Purchase of dual photon imaging systems

All nuclear medicine physicians, assisted by a nuclear medicine physicist, acquire some experience during their careers in purchasing gamma cameras and other accessories for a nuclear medicine service. The decision making process, leading to the purchase of a system performing dual photon imaging, calls for knowledge of the basic physics of coincidence detection and of the differences between 2-D and 3-D acquisition in terms of sensitivity, the ratios between the true and the random events, and scatter fraction, as well as the different methods to overcome these problems.

There are a number of different ways to increase the sensitivity of the system and physicians should work closely with a physicist who has extensive knowledge of these areas. It is recommended that they should visit or contact a site that is already functioning. They should be allowed to review the TABLE 4.1. COMPARISON OF PET VERSUS SPECT SYSTEMS

Characteristic SPECT Hybrid

PET–SPECT

PET (2-D)

PET (3-D) Sensitivity relative to SPECT ×1 (single high

resolution)

×6 (triple head fanbeam)

ª×7 ×15 ×75

Reconstructed resolution (mm) 8–10 (head) 5–7 4–6 4–6

Scatter fraction (%) ª30 ª30 ª15 ª40

Attenuation factor (thorax) ×7 (Tc-99m) ×30 ×30 ×30

4.4. DUAL PHOTON IMAGING

performance of the system from the work scheduling book in terms of its uptime and downtime. They should also have an opportunity to observe on the workstation the studies performed. The nuclear medicine physicist should be able to review the results of the various quality control tests performed.

There are many aspects of purchasing dual photon imaging systems that are common to the purchasing of single photon imaging systems; these have been covered in an earlier section of this chapter. In addition to specific advice on contractual arrangements, warranty and service, the reader should bear the following points in mind when purchasing dual photon imaging equipment.

In most cases the primary purpose of purchasing the equipment is to perform oncology studies, although specific centres may have research require-ments in other areas. The main dilemma facing a purchaser is what type of system to purchase. At the time of writing there are basically two types of system: dedicated PET systems (including full ring systems and lower cost partial ring systems) and hybrid SPECT–PET systems based on standard gamma camera technology, and there is a significant difference in the cost of these systems. The main considerations in choosing between the systems can be summarized as follows.

(a) Sensitivity

In a hybrid PET–SPECT system, sensitivity is a primary concern and a thicker crystal than that normally used for SPECT is required. The effect of increasing crystal thickness on routine single photon nuclear medicine studies should be considered. Although a slight decrease in resolution is demonstrated in bar phantom studies, it has little effect on routine clinical studies. An advantage is the additional increase in the sensitivity for such radionuclides as

67Ga, ¹¹¹In and ¹³¹I. The trade-off between resolution and sensitivity in PET versus SPECT applications for these thick crystal systems is still under evaluation. Sensitivity is improved by using 3-D rather than 2-D acquisition as outlined in the sections earlier in this chapter. The exact trade-off in useful counts (with scatter correction) for whole body applications continues to be evaluated.

(b) Count rate

The use of a large single detector results in specific count rate limitations.

There are several approaches to improve count rate capability with specific circuitry designed to enhance the performance of gamma camera based systems. Nevertheless, there is a limit to the activity that can be administered.

(c) Time for acquisition and processing

Both the relatively low sensitivity of hybrid systems and the count rate limitations result in the need for relatively long acquisition times, particularly for whole body imaging studies. A further constraint is the period required to measure attenuation in these studies. This makes the total time required for whole body acquisition a critical factor in determining the utility of a system. In addition, since iterative reconstruction is commonly used instead of filtered back-projection, processing can be relatively slow. The total time of examination including processing should be taken into consideration.

(d) Flexibility

Despite some constraints on performance, hybrid systems are consid-erably more flexible than dedicated systems. This can be a major consideration in situations where patient numbers or radionuclide supply may be limited.

Hybrid systems have considerable appeal for certain centres. New develop-ments in detector technology are likely to result in a wider range of hybrid systems.

It should be noted that the technology used in dual photon imaging is changing rapidly. As it develops, additional factors might need to be taken into consideration.

4.4.7. Specification

A new NEMA publication (Performance Measurements of Positron Emission Tomographs (see the bibliography to this section)) has been finalized on the specification of dual photon imaging systems and is applicable to both conventional PET systems and gamma camera based coincidence detection systems. The emphasis of this document is on instruments designed for whole body applications, although additional tests are included that provide comparative information related to other types of application. The major advance in this document is that no distinction is made between conventional and gamma camera based systems. A more direct comparison between the specifications should therefore be possible in the future.

The parameters specifically defined in the new document include those listed below:

4.4. DUAL PHOTON IMAGING

(a) Spatial resolution

The report includes the radial and tangential FWHMs at the centre and 10 cm off-axis as well as the axial resolutions at the same positions.

(b) Scatter fraction, count losses and random events

This includes count rate plots for true, random, scatter and total events as well as noise equivalent count rate. Peak counts are specified for each type of event as well as scatter fraction.

(c) Sensitivity

System sensitivity is specified, with extrapolation to a zero attenuation situation. The axial sensitivity profile is also presented.

(d) Accuracy — corrections for count losses and random events

The deviance of the corrected count rate from the expected true count rate is specified, as are image quality and accuracy after attenuation and scatter corrections. The contrast is calculated for spheres in a whole body phantom.

The additional tests suggested for applications other than whole body studies are:

Scatter fraction

Count loss and random event measurements (dead time and true event rates) should be made.