Philips TDC User manual

TDC (tomo density control) and how it works

As in all AEC (automatic exposure control) techniques the generator sets a density voltage according to

- the chamber type

- the kV value

- the FSC = film-screen-combination (using cassettes, imaging plates or digital detectors) or the image
intensifier.
This density voltage should be achieved during the exposure.

It is coming in via the Amplimat input plugs EZX 21/22/31/32/41 of the generator, controlled by "dose rate
control" which is physically located on-board of CU EZ139.

For AEC falling load or kV-fixed current the dose signal will more (fixed current) or less (falling load) be
constant until the dose ramp reached the density voltage setpoint value and the exposure is terminated.
The resulting exposure time is determined by the patient size and of course the application.

At TDC the density voltage setpoint is known by the exposure parameters and the FSC.
The exposure time is always fixed depending on the time which is assigned to various trajectories.

APR data for TDC have to be set according to the sensitivity of the film-screen-combination similar to the
APR settings for straight Bucky use.
The average mAs value of the application and patients sizes must be programmed, the mA value with which
the exposure will be started is calculated by the generator.
During the exposure dose rate control tries to drive an emission current to achieve a dose ramp which is
almost linear throughout the exposure.
At the end of the tomo exposure time (fixed) the density setpoint value should be achieved.

If exposure parameters are not exactly known one can easily set them during an examination:
Tomo series are typically started with a Bucky exposure.
Bucky and tomo RGDV chambers are the same, the selected FSC data set will also be the same.
Multiply the post indication mAs value of the Bucky exposure with 1.2 …1.4 to set the start mA or mAs value
for the tomo exposure.

The advantage of TDC is that if the first tomo sweep has been carried out with parameters far out of range
finally the second exposure will get the customer a perfect tomo exposure with a linear dose throughout the
exposure (as long as he does not push the APR button which will get him the default data set again).

The following pages show all data, details and their explanations one can see during TDC operation of a
tomo system hooked up to an Optimus.
A Bucky TH2 system with sensing was used in this case, but it can be any geometry.

A series of 10 TDC exposures has been carried out with properly preset TDC APR and start values out of
range to show the abilities of TDC.

Oscilloscope screenshots shall clarify what happens in the generator and what one can see as result in the
form of a dose signal.

TDC-calculation-process-pictures.doc Mar – 30 – 2005 page 1 - 20

Basic test conditions:

Optimus RAD 80kW with SRO33100 tube
110cm SID
40° tomo sweep 2000ms
collimator opening size 15x15cm, only center field of Amplimat chamber 9890 000 01615
50mm Al filter at the collimator.

A random APR

was selected.

To see what density voltage and other TDC relevant parameters one should expect, select path:

TDC-calculation-process-pictures.doc Mar – 30 – 2005 page 2 - 20

TDC initial APR Calculation
Fr 11. Mrz 07:58:45 2005

U nominal : 73 kV
I start : 16 mA
t backup : 2000 ms
C eff ht : 4.550 nF
dose measurement input : EZX21
film screen comb. : 1
dose nominal : 1 OD
I max : 410.958 mA
I min : 1 mA
I max NOMO : 234.109 mA
dose calculated : 322
kV factor : 0.960
U off : 0.785 V
t corrected : 1994.811 ms
number of steps : 49
time/sample : 40.710 ms

Explanation of the calculation screen:

U nominal kV setpoint = 73kV

I start emission current start value; in this case calculated by the selected manual technique RUQt

(radiographic kV-mAs-ms) – setpoint 32mAs and 2000ms = 16mA

t backup time at which CU is finally terminating the exposure if DRC (dose rate control) didn’t

C eff ht programmed cable capacity (length dependent) and HT transformer capacity; determines

the exposure termination kV and load dependent to pass the 75% high tension within the
exposure time set during kV discharge

t corrected (next page) is the calculated high tension off command time

dose measurement input physical input plug of the dose sensor

film screen comb. one out of five possible FSC data sets

dose nominal indicates the density correction value with + or – correction; 1.00 without correction

“dose nominal : xx OD” optical density correction values at 12.5% steps programmed

- 4 - 3 - 2 - 1 no + 1 + 2 + 3 + 4

0.63 0.71 0.8 0.9 1 1.1 1.25 1.4 1.6

I max max emission current kV dependent of the selected focus (might be limited by the max

emission current of the generator e.g. 650mA Optimus 50)

