COMISIÓN EJECUTIVA O DELEGADA Breve descripción

3) Screen efficiency, the proportion of the light produced in the screen that reaches the film (50%).

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The IF can be increased by increasing X-ray absorption by using thicker screen.

ƒ ↓ Exposure.

ƒ The same number of X-ray photons are absorbed in the screen for the same film density → No change in noise (quantum mottle)

→ Worse blurring or resolution will be with the thicker screen.

The IF can also be increased by increasing the conversion or screen efficiency.

ƒ ↓↓ Exposure & patient dose.

ƒ ↓↓ Number of X-ray photons → ↑ Noise.

ƒ ↑↑ Screen efficiency "e.g. using a reflecting layer"→ ↓↓ resolution, but ↑↑ conversion efficiency does not affect it.

Table 3.3 Summary of effects on image quality of increasing screen speed

Screen change Noise Resolution X-ray absorption Same Same

Density Same Same

Screen thickness Same Worse

Conversion efficiency Worse Same

Screen efficiency Worse Worse

3.8 CHOICE OF EXPOSURE FACTORS

The controls of an X-ray set usually include kV and two out of the three factors: mA, exposure time, and mAs (Table 3.2).

• In making a particular radiological examination using film-screen system: Film dose is approximately proportional to kV4 x mAs

• As a rule of thumb, since 904 in twice 754, increasing the tube voltage by 1/4 "15 kV" allows the mAs to be halved when imaging a given subject.

Table 3.2 Examples of exposure factor's

Examination Exposure factors

Barium meal (screening) 90 kV

0.5 mA (up to 5 mA if necessary) Adult chest X-ray 70 kV

10 mAs comprising a high mA (e.g. 300-500 mA) and a short exposure time of only a few milliseconds (e.g. 0.02 s) Chest X-ray

(high-kV technique) 120 kV 4-5 mAs (high mA & short exposure time)

Kilovoltage

• In general, as high as possible a kV will be used so as to

1) increase the penetration of the beam and reduce patient dose. 2) increase the latitude of exposure and range of tissues displayed.

3) reduce the mAs needed → allow shorter exposure times, within the rating of tube. • But not so high a kV that insufficient contrast results in the area of diagnostic interest.

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• Having chosen the kV for a particular examination, this determines the required mAs, which is then subdivided into

1) as short as possible an exposure time (to arrest motion) and

2) a correspondingly high mA - just within the rating of the tube (Section 2.7.3). Exposure time

• The necessary exposure time can be reduced by selecting 1) a higher kV and

2) a larger focal spot.

• Focal spot size, exposure time and screen speed should be chosen together, to

give minimal total blurring → occurs when the separate blurring components are approximately equal.

• The necessary exposure time can also be reduced by ↑ tube rating "allowable mA" using: 1) a three-phase generator, rather than single phase.

2) full-wave single-phase rather than self-rectification. 3) a higher speed and larger-diameter anode disk. High- Kilovoltage Radiography

• Carried out at maximum kV (say, 150 V), the mAs selected according to the thickness and nature of the part being radiographed.

• In consequence:

ƒ Subject contrast is low → exposure latitude is wide "wider range of tissue can be imaged on a film" → the choice of mAs less critical "↓ number of repeat exposures"; ƒ Skin dose is reduced;

ƒ Efficiency of X-ray production is high, thus reducing the heat loading and allowing very short exposure times;

ƒ ↑↑ Scattered radiation → grids are less effective & air gap is generally preferred.

Automatic exposure control (AEC)

• The exposure is controlled automatically by placing 3 thin sensor on top of the cassette to measure the intensity of the X-rays.

• The sensors "control device" may be either:

ƒ a flat parallel-plate ionization chamber(see Section 1.8); or

ƒ a phosphor, which converts the X-rays to light, coupled to a photo-multiplier, which measures the light.

• 3 sensors are placed at different locations in the X-ray field → allows control of the exposure to one region of interest or an average exposure over two or three areas.

• After selecting the kV → AEC terminate the exposure automatically when the correct mAs has been delivered to the film, irrespective of any misjudgment of the ideal kV (in respect of patient thickness, etc.) or fluctuations in the mains supply.

• The radiologist's preference for 'light' or 'dark' films is accommodated by a sensitivity or density control.

• The control device:

ƒ Should not attenuate the beam very much. ƒ Generally larger than the cassette.

ƒ Must not itself produce an image on the film.

In some designs of sensor, this last requirement would not be met, and it must be placed behind the cassette, which should then not incorporate the usual heavy metal backing.

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• Reject analysis of all films which are not of diagnostic value should identify which is at fault & actions can be taken accordingly:

1. Positioning.

2. Patient movement. 3. Choice of exposure factors. 4. Equipment. 5. Processing.

• To avoid repeat films and unnecessary patient dose; the following parameters should be checked on each tube and generator after each service and whenever there appears to be a lack of consistency of exposure.

KILOVOLTAGE:

• The actual tube kilovoltage should be within ±5% of the set value. • The kV can either be measured:

9 Directly (invasively) by potential divider applied across the high tension leads 9 Indirectly (non-invasively) by a penetrameter method.

• Penetrameter method:

8 Employs 2 copper absorbers of different thicknesses, beneath each of which is a photodiode.

8 The unit is positioned in the middle of the X-ray field, some 50-75 cm from the tube. 8 Attached to the tube is a 0.5 mm copper filter which transmits mainly X-rays near to

the peak kiloelectronvolt value.

8 Tube kV, mA, and exposure time are selected, and an exposure made.

8 Each detector produces a different current, proportional to the X-ray intensity falling on it. The higher the tube kV, the smaller the ratio of the two currents.

8 The current ratio is displayed, as the equivalent kV, on a digital read-out. The type of the waveform (pulsating or constant) can also be displayed.

8 The kV is checked at several settings of the kV control.

8 The kV meter itself needs to have the accuracy of its readings checked periodically at a national or secondary standards laboratory.

• In the same meter, the current from a single diode may be used to: 1. Monitor X-ray output.

2. Used to start and stop a built-in digital timer to measure pulse length or check timer accuracy.

3. Displayed on an external storage oscilloscope → showing the waveform more precisely, how quickly the X-rays switch on and off, and the exposure time.