Siemens Mammomat 3000 User manual

MAMMOMAT 1000/3000

Service Instructions

New AEC

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Replaces: n.a.

0 - 2 Revision

Chapter Page Revision

1all01 2all01 3all01 4all01 5all01 6all01

MAMMOMAT 1000/3000 Register 5 SPB7-230.898.01 Page 2 of 4 Siemens AG

Rev. 01 04.00 TD SD 24 Medical Engineering

Contents 0 - 3

Page

1 _______Functional description __________________________________________1 - 1

Overview ........................................1-1

MainfunctionoftheMAMMOMAT............................1-1

Tube.........................................1-1

Filter.........................................1-2

ObjectTable.....................................1-2

FilmandScreen...................................1-3

Detector.......................................1-3

MeasuredSignal...................................1-3

Gain.........................................1-3

DoseMeasuring...................................1-3

DoseRateMeasuring................................1-4

TimeDependency..................................1-4

TwoWings......................................1-4

UserChoices ....................................1-4

Exposurecontrol ....................................1-4

ControlofOneExposure ..............................1-5

InitialDose......................................1-5

DoseCorrection...................................1-5

EntireSetofControlParameters..........................1-7

CorrectionTables..................................1-7

SensitivityCorrection................................1-8

Sensitivity......................................1-8

2 _______Error messages ________________________________________________2 - 1 3 _______Parts replacement ______________________________________________3 - 1

ActionlistforMAMMOMAT1000/3000withnewAEC.................3-1

4 _______Description of LED’s and measuring points_________________________ 4 - 1

MeasuringpointsonAECboardD701.........................4-1

SwitchesonAECboardD701..............................4-1

JumpersonAECboardD701..............................4-1

LEDsonAECboardD701 ...............................4-2

5 _______Tests_________________________________________________________5 - 1

Check the AEC according to "Installation and start-up instructions"

forMAMMOMAT1000/3000............................5-1

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0 - 4 Contents

Page

6 ______ Problems during Installation/ Service ______________________________6 - 1

AECCorrectiontables.................................6-1

Measured dose rate exceeds +

Errorduringbackupfromdisc/floppy .........................6-2

Errorduringresetofinstallationparameters......................6-3

Errorsduringoffsetcompensationtest.........................6-4

DetectorNormalization.................................6-6

Normalizationfactoroutofrange..........................6-6

Miscellaneous .....................................6-7

CommunicationErrorswithServicePC ......................6-7

30%........................6-1

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Rev. 01 04.00 TD SD 24 Medical Engineering

Functional description 1

Overview 1

This document describes the control model of the AEC unit for M1000/3000. A view of the function of the Mammomat is given together with a summary of possible exposure settings, their effects and characteristics of Mammomat performance. The function of dose control is described together with an explanation of measurements done during the exposure, on which it is based.

Main function of the MAMMOMAT 1

Picture below gives a schematic view of the exposure system.

X-ray Tube

ilter

1-1

Target - Breast

Obj ect Tab le

Screen and F ilm

Detector

Schematic view of the Mammomat

The generator supplies a voltage across the x-ray tube. The resulting radiation is filtered before it reaches the target. Then it passes through an object table and reaches the fluorescent screen, which emits visible light giving the desired picture on the film. The part of the radiation, which is not attenuated in the screen, reaches finally the detector and results in a signal measured by the AEC unit. The main purpose of the AEC unit is to stop the exposure so that the resulting picture is given the desired average optical density based on current exposure settings and the measured signal from the detector.

Tube 1

When a voltage is applied acrossthe x-ray tube, the electrons, hitting the anode, initiate radiation. The tube contains two different anode materials - molybdenum and tungsten giving different kind of x-ray spectra. There are actually two different anodes for each mentioned material, used to give large and small focus for the x-ray beam. Small focus is usedtogether with magnifying objecttables. Large focus is used for all other object tables.

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1 - 2 Functional description

Theoperator isallowedto choosea desiredtubevoltagefrom a rangebetween23 and35 kV with 1 kV resolution. The voltage influences the energy spectrum and gives the operator a control of how hard x-rays are used during the exposure. The current through the tube, which is directly proportional to the overall intensity of the beam, is a function of the chosen voltage, the maximum power setting and the maximum current limit.

