NEMA NU 2 Performance Tests for Scanners with Intrinsic Radioactivity

• background
• intrinsic radioactivity
• lutetium oxyorthosilicate
• National Electrical Manufacturers Association
• NU 2
• PET

The NU 2-1994 and NU 2-2001 standards published by the National Electrical Manufacturers Association (NEMA) (1,2) are widely used for characterizing the performance of PET scanners (3,4). These documents specify procedures for acquiring and analyzing data using standard phantoms and sources. They also specify criteria that are intended, in part, to ensure that background event rates are negligible during certain of these acquisitions. An example is the limit on random coincidence rate specified for the measurement of scatter fraction in NU 2-2001 (section 4). Although scanners using conventional scintillator detector materials such as bismuth germanate or NaI can generally meet these standards, they may not all be met when the detector material emits significant intrinsic radiation, as is the case with the new lutetium oxyorthosilicate (LSO) scintillator. This intrinsic radiation gives rise to significant background event rates that cannot be eliminated by following the current NU 2 procedures. Thus, not all these standards can be applied directly to LSO-based scanners without error. The purpose of this communication is to propose certain modifications to the NEMA NU 2 standards that properly account for the effects of the intrinsic radiation and permit accurate measurement of the performance of LSO-based scanners. We explicitly discuss the NU 2-2001 standard measurements; analogous adjustments can be developed for the NU 2-1994 measurements.

LSO contains natural lutetium, of which the radioactive nuclide ¹⁷⁶Lu composes 2.6%. ¹⁷⁶Lu decays by β⁻ emission, followed by a cascade of γ- and x-rays over a broad energy spectrum. The resulting background counting rate depends on the construction of the detector and the energy window used for discrimination. For the LSO block detectors used in the ACCEL dedicated PET and LSO PET/CT scanners (CPS Innovations), the background single-event rate is typically about 7.5 kcps, for an energy window of 350–650 keV. The scanner cannot distinguish between these intrinsic events and events arising from extrinsic sources such as a patient; each contributes in the same way to measured random coincidences. The total block singles rate in clinical scans may range up to 160 kcps or higher. Considering that the random coincidence rate is approximately proportional to the square of the singles rate, one can estimate (considering also dead-time losses) that the presence of the intrinsic singles increases the randoms rate by about 8% at these higher counting rates, compared with what it would be without the background radiation. The relative contribution of the intrinsic randoms is of course greater at lower counting rates. This is illustrated in Figure 1, which shows a plot of the randoms-to-trues ratio (RTR) versus the average block-detector single-event rate for a NEMA NU 2-2001 (section 4) counting-rate test on an ACCEL. The minimum value is 0.17 at a singles rate of 15 kcps per block. In the absence of the intrinsic or other background, the RTR would have fallen monotonically to zero as the activity diminished. Clearly, the NU 2-2001 criterion that the RTR be less than 0.01 in the last frames of this test cannot be met on such scanners. This would have serious consequences for the estimation of scatter from this test, as has been discussed in detail (5).

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FIGURE 1.

RTR for a NEMA NU 2-2001 counting-rate test performed on an ACCEL, as a function of the average singles rate in a block detector.

In addition to random coincidences, the intrinsic radiation may also give rise to a small number of true coincidences. An intrinsic true event may result when a ¹⁷⁶Lu nuclide undergoes β-decay in one detector, depositing energy locally in that detector, and also emits a prompt γ-ray (isotropically) that is absorbed in a second detector. The total rate of such events for the ACCEL or LSO PET/CT scanner, for an energy window of 350–650 keV, is only about 600 cps in a scan performed with no objects in the field of view (FOV) of the scanner. These events are distributed uniformly across the FOV. The rate may be significantly higher if lower, wider energy windows are used (5). When an object such as a patient is in the scanner, the intrinsic trues are attenuated by the object, similarly to the annihilation radiation. Generally, these events are of no consequence in the clinical scanning regime for the ACCEL or LSO PET/CT scanner, for which total true-event rates may be more than 1,000 times higher than the (attenuated) intrinsic trues contribution. In fact, because of their very low level, current commercial scanners do not make an explicit correction for the intrinsic trues. On the other hand, the effect of intrinsic trues can be significant and should be compensated when scanner sensitivity or scatter fraction is measured using very low activity levels.

