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The image intensifier is comprised of a large cylindrical, tapered tube with several internal structures in which an incident x-ray distribution is converted into a corresponding light image of non-limiting brightness. A picture of an image intensifier television (II-TV) system is shown below. X-ray to light amplification is achieved in several sequential steps. First, x-rays incident on and absorbed by a cesium iodide (CsI) structured phosphor produce a large number of light photons resulting from the energy difference of x-rays (30-50 keV average) to light photons (1 -3 eV average). Absorption and conversion efficiency is on the order of 60% and 10%, respectively. A fraction of the light photons interact with an adjacent photocathode layered on the backside of the input phosphor, releasing a proportional number of electrons (typically on the order of 5 light photons / electron). Being negatively charged, the electrons are accelerated through a potential difference of approximately 25,000 volts towards the positive anode positioned on the tapered side of the evacuated tube. Electro-magnetic focusing grids maintain focus and at the same time minify the electron distribution as it interacts at the output phosphor structure, producing a large increase in the light intensity compared to the amount of light originally produced at the input phosphor. Overall brightness gain of the II is achieved through the acceleration and kinetic energy increase of the electrons impacting on the output phosphor (known as electronic or flux gain) as well as the geometric area reduction of the electron density from the large area input phosphor to the small area output phosphor (known as minification gain, equal to the ratio of the input to output phosphor areas, or ratio of the square of the diameters). The combination of electronic and minification gain results on the order of 5000X increase in brightness. Variable brightness gain occurs with a change in the input phosphor active area; as the field of view (FOV) is reduced, the minification gain is reduced, decreasing the overall brightness gain (and vice-versa). Optical coupling of the output phosphor to a TV camera or photospot, cine, or other light detector allows the detection of the image and subsequent display.

Control of the II "speed" is achieved with the inclusion of a light-limiting aperture in the conjugate lens system. For situations requiring high dose (i.e., digital subtraction angiography), the aperture diameter is reduced (large f number), while for low-dose fluoroscopic applications (i.e. upper GI fluoro), the aperture diameter is increased (small f number).

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Figure E illustrate a typical overtable II/TV system, housing, carriage (allows vertical and horizontal positioning, and table (the x-ray tube is mounted under the table with a fixed geometry relative to the II detector). Figure F shows an internal cross-section of the II, and important structures including the image intensifier envelope, the input phosphor comprised of structured cesium iodide (CsI) scintillator material, the photocathode comprised of a light-sensitive, electron emitting material (Sb2Cs3), the electron focusing electrodes, the anode structure, the output phosphor comprised of zinc cadmium sulfide (ZnCdS:Ag), the tandem conjugate lenses with light limiting apertures and partially silvered mirror (to partially reflect light), and light photon detectors (film, TV camera, CCD camera) to capture the output image and convert into a useful image for viewing.

Image intensifiers come in a range of input field of view (FOV) diameters for diagnostic imaging applications, from 6 inches (15 cm FOV) to 16 inches (40 cm FOV), and many dimensions inbetween, depending on the type of imaging procedure. II's have a spherical input phosphor structure, with a curvature designed to withstand the large force on the II enclosure, resulting from the internal vacuum required for operation. The output phosphor dimension is typically about 1 inch (2.54 cm) diameter. The difference in size between the input and the output results in minification of the output image, whereby the electrons emerging from the photocathode of the input are focused and minified during acceleration through the evacuated tube. Brightness gain achieved by minification is equal to the area of the input phosphor to the area of the output phosphor, resulting from the increased electron density and corresponding increased light intensity at the output phosphor. In Figure G, an illustration of two FOV examples demonstrate the electronic "magnification factor" that is available on most image intensifier systems.

The choice of technique in Figure I was 75 kV and 2.4 mA, which resulted in an entrance skin air kerma of 35 mGy/minute. When the image was magnified by a factor of 1.5 (Figure J), the system increased the x-ray tube voltage to 85 kV and used a tube current of 2.7 mA, which increased the entrance air kerma rate to 50 mGy/minute. When the image was magnified by a factor of 2.5 (Figure K), the system further increased the x-ray tube voltage to 94 kV and used a tube current of 2.8 mA, which increased the input air kerma to 61 mGy/minute.

