Physics and Radiobiology of Nuclear Medicine

The transaxial spatial resolution typically ranges from 5 mm to 7 mm at 10
cm and the axial spatial resolution from 5.4 mm to 6.6 mm for commercial
PET scanners. The transverse resolution is best at the center of FOV and
worsens toward the periphery of the scanner.

Sensitivity

The sensitivity of a PET scanner is defined by the number of counts per
unit time for each unit of activity and is normally given in counts per second
per microcurie (or megabecquerel) (cps/mCi or cps/MBq). Commercial
vendors give this in units of volume sensitivity, kcps/mCi/cc or cps/Bq/cc.
Sensitivity depends on the geometric efficiency, detection efficiency of the
detector, pulse-height window, and dead time of the detector. These factors
have been discussed in detail in Chapter 10 for conventional cameras.
However, the geometric efficiency needs further consideration because of
the specific configuration of the PET scanners. The geometric efficiency
depends on the distance dbetween the detector and the source, the diam-
eter Dof the ring, and the number of rings nin the scanner. Increasing the
distance dand the diameter Dreduces the solid angle subtended by the
detector at the source and, thus, decreases the geometric efficiency. With all
these factors taken into consideration, the sensitivity Sof a single-ring PET
scanner is given by (Budinger, 1998)

(13.10)

where Ais the detector area seen by a point source to be imaged,e=detec-
tor efficiency,m=linear attenuation coefficient of 511 keV photons in detec-
tor material,tis the thickness of the detector, and Dis the diameter of the
ring. The sensitivity increases with the number of rings in the scanner.
The sensitivity of the PET scanners in 2-D mode acquisition is about 0.2
to 0.5%, whereas it is 2 to 10% in 3-D acquisition due to the absence of
septa between the rings. However, the latter contains a large number of
random and scatter coincidences.

Noise Equivalent Count Rate

As discussed in Chapter 10 under contrast, noise degrades the image con-
trast and primarily arises from the statistical variation of the count rates.
Noise is given by where Nis the count density. An important para-
meter related to noise is the noise equivalent count rate (NECR) that is
given by

NECR (13.11)

T

TSR

=

++

2

1 N,

S

Ae

D

t

=

⋅⋅ ⋅ ⋅

()

e −

p

m

24 m

2

37 10.

cps Ci

204 13. Positron Emission Tomography