It is useful to define a characteristic extrinsic activity threshold, a_ref, above which the response of the system is determined more by the radiation originating external to the detectors than by the internal activity. Considering Figure 1, the point at which RTR is minimized seems like a natural choice. Neglecting counting losses, the random coincidence rate is proportional to s², where s is the total singles rate. Similarly, the true coincidence rate is proportional to (s − s_int), where s_int is the intrinsic singles rate. From this, it is easy to show that the minimum of the RTR should occur at 2s_int. We thus define a_ref as the external (to the detectors) source activity, or activity concentration when appropriate, at which s = s_ref = 2s_int. This is illustrated in Figure 2, which clearly shows that a_ref is the intrinsic equivalent external activity, that is, the activity level in the object being scanned that would give rise to the same singles rate as that produced by the intrinsic radiation. For the present example, a_ref = 0.61 kBq/mL in the 22-L, 70-cm-long phantom, or a total activity of 13.4 MBq, most of which would be outside the FOV of the camera. This activity is approximately 4% of that at which the peak noise equivalent counting rate (NECR) occurs. The reference activity a_ref is a key concept in our recommendations. The value of a_ref corresponding to s_ref will vary from one test to another, depending on the emitter and attenuation distributions in the source. s_int can easily be measured by removing all external activity from the vicinity of the scanner and performing a brief acquisition.

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FIGURE 2.

A plot of the singles rate trend at low activity, showing the definition of a_ref.

The NEMA standards specify that certain measurements be performed with RTR < 0.01. Although the minimum RTR depends somewhat on the source configuration, for the NEMA test configurations the randoms rate are not likely to fall below 1% of the trues rate. However, the intent of the NEMA procedures in these cases is not to achieve a low randoms rate per se but, rather, to isolate one physical characteristic of the scanner from another; for example, to measure the intrinsic spatial resolution of the system without the complications of pileup and mispositioning that may occur at higher counting rates, or to measure the scatter component of the true coincidences without contamination from random coincidences. These goals are still achievable on LSO-based machines with appropriate acquisition and analysis techniques.

The NEMA tests for spatial resolution and sensitivity are to be performed at low counting rates to avoid biasing results through inaccurate corrections for dead time, pileup, and randoms. Bias due to randoms correction is not an issue with scanners that use a delayed coincidence window technique for measuring and subtracting random coincidences. Thus, the presence of additional randoms due to the intrinsic radiation will not bias results but may require acquisition of 10%–20% more counts at low activities to achieve an equivalent integral signal-to-noise ratio in the data, compared with non-LSO systems (based on a simple NECR estimate). In fact, the effect of randoms is probably much less for these tests, since the randoms will be distributed more uniformly than are the spatially localized trues.

Another consideration in the case of intrinsic radiation is its contribution to count losses. Several of the NEMA tests require that these be <1%. Dead time in the system will in principle be greater at a given extrinsic source activity with the intrinsic events present, because of competition for processing time and multiplexed data channels. However, experimentally, dead time is defined in terms of the deviation of a response (e.g., trues counting rate vs. source activity) from a linear trend extrapolated from “low-counting-rate data.” Somewhat ambiguously, low-counting-rate data are defined as those having <1% dead time. The intent appears to be to operate in a low-activity regime in which the maximum deviation from linearity is less than 1%. Empirically, we find that this goal can be met for the LSO-based scanners mentioned above over a reasonably broad range of activities in the vicinity of a_ref. Figure 3 shows total trues versus activity for the NU 2-2001 70-cm phantom on an ACCEL (same data as for Fig. 1). A linear regression to the data from a_ref = 0.61 kBq/mL down to an activity lower by a factor of 4 (2 half-lives) is also shown. The maximum deviation of these data from this linear trend is <1%. The linearity of the system’s response at singles rates below s_ref is an important criterion for the definition of a_ref. If, on some system, the trues counting rate were found not to be linear to within 1% up to s_ref = 2s_int, then the value of s_ref, and hence a_ref, for that system should be lowered appropriately. For the purpose of extrapolating a linear trend to higher activities in order to evaluate count losses, we believe a least-squares fit of a linear function in this range is preferable. To ensure adequate statistics, we suggest acquiring 8 frames in the interval from a_ref to a_ref/4, each one-quarter half-life long.

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FIGURE 3.

Trues rate at low activity from a NEMA NU 2-2001 counting-rate test on an ACCEL. Also shown is a linear trend fitted to the data below 0.61 kBq/mL.

The modifications to the standards proposed here have served as the basis for the measurement of the complete NEMA test suite for an LSO PET/CT scanner that is presented in an accompanying article in this issue (6). The NU 2-2001 corrections accuracy (section 6) and image quality tests (section 7) require no special implementation on LSO-based scanners, although if a correction for intrinsic trues is available it may be applied for these reconstructions.