the mechanical installation can be checked accurately, but sometimes available only on installation.

Procedure (1) Check the system for damage.

(2) Rotate the scintillation camera. Check for constancy of speed, vibration, the presence of mechanical noises and whether it stops correctly at the end of rotation. Check both clockwise and anticlockwise rotation. In particular, use the stopwatch to check if rotational speed changes as a function of angular position. The motors may have to work much harder to lift the head, in contrast to when the head is descending. Ensure, where possible, that the head is adequately counterbalanced for the different collimators available.

(3) Where appropriate, check that the system returns to its ‘home’ position accurately and reliably. (4) For a step and shoot system, check that the correct number of angles is used, for each possible angular increment that can be selected. (5) Check that the head is mechanically centred with respect to the axis of rotation to within the manufacturer’s specifications. Typically, this should

be accurate to within 1–2 mm. If the errors are taken into account by the reconstruction software and if such errors are consistent, then errors up to 1 cm may be accepted if the test for tomographic resolution gives acceptable results (see Section 4.3.4). With errors larger than 1 cm, electronic correction of the centre of rotation offset becomes difficult.

(6) Check that the Y axis of the head is parallel to the axis of rotation. This may also be difficult to measure mechanically without special purpose equipment and the manufacturer must check this at the time of installation. The test for centring given in Section 4.3.6 can provide information about the alignment of the Y axis to the axis of rotation.

(7) Check that the long axis of the bed is reasonably parallel to the axis of rotation, where appropriate, and centred. This may be checked by measuring the distance from the bed to the camera with the head at 90° and then at 270°. These two distances should be within 1 cm of each other. This should also be checked for two extreme positions of the bed (where appropriate), which will confirm that the bed is reasonably parallel to the axis of rotation.

(8) Check that any readings of head position, for example, angle, or distance from the axis of rotation, are accurate by measuring the corresponding distances, angles, etc. The radius of rotation is most easily performed by taking a measurement from the head to some fixed point, for example, on the bed, then rotating the head by 180°, repeating the measurement and dividing the sum of the distances by two.

(9) Check that the gantry is vertical. This can be ensured by placing a spirit level on the collimator at 0° and at 180° along the Y axis. The reading should be the same in both cases. If not, the manufacturer should rectify the situation. This is important when adjusting the angle of tilt, if a spirit level is used. Errors in adjusting the verticality of the gantry will show up in the centring test (see Section 4.3.6).

(10) For a system using a spirit level on the back surface of the detector head, check if the collimator face is parallel to the back surface of the detector. Place a spirit level on the collimator with the detector head facing down and compare with the spirit level on the back surface of the detector. The two surfaces should be parallel.

(11) In many systems, as the radius of rotation is changed, the head tilt angle is adjusted to ensure that the head remains parallel to the axis of rotation. Check that the head remains level as the radius of rotation is increased.

(12) Visually check the collimators for damage. (13) Check that the emergency stop button and any patient safety devices

function. (14) For non-circular orbits, check that the system performs the correct mechanical motions for several different selected orbits. Tests that are more detailed will normally be required and should be indicated in the manufacturer’s documentation.

(15) Check that any cables do not become twisted or damaged when the system rotates. Check all such electrical connections, for example, if a slip ring is used, visually inspect it.

Data analysis None. Observations

This test is intended to be performed as a reference test and daily.

The capability of the system to perform the mechanical movements required without vibration is important. Many problems can be indicated by noises generated by the system, or from observing the time taken to rotate from position to position.

The radius of rotation as indicated on some readout display may indicate distance from the axis of rotation to the front of the collimator, or alternatively, to the front of the crystal. Note which of these is, in fact, the case.

Interpretation of results If a SPECT system is not set up very carefully mechanically, it will not be

capable of providing good SPECT images. The tolerances permitted are far less than for a conventional scintillation camera. Most of the errors resulting from such mechanical problems will become more obvious by the centring test (see Section 4.3.6).

Limits of acceptability In order to perform good SPECT studies, the mechanical positioning of

the head needs to be such that the centre of rotation offset is less than 1 mm. However, when adequate software or electronic centring methods exist, the centre of rotation offset needs to be less than 1 cm and reproducible to within

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

not, indicate the follow-up action taken. If the mechanical accuracy is not within tolerance and cannot be corrected by software, then the system should not be used for SPECT.