Neither the energy spectrum of the beam nor it's intensity are constantduring the exposure, due to raising and falling times for the voltage and the tube current. The voltage has typical raising timesof app. 50 ms and the tube current 5 ms.

Filter 1

Different filtering is used to give the beam desired energy spectrum. For the molybdenum anode, two filter selections are possible: molybdenum and rhodium. For the tungsten anode, only rhodium is used as the filter material. This gives three possible anode/filter combinations - Mo/Mo, Mo/Rh and W/Rh - resulting in three main kinds of x-ray beam. Within each, a fine adjustment of the energy spectrum, i.e. the hardness of the beam, is possible by adjusting the voltage.

Object Table 1

There are totally 7 different kinds of object tables that can be used described in the table below.

Name Type Used beam focus Size Magnification

Grid 18 x 24 Grid Large 18 x 24 cm Grid 24 x 30 Grid Large 24 x 30 cm NoGrid 18 x 24 Grid-less Large 18 x 24 cm NoGrid 24 x 30 Grid-less Large 24 x 30 cm Mag 1.5 Magnification Small - 1.5 Mag 1.8 Magnification Small - 1.8 Biopsy

table

According to their x-ray characteristics and use, the object tables are ordered into four groups: grid, grid-less, magnification and biopsy:



Gridtablesareusedtotakeordinarypicturesandareequippedwithamovinggridthat

reduces the amountof secondaryradiation, i.e.scatteredradiationfrom thetarget

object.



The grid-less tables are not equipped with a grid.



The Magnification tables have enlarged distance between the object and the film, giving

togetherwithsmallfocusof theradiation thedesiredmagnificationin thepicture.

Grid-less with biopsyunit

Large



The biopsy table, also called stereotactictable, isvery similar to the grid-less tables.

Itis equippedwitha biopsyunit thattogether withstereotacticsisusedto identify the

3D-position of the biopsy target within the breast and perform the biopsy. The pictures

are smaller and can be taken from a direction not perpendicular tothe object table.

Angles of+

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10° are used.

Rev. 01 04.00 TD SD 24 Medical Engineering

Functional description 1 - 3

Film and Screen 1

A considerable part of the radiation, which passes the target and possibly the grid of the object table, is attenuated in the fluorescent screen. The energy is than emitted as visible light and gives the desired picture on the film. The film is also sensitive to the x-rays, however the x-rays do not contribute to the picture significantly, compared to the visible light emitted by the screen.

Detector 1

After passing the filter, the breast, the object table with screen and film, the radiation finally reaches the detector. It is a semiconductor device that acts as a current source supporting a current proportional to the overall intensity of the attenuated radiation. The attenuation of the detector varies of course with the energy of the radiation, which makes the generated current dependent on the energy spectrum of the beam. The generated current is amplified and converted to a voltage level, which is the signal received from the detector by the AEC unit.

Measured Signal 1

The signal from the detector is amplified on the AEC board and than converted to pulses with a frequency proportional to the voltage level of the signal. The AEC unit contains a PLD (Programmable Logic Device), programmed to register the detector signal by counting the pulses that the signal is conver ted to.

Gain 1

Before every exposure, there is a possibility to alter the gain applied to the detector signal by the AEC board. Higher gain gives stronger signal to the V/F-converter, i.e. more pulses and better precision in the counters of the PLD. There is however an upper bound for the voltage level of the amplified signal - the V/F-converter functions for signals up to 10 V and all voltages above this limit does not contribute to larger frequency of out-coming pulses.

Dose Measuring 1

ThedoseismeasuredbycountingthepulsesfromtheV/F-converterduringatimeof interest. This dose measure is not comparable to any conventional dose units because it gives varying responses depending strongly on the energy spectrum of the radiation registered by the detector. However, for the same anode, filter, tube voltage, object table, screen and object, this dose measure is directly proportional to the dose received by the screen.

