Example: Clinical study — collimator star pattern

This¹³¹I image of a patient with metastatic thyroid tissue was acquired using a high energy collimator.

Results: An intense star pattern is seen. This is produced by photon penetration of the hexagonal array of collimator holes and septa. Certain directions of travel through the collimator contain a lesser total septal thickness and produce the arms of the star.

Comments: The image shows parallel arms because there are two separate hot regions. The star pattern is an effect usually associated with foil collimators rather than cast collimators, and is inevitable when imaging very hot foci of activity.

2.2.10. Artefacts arising from sources/phantoms 2.2.10.1. Fillable flood sources

General comments

The use of fillable flood sources requires care in order to ensure uniform mixing, to ensure that there are no air bubbles in the area used to assess scintillation camera performance and to ensure that there is no bulging or overfilling with liquid. Bulging can be avoided by using a phantom with thick walls, e.g. 1 cm of Perspex (Lucite). To avoid overfilling, a phantom that is filled horizontally is preferable to one that is filled vertically.

In addition, the seal must be perfect to ensure that there is no leakage of the radioactive contents that would lead to radioactive contamination. Distilled (de-aerated) water should be used if the contents of the phantom are not disposed of after use (after the radioactivity has decayed). The solution should be replenished regularly in order to avoid the growth of algae, which can bind with ^99mTc compounds, causing hot artefacts. The use of ^99mTc pertechnetate is advised. Other ^99mTc radiopharmaceuticals may cause problems.

Careful filling and secure knowledge that a fillable flood phantom is indeed uniformly filled are especially important when the phantom is intended to be used to obtain uniformity (or sensitivity) correction maps and calibration data.

The construction of the flood source can influence the uniformity measured. Phantoms with thick walls and a large volume have shown an increase in counts in the centre of the FOV, which is possibly due to scatter and septum penetration (depending on the collimator and radionuclide used). These effects cause a non-uniform result from a camera which on other tests appears uniform. This is more likely to be a problem when uniformity is very good. The dimension of the flood source should be such that it is several centimetres larger than the collimated FOV. It is often impossible to obtain a good fillable flood source for use with modern scintillation cameras. Larger detector size results in a heavy source, and multiple detectors may prevent sources from being positioned as required. The user is advised to always exercise caution when using fillable flood sources.

2.2.10.1A. Examples: Fillable flood sources — air bubble

Extrinsic routine QC images using a fillable flood source, ^99mTc, 20% energy window.

Results:

A

Uniformity image. A discrete circular cold area is seen in the middle of the FOV. Visual inspection of the flood source revealed a large air bubble. The cold indentation at the top left edge of the FOV was also due to an air bubble.

B

Spatial resolution image using a four quadrant bar pattern. A discrete large circular cold area is seen in the lower right quadrant. By changing the position of the flood source, the position of the cold area was also moved. The cold area was due to an air bubble in the solution that was clearly visible on physical inspection.

C

The image shows an elongated cold area, due to an air bubble, at the left edge of the FOV. This air space was clearly seen and was due to air left in the phantom after filling.

Note:The distinct delineation of the cold area is indicative of an air bubble rather than a defect in the detector.

2.2.10.1B. Example: Fillable flood source — non-uniform mixing

Two different examples of a routine extrinsic uniformity image using ^99mTc in a fillable flood source.

In each situation the ^99mTc was injected into the water in the flood source and was left to disperse over a period of about one hour.

Results: The non-uniformity shows that dispersion was insufficient to produce a homogeneous distribution of ^99mTc, and that actual mixing must take place. The right image also shows a small air bubble in the centre of the FOV.

2.2.10.1C. Example: Fillable flood source — bulging sides

Routine uniformity image obtained with a fillable flood source. The phantom was filled with too much water, which produced bulging of the centre part of the phantom.

Results:The image shows a dome shaped increase in counts at the centre due to the bulging sides of the phantom.

Comments:This phantom had thin walls and, when upended for the purpose of filling, was prone to bulging if overfilled with water. The effect could be avoided by filling the phantom in a horizontal position and with only the correct volume of water.

2.2.10.1D. Example: Fillable flood source — old solution containing algae

Routine flood image obtained with ^99mTc in a fillable flood source.

Results: The^99mTc flood image shows an intense small hot spot in the lower right quadrant of the image. This was due to algae.

Comments: Algae may adsorb ^99mTc radiopharmaceuticals. Therefore, when water is allowed to remain in a phantom for a long period of time, algae formation may occur. The use of distilled water is preferable to using tap water. Alternatively, the phantom should be emptied after use (taking the appropriate radiological protection precautions, or after the radioactivity has decayed). It is recommended that ^99mTc pertechnetate be used to minimize problems with algae and adsorption. It is not recommended to use colloids or chelates.

2.2.10.1E. Example: Fillable flood source — adherence of activity to the container at the filling site (algae)

The circular fillable flood phantom was upended to enable radioactivity to be added to the water already within the phantom. The water level was well below the filling hole. Since ^99mTc pertechnetate was not available, ^99mTc MDP was used and was injected with a syringe in a steady stream against the side of the phantom into the water. After removal of air and mixing, flood field uniformity images were obtained (20% energy window, 3 million counts each image). The images show the phantom in two different orientations.

Results: The left image shows an artefact on the left side of a hot line with a less intense line crossing it perpendicularly (forming a cross). When the phantom was rotated by about 90°, this cross also rotated (right).

The artefact was assumed to be due to the MDP pharmaceutical which had adhered to algae on the side of the Perspex (Lucite) material of the phantom as the ^99mTc MDP was injected. When a similar filling process was tried in the same phantom using ^99mTc pertechnetate, no such artefact was observed. Also, when a similar cross was created with ^99mTc MDP on a clean sheet of Perspex (Lucite), which was afterwards immersed in water, there was no remaining image of the cross visible.

It was concluded that ^99mTc in the form of MDP adhered to the algae, and that in the future it would not be used.

Comments:The importance not only of clean phantoms, but also of the radiopharmaceutical used in the solution, is illustrated here as well as in the previous example (D). It is always preferable to use distilled water rather than tap water, and to clean phantoms periodically. The addition of chlorine can prevent the growth of algae. However, cleaning should be done with care, since solvents can cause damage (which is not always seen initially but can result in cracking later).

2.2.10.2. Cobalt sheet sources