I min smallest possible emission current, always 1mA

I max NOMO if the exposure would be set

non-AEC in kV-mAs-ms or
kV-ma-ms technique the max
emission current in this case is

234.109mA for 2000ms at 73kV

TDC-calculation-process-pictures.doc Mar – 30 – 2005 page 3 - 20

dose calculated DRC calculation parameter (value without dimension calculated from the FSC speed)

kV factor kV correction factor relative to 70kV (determined by the chamber and screen characteristic)

U off density voltage kV dependent (determined by the chamber and screen characteristic)

t corrected see C eff ht explanation

number of steps number of calculation steps throughout the set exposure time

time/sample time of a sample step (multiply number of steps and time/sample = exposure time)

With the initial default APR data set one gets the following dose ramp and filament current curve:

If a deviation of the emission current has to be compensated or if the dose has to be controlled during a TDC
exposure the filament control unit uses only two filament current values to achieve the fastest change of the
emission current:

- minimum filament current 500mA

- maximum filament current; tube type and focus specific; to see the max value go path:

TDC-calculation-process-pictures.doc Mar – 30 – 2005 page 4 - 20

tube 1 tube 2 tube 3
large focus
small focus

1 3 5
2 4 6

small focus data SRO33100

Ifmax [mA] max filament current for the small focus SRO33100

Ifregelmax [mA] offset value to compensate the filament cooling due to the work

function; offset value only active during high tension

A perfect TDC exposure is when the dose ramp is a straight line from the exposure start to the density
voltage setpoint at the end of the exposure.
A second indication is when the filament current value at the end is the same as in the beginning.
There might of course be a lot of control amplitudes up and down throughout the exposure caused by the
non-linear penetration of the exposed region.

A trace data table can be read from the generator after the exposure:

