Detection Of Nodes With A Low Uptake Sensitivity

Sensitivity is defined as the fraction of gamma photons emitted by the source that is detected by the probe. It is specified as the count rate per amount of activity (cps/Bq or cps/kBq). Sensitivity should be as high as possible. Sensitivity is a combination of intrinsic detector efficiency and geometric factors such as detector area and collimation. Differences in sensitivity between different types of probes may be considerable. We measured values between 1 and 43 cps/kBq for the probes in Table 1 at a distance of 1 cm from a source (Table 2, measured between

Table 2 Sensitivities Measured in Air for a 57Co Source Using the Full Energy Spectrum, at a Distance of 1 cm and 5 cm (Measured Relative to the Tip of the Probe or Collimator)

Sensitivity (cps/kBq)

Table 2 Sensitivities Measured in Air for a 57Co Source Using the Full Energy Spectrum, at a Distance of 1 cm and 5 cm (Measured Relative to the Tip of the Probe or Collimator)

Sensitivity (cps/kBq)

Probe

Collimator

at 1 cm

at 5 cm

C-Trak,a Nal

None

16

1.4

With

8.4

1.3

Neoprobe, CdZnTe

None

43

3.5

Shield

36

2.8

Cone

10

1.5

RMD CdTe

None

1.2

0.08

Gammed CdTe

None

39

0.25

Gammed Csl

None

5.2

0.45

Note: Values for 99mTc will be lower, in particular for the probes with CdTe crystals. a C-Trak also supplies a MiniProbe which has a low weight, but the sensitivity of this probe is about three times less than that of the standard probe. Source: Ref. 4.

Note: Values for 99mTc will be lower, in particular for the probes with CdTe crystals. a C-Trak also supplies a MiniProbe which has a low weight, but the sensitivity of this probe is about three times less than that of the standard probe. Source: Ref. 4.

July 1996 and April 1997). Also given in Table 2 are the (reduced) sensitivities that were measured when additional collimators were used.

If a probe has no collimation at all, the crystal can be brought very close to the node. Increasing the distance between node and probe will mean that the sensitivity decreases with the square of the distance (double the distance: fourfold reduction of the sensitivity). A collimator may shield part of the crystal, which will reduce the sensitivity. In addition, the minimum distance between node and crystal will be larger (of the order of 1 cm depending on the length of the collima-tor; see Fig. 1). This means that the maximum sensitivity (with the probe as close as possible to the node) is considerably reduced by a collimator, but if the probe is far from the node, the collimated sensitivity will be relatively closer to the uncollimated value (compare the values for 1 cm and 5 cm in Table 2 for the same probes with and without the collimator). Closer than 1 cm to the source, the differences are even larger: At about 1 mm, the sensitivity of the Neoprobe 1000 with shield is about 175 cps/kBq, but with the cone collimator, it is only 35 cps/kBq. The sensitivity of the C-Trak probe without an additional collimator is about 40 cps/kBq. These data were measured using 57Co. For the Neoprobe, the sensitivity for 99mTc is 20-30% lower.

Sensitivity may be of crucial importance during intraoperative use of the probe because the uptake of the tracer in the sentinel node may be very low or the node may be at a large distance from the probe. When the large amount of radioactivity at the injection site is nearby, the combination of collimation, scatter suppression, and sensitivity becomes important. This will be discussed in one of the next sections. Therefore, sensitivities of the probe in different configurations should be carefully evaluated. Manufacturers should specify the sensitivity in cps/kBq for a specified radionuclide in well-defined geometric configurations with the different collimators available.

Was this article helpful?

0 0
How To Prevent Skin Cancer

How To Prevent Skin Cancer

Complete Guide to Preventing Skin Cancer. We all know enough to fear the name, just as we do the words tumor and malignant. But apart from that, most of us know very little at all about cancer, especially skin cancer in itself. If I were to ask you to tell me about skin cancer right now, what would you say? Apart from the fact that its a cancer on the skin, that is.

Get My Free Ebook


Post a comment