Phantom Laboratory Catphan 503 - Sensitometry (CT Number Linearity)

15
Sensitometry (CT number linearity)
The CTP404 module has sensitometry targets made from Teon®, Delrin®, acrylic,
Polystyrene and low density polyethylene (LDPE), polymethylpentene (PMP) and air.
The Catphan® 600 is also equipped with a small vial which can be lled with water
and inserted into the top hole of the CTP404 module. The CTP401 module has Teon,
acrylic and low density polyethylene (LDPE) and air targets. These targets range from
approximately +1000 H to -1000 H.
The monitoring of sensitometry target values over time can provide valuable information,
indicating changes in scanner performance.
Nominal material formulation and specic gravity
Material Formula Z
eff
1
Specic Gravity
2
HU range
3
Air .78N, .21O, .01Ar 8.00 0.00 -1046 : -986
PMP [C
6
H
12
(CH
2
)] 5.44 0.83 -220 : -172
LDPE [C
2
H
4
] 5.44 0.92 -121 : -87
Water [H
2
O] 7.42 1.00 -7 : 7
Polystyrene [C
8
H
8
] 5.70 1.03 -65 : -29
Acrylic [C
5
H
8
O
2
] 6.47 1.18 92 : 137
Delrin® Proprietary 6.95 1.42 344 : 387
Teon® [CF
2
] 8.43 2.16 941 : 1060
Electron density and relative electron density
Material Electron Density Electron Density Relative Electron
(10
23
e/g) (10
23
e/cm
3
) Density
4
Air 3.002 0.004 0.001
PMP
5
3.435 2.851 0.853
LDPE
6
3.435 3.160 0.945
Water 3.343 3.343 1.000
Polystyrene 3.238 3.335 0.998
Acrylic 3.248 3.833 1.147
Delrin® 3.209 4.557 1.363
Teon® 2.890 6.243 1.868
1
Z
eff
, the efective atomic number, is calculated using a power law approximation.
2
For standard material sensitometry inserts The Phantom Laboratory purchases a multiple year supply of
material from a single batch. Samples of the purchased material are then measured to determine the
actual specic gravity. The specic gravity of air is taken to be .0013 at standard temperature and
pressure. For custom cast materials the specic gravity of each cast batch is noted and supplied
with the phantom.
3
These are maximum and minimum measured values from a sample of 94 scans using different
scanners and protocols. HU can vary dramatically between scanners and imaging protocols and
numbers outside of this range are not unusual.
4
Relative Electron Density is the electron density of the material in e/cm
3
divided by the electron density of water (H
2
O) in e/cm
3
.
5
Polymethylpentene
6
Low Density Polyethylene