TDC-calculation-process-pictures.doc Mar – 30 – 2005 page 5 - 20

TDC Trace
Fr 11. Mrz 08:05:02 2005

idx dose nominal dose actual dose rate reg in reg out new I

----------------------------------------------------------------------------
0 +0.0E+0 +0 +0.0E+0 +0.0E+0 +0.0E+0 +0.0E+0
1 +0.0E+0 +0 +0.0E+0 +0.0E+0 +0.0E+0 +1.6000E+1
2 +1.8E+0 +2 +2.0E+0 +0.0E+0 +1.000E+2 +1.6000E+1
3 +7.4E+0 +5 +3.0E+0 +6.0E-1 +1.081E+2 +1.7296E+1
4 +1.48E+1 +12 +7.0E+0 +1.03E+0 +1.035E+2 +1.7895E+1
5 +2.22E+1 +19 +7.0E+0 +1.18E+0 +1.020E+2 +1.8247E+1
6 +2.96E+1 +27 +8.0E+0 +9.6E-1 +9.96E+1 +1.8167E+1
7 +3.70E+1 +33 +6.0E+0 +2.00E+0 +1.100E+2 +1.9976E+1
8 +4.44E+1 +39 +6.0E+0 +3.45E+0 +1.138E+2 +2.2725E+1
9 +5.19E+1 +47 +8.0E+0 +3.13E+0 +1.003E+2 +2.2788E+1
10 +5.93E+1 +55 +8.0E+0 +2.15E+0 +9.65E+1 +2.1982E+1
11 +6.67E+1 +64 +9.0E+0 +9.9E-1 +9.51E+1 +2.0902E+1
12 +7.41E+1 +74 +1.00E+1 +0.0E+0 +9.55E+1 +1.9959E+1
13 +8.15E+1 +82 +8.0E+0 -5.0E-2 +1.002E+2 +1.9989E+1
14 +8.89E+1 +91 +9.0E+0 -5.2E-1 +9.69E+1 +1.9360E+1
15 +9.63E+1 +99 +8.0E+0 -9.9E-1 +9.65E+1 +1.8682E+1
16 +1.037E+2 +107 +8.0E+0 -1.22E+0 +9.77E+1 +1.8250E+1
17 +1.111E+2 +114 +7.0E+0 -1.07E+0 +1.000E+2 +1.8250E+1
18 +1.185E+2 +121 +7.0E+0 -9.2E-1 +9.99E+1 +1.8231E+1
19 +1.259E+2 +129 +8.0E+0 -1.14E+0 +9.74E+1 +1.7756E+1
20 +1.333E+2 +136 +7.0E+0 -9.9E-1 +1.000E+2 +1.7750E+1
21 +1.408E+2 +143 +7.0E+0 -5.5E-1 +1.021E+2 +1.8122E+1
22 +1.482E+2 +151 +8.0E+0 -1.03E+0 +9.58E+1 +1.7358E+1
23 +1.556E+2 +160 +9.0E+0 -2.20E+0 +9.17E+1 +1.5909E+1
24 +1.630E+2 +167 +7.0E+0 -2.00E+0 +9.99E+1 +1.5884E+1
25 +1.704E+2 +174 +7.0E+0 -1.80E+0 +9.92E+1 +1.5752E+1
26 +1.778E+2 +181 +7.0E+0 -1.18E+0 +1.026E+2 +1.6159E+1
27 +1.852E+2 +186 +5.0E+0 -8.0E-2 +1.080E+2 +1.7442E+1
28 +1.926E+2 +193 +7.0E+0 +0.0E+0 +9.98E+1 +1.7405E+1
29 +2.000E+2 +201 +8.0E+0 -1.0E-1 +9.93E+1 +1.7278E+1
30 +2.074E+2 +209 +8.0E+0 -4.0E-1 +9.79E+1 +1.6907E+1
31 +2.148E+2 +216 +7.0E+0 -1.2E-1 +1.019E+2 +1.7221E+1
32 +2.222E+2 +224 +8.0E+0 -4.5E-1 +9.74E+1 +1.6772E+1
33 +2.297E+2 +232 +8.0E+0 -5.7E-1 +9.90E+1 +1.6602E+1
34 +2.371E+2 +240 +8.0E+0 -1.07E+0 +9.60E+1 +1.5936E+1
35 +2.445E+2 +246 +6.0E+0 -3.7E-1 +1.049E+2 +1.6710E+1
36 +2.519E+2 +252 +6.0E+0 +0.0E+0 +1.021E+2 +1.7060E+1
37 +2.593E+2 +257 +5.0E+0 +5.7E-1 +1.049E+2 +1.7895E+1
38 +2.667E+2 +264 +7.0E+0 +9.9E-1 +1.034E+2 +1.8498E+1
39 +2.741E+2 +272 +8.0E+0 +5.2E-1 +9.75E+1 +1.8027E+1
40 +2.815E+2 +280 +8.0E+0 +3.7E-1 +9.98E+1 +1.7983E+1
41 +2.889E+2 +288 +8.0E+0 +9.0E-2 +9.85E+1 +1.7710E+1
42 +2.963E+2 +296 +8.0E+0 +0.0E+0 +9.97E+1 +1.7656E+1
43 +3.037E+2 +302 +6.0E+0 +4.2E-1 +1.035E+2 +1.8267E+1
44 +3.111E+2 +309 +7.0E+0 +5.2E-1 +1.009E+2 +1.8424E+1
45 +3.186E+2 +316 +7.0E+0 +9.6E-1 +1.038E+2 +1.9119E+1
46 +3.260E+2 +322 +6.0E+0 +2.00E+0 +1.095E+2 +2.0925E+1
47 +3.334E+2 +331 +9.0E+0 +6.0E-1 +9.21E+1 +1.9267E+1
48 +3.408E+2 +338 +7.0E+0 +1.03E+0 +1.049E+2 +2.0203E+1
49 +3.482E+2 +344 +6.0E+0 +2.10E+0 +1.098E+2 +2.2179E+1
50 +3.556E+2 +352 +8.0E+0 +1.80E+0 +9.94E+1 +2.2036E+1
51 +3.630E+2 +0 +0.0E+0 +0.0E+0 +0.0E+0 +0.0E+0

Explanation of trace screen columns:
(some values appear as exponential values, e.g.: +2.2036E+1 = 2.2036 x 101 = 22.036)

idx index number of step

dose nominal dose setpoint value generated from the dose calculated parameter of the TDC calculation

screen; typically slightly higher compared to the calculated value

TDC-calculation-process-pictures.doc Mar – 30 – 2005 page 6 - 20