LIGHT IMAGE RECORDERS

Conventional Spot Film Recorder

• Spot film devices interpose an x-ray film cassette between the x-ray beam and the image intensifier tube.

• The standard 91/2-in. square cassette is now replaced with several sized cassettes. • During fluoroscopy, the radiologist can at any time move the cassette from its park

position (shielded by lead) to a position ready for exposure → But, there is a delay before a film moved into position and an exposure made (3/4 - 1 sec).

Several factors make this delay necessary:

ƒ First, Heavy cassette.

ƒ Some changes at the x-ray factors are required.

8 Fluoroscopy is conducted at about 80-90 kVp and 1-3 mA of tube current. 8 Spot film uses fluoroscopic kVp, but requires much higher mA (300-400 mA) 8 So, time is required to increase x-ray tube filament heating, and the x-ray tube

anode rotation speed.

8 A phototimer controls the length of exposure. Spot "or Photospot" Film Cameras

• Records the image output of an image intensifier on a film.

• Produce reduced size images: typically, the film is roll film of size 70 or 105 mm, or cut film 100 mm in size.

Recall that the light from the output phosphor of the image intensifier is converted into a parallel beam image by a lens placed close to the output phosphor (refer back to Fig. 12-5). A semitransparent mirror placed in this parallel beam will allow about 10% of the light to travel on to the TV camera, and reflect the remainder to a photospot camera or a cine camera

• Originally, it was confined to gastrointestinal fluoroscopy → now used for all fluoroscopic images, including angiography.

This has been made possible by the improved resolution of cesium iodide image

intensifier tubes.

Large input phosphor diameters up to 16 in. allow large areas of the body to be imaged without moving the image intensifier → a single photospot film can cover almost the entire abdomen (this could not be done with a 9-in. tube).

Advantages:

1. Most significant → Marked reduction in patient exposure.

ƒ The recommended per-frame exposure for photospot film cameras is 100 µR. The exposure for a comparable spot film is 300 µR, three times as much.

ƒ Generally, the dose to the patient is 3-5 times smaller than with full-size (cassette-loaded) film.

N.B. 100 mm film requires a greater patient dose than 70 mm.

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ƒ The film does not have to be changed between exposures.

ƒ The delay between initiation and completion of an exposure is shorter. ƒ Exposure times are shorter (in the 50-ms range) → ↓↓ motion.

ƒ High framing frequency = 12 frames/second.

3. it is possible to record and view the image at the same time → since the camera is

recording directly from the output phosphor.

4. The resolution of the resulting films is that of the image intensifier, about 4 line pairs per mm (a range of 3 to 5 Ip/mm).

Resolution of routine spot films is theoretically greater than that of spot film camera films → but longer exposure times required for spot films may degrade the image because of motion unsharpness.

5. Savings in film costs of greater than 80%. Disadvantages:

• The little films are a nuisance to process and store.

• It needs practice to feel comfortable looking at angiograms on small format. • The sharpness or resolution, being that of the image intensifier, is less good. CINEFLUOROGRAPHY

• Cinefluorography is the process of recording fluoroscopic images on cine film. • Records from the output phosphor of the image intensifier.

• A beam splitting mirror allows simultaneous cine recording and television viewing, just as with spot film camera recording.

• Two film sizes: 16 mm and 35 mm.

• In the United States, 98% of all cine is done on 35 mm film, and 95% of all cine studies involve the heart. Therefore, our major emphasis will be on 35 mm cardiac cinefluorography.

Cine Camera

• The basic components of the camera are: ƒ Lens.

ƒ Aperture

ƒ Shutter.

ƒ Pulldown arm.

ƒ Pressure plate.

ƒ Film transport mechanism. • Light enters the camera through the lens and is restricted by the aperture.

• The shutter is a rotating disc located in front of the aperture with a sector cut out of its periphery.

As the shutter rotates → it interrupts light flow into the camera.

• While the shutter is closed, the pulldown arm advances the film to the correct position for the next exposure (Fig. 13-13).

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• The framing frequency = 60 "divided or multiplied by a whole number (e.g., 71/2, 15, 30, 60, or 120)".

• The combination of the framing frequency and shutter opening determines the amount of time available for both the exposure and pulldown.

For example, with a 180° shutter opening and 60 frames per second, the time available for both the exposure and pulldown is 1/120 sec.

Framing

• Framing is controlled by the lens of the cine camera.

• Exact framing means that the entire image (the output phosphor of the II) just fits

on the cine film.

• Over-framing means that only a portion of the image is recorded on the film →

so, some of the output image is not recorded.

ƒ The film image with overframing is larger than the film image for exact framing, and this increased size is usually desirable.

ƒ Extremes of overframing are generally avoided because patient exposure in areas not recorded is undesirable.

• Framing characteristics are established when the cine system is installed.

X-Ray Exposure

• The timing and intensity of the x-ray exposure are controlled during cinefluorography by two electrical signals that originate from within the cine system.

ƒ One signal coordinates the x-ray exposure with the open time of the camera shutter (synchronization).

ƒ The other maintains a constant level of intensifier illumination by varying the exposure factors for areas of different thickness or density (automatic brightness

control, mentioned before). Synchronization

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camera shutter was closed.

• These continuous exposures had two serious disadvantages:

ƒ Large patient exposures.

ƒ Decreased life expectancy of the x-ray tube.

• In all modern cinefluorographic systems → the x-ray output is intermittent & the synchronized with the open time of the camera shutter.

• The shutter of the camera is timed by 60 Hz power → permits shutter speeds that are fractions or multiples of the number 60 (e.g., 7 1/2 , 15, 30, 60, and 120 frames exposed per second).

At low framing frequencies the image flickers because the x rays are pulsed, but flicker does not interfere with monitoring.