(a) higher voltages will reduce the entrance skin air kerma which needs to be kept below 90 mGy/minute (10 R/min) for regulatory purposes. Adjusting the x-ray tube voltage with increasing magnification resulted in only relatively modest increases in the entrance air kerma rate (35 mGy/minute -> 50 mGy/minute -> 61 mGy/minute).

(b) the tube current needs to be kept below ~5 mA to minimize the power input into the x-ray tube anode permit continuous fluoroscopy operation without overheating the x-ray tube. The increased in power input to the anode (power is kV x mA watt) was also relatively modest (190 watt -> 230 W -> 260 W).

By contrast, maintaining a constant x-ray tube voltage with an increase in magnification of x 2.5 would have required an increase in entrance skin air kerma (and power loading to the x-ray tube anode) of 625% (i.e., 2.5^2) because of the six fold reduction in exposed area of the input phosphor.

(a) Increased x-ray tube voltage will reduce the amount of image contrast and the corresponding contrast to noise ratio. Maintaining the contrast to noise ratio is desirable as this improves lesion detectability.

(b) Patient dose consideration is of much less concern than in fluoroscopy for several reasons. Diagnostic image quality is normally the paramount concern, and one does not wish to compromise diagnostic performance by using too little radiation. Furthermore, there are no dose limits in radiography, which is used for diagnosis, whereas there are dose limits in fluoroscopy (entrance air kerma must normally be < 90 mGy/minute). Even though the patient dose per frame is high compared to fluoroscopy, the total number of photospot images acquired is very low.

(c) There are no x-ray tube heating problems in digital photospot imaging. The total energy deposited into the anode is a product of the power (kV x mA) and the total exposure time (s) (i.e., Energy (J) = kV x mAs). The total energy deposited in the three examples shown above ranges from 0.6 kJ (65 kV and 9 mAs) to 2.2 kJ (65 kV and 33 mAs). The anode tube capacities of x-ray tubes in typical fluoroscopy/radiography imaging systems is typically hundreds of kJ and x-ray tube heating is generally not an important issue.

In contrast to the variation of x-ray tube voltage in magnification fluoroscopy, the x-ray tube voltage is normally kept approximately constant in magnification modes for digital photo spot imaging. As a result, the entrance skin air kerma and energy deposition into the x-ray tube anode are approximately inversely proportional to the exposed area of the input phosphor of the image intensifier. Halving the field of view, which doubles the magnification and corresponding spatial resolution, would be expected to (approximately) quadruple the entrance air kerma in digital photospot imaging.

In this example, note that the Figure O (25 cm field of view) includes a small region around the skull that consists of the image intensifier being directly irradiated by the x-ray beam. This is generally undesirable (see collimation section below), and will result the selection of technique factors that take into account the relatively large detected signal from these directly irradiated regions. Predicting changes in the selected radiographic techniques (kV/mA), patient dose and x-ray tube loading under such conditions is particularly tricky.

The idea is to build a tube that is long enough for your lenses working distance yet narrow enough to create an interesting design. The reflection occurs naturally when aimed at your subject then accentuated when the flash is triggered.

Fortunately, you can now purchase the reflection tubes online. Coming in a variety of lengths and apertures to allow you to shoot with a 60mm/100mm or 105mm, full frame, APS-C even compact cameras and diopters can be used.

The store-bought devices in this image have a 67mm threaded base so that they can be attached to a standard flip adapter as opposed to mine which I hand held against my lens port.As with any image, lighting is everything and how you apply the light to your image will set it apart from the others. When your shooting this style of image, you will quickly notice that you can get multiple different effects by changing the lighting angle.

I made this tube about 4 years ago and used it a few times with my 60mm lens. I discovered nearness to the subject with the tube can give it the appearance that the subject is nearly in-side of the tube itself.

After lining up on this purple Rhinopias and shooting a few times, I decided to try something different. Using the blue mode on my torch for backlighting, and taking advantage or the particulate in the water, I was able to capture a photo that I thought was pretty interesting. For me, it looks like the rhinopias is in space!

Creating something exotic from something as familiar as a blue ribbon eel can pose all kinds of challenges. I used my snoot to help eliminate the sand and rocks behind the subject then remained patient and waited for the eel to move into the right position. When my strobe flash hit the subject the first time, I was hooked. The cobalt blue and gold came through nicely in the tube. 2351a5e196