A logarithmic scale for the dose is also used. It is based on the usual definition of relative exposure and the unit exposure points (EP).

Dose

ΕΦ

DoseDoseEP

log10 log log10

Dose

The used logarithmic scale is however absolute in the sense that 0 EP is defined to be equal to 100 counts:

Dose

counts

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10100

i.e.

Dose

log10

Dose

100

counts

1 - 4 Functional description

Dose Rate Measuring 1

In order to measure the dose rate, the pulses are counted during a period of time and the resultingdoseisthendividedbythelengthofthetimeperiod.Theresultingdoserateis an average over the chosen time interval.

Time Dependency 1

Due to the raising and falling times of the tube voltage and current, the detector signal is also time dependent and has typical raising times of 50 ms.

Two Wings 1

The Mammomat can have one or two wings using separatedetectors.

User Choices 1

The following table summarises possible user choices affecting the exposure:

User choice Range Change consequence

Anode/ filter Mo/Mo, Mo/Rh, W/Rh Rough change of spectral properties of the

beam, possibly affecting power settings and automatic choice of tube current.

Tube voltage 23 - 35 kV Fine change of spectral propertiesof the

beam, possibly affecting power settings and automatic choice of tube current.

Speed H or D Choice between two sets of parameters for

two different film/ screen-combinations.

Density correction -24/8 to + 24/8 EP Relative adjustment of exposurelength,

0 = nominal AEC exposure. Object table One of 7 See section Object Table Wing 1 or 2 Different detectors in both wings.

Exposure control 1

Thecontrolmodelforthe AEC-unitis typically subdividedinto twoparts.Firstpart handles the control of a single exposure with one set of exposure settings such as kV, anode/filter etc. It is based on the knowledge of these parameters before the start and measurements done during the exposure. Second part puts the control model for one exposure into a systematic approach to all possible exposure settings for the Mammomat. It explains how the entire set of control parameters for the AEC-unit is build up in order to cover all possible combinations of kV, object tables, anode/filter choices and up to 2 film/screen combinations.

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Functional description 1 - 5

Control of One Exposure 1

Before start of exposure, the AEC-unit is only aware of exposure settings chosen by the operator and has no knowledge of thickness or density of the actual breast. This implies a two step approach to control of the entire exposure. First step is done before the exposure start and contains calculation of an initial dose that will be executed before the AEC will decide what to do next. During the execution of the initial dose, the unit measures the dose rate and uses it to calculate a correction dose that has to be executed before the exposure stops. When the AEC-unit has registered a total dose being equal to the sum of the initial dose and the correction dose, it will stop the exposure.

Initial Dose 1

The initial d ose is chosen in EP. It co nsists of an estimated dose, a sensitivity, a sensitivity correction and a density correction:

DcScSDoseDose

ΗΗΗΖ

EPEPEPEPestimatedEPinitial

,,,,,

where: Dose

estimated,EP

- the estimated dose, a value specifically chosen for the used film/screen

combination, anode/filter combination, object table group and tube voltage.

- the sensitivity, a value controlling an overall level of target optical density for all

AEC-exposures.

SC,

- the sensitivity correction, chosen as a part of the calibration of specific equipment

(Mammomat, object tables).

DC,

- the density correction, a value that is chosen by the operator for each exposure.

The estimated dose is usually chosen, so it will give an OD = 1,5 for a 5 mm thick PMMAphantom.

Dose Correction 1

In order to measure the dose rate with the detector, the following procedure is used. The initial dose is subdivided into a 60%-part and a 40%-part. Both parts are handled in the AEC-unit by two separate counters, implemented in the PLD. At the exposure start, the 60% counter starts counting down at the rate of pulses from the V/F-converter that handles the amplifiedsignal from the detector. During counting down the 60% of the initial dose, time is measuredby a time monitor. Whenthe 60% counterhas reached 0, the time is read and used together with the actual value of 60 % of the initial dose as an average dose rate:

Dose

6,0

%60

ΕΦ

DoseTime

6,0

countsinitial

where

DoseTime,6,0

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- time as a function of 60 % of initial dose in counts.

countsinitial

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