Let’s continue discussing remaining components of The Image Intensifier Tube.
The Output Phoshpor is also composed of Cesium Iodide, so when the electrons from
the photocathode interact with it, their kinetic energy is transformed into visible light.
As stated previously, since the electrons reaching the output phosphor are more highly
concentrated, the result is a brighter visible light image produced. It should be noted that
the image received at the output phosphor is upside down compared to the image received
at the input phosphor. This is because of the focal point created at the anode within
the image intensifier. In the image on the left, imagine an x-ray photon to the left
of the image intensifier representing imagery of the patient’s stomach. As we follow the
path the energy takes through conversion through the image intensifier, it reaches a focal
point, and continues traveling to the opposite side of the output phosphor. This is easily
correctable within the display components we will discuss later.
Let’s talk about brightness gain. There are two processes that contribute to intensifying
the image, ultimately resulting in the final brightness achieved in the light image emitted
from the II toward the TV camera or other recording devices. These are flux and minification.
Minification Gain is the ratio between input phosphor screen size, or diameter, to the
output phosphor screen size. The mathematical formula can be expressed as the diameter of
the input phosphor squared, divided by the diameter of the input phosphor squared. In
your textbook, the example given has a 30 cm input phosphor and a 2.5 cm output phosphor.
The formula for diameter is pi times radius squared. For the input phosphor, that’s
3.14 x 15 squared, which equals 706.5 square centimeters. For the output phosphor, that’s
3.14 x 1.25 squared, which equals 4.9 square centimeters. These round to 707 and 5 respectively.
The result is 141.4, meaning the brightness of the light emitted from the image intensifier
will be 141 times greater than the light originally emitted from the input phosphor from the minification
process alone.
Flux gain is also used to calculate total brightness gain. If you recall, flux gain
was the ratio of the number of light photons at the output phosphor to the number of x-ray
photons at the input phosphor. Your textbook gives an example where a particular flux gain
for an image intensifier is equal to 70. To find Brightness gain, you would multiply the
flux gain, which was 70 by the minification gain in the previous example of 141. The result
is that the total brightness, or Brightness Gain is 9,870, meaning the light at the output
is 9,870 times brighter than the original brightness at the input phosphor. Typical
brightness gains can range from 5,000 to as much as 20,000. Now think of those radiologists
in the first video in this section who were being bombarded by x-ray photons coming from
the patient to their faces. Think of the patient exposures that must have been happening without
this technology, and the reduction in dose we experience today because of it. It’s
no lie that today is the safest it’s ever been to be in the field of radiography.
The international commission of radiologic units and measurements recommends evaluating
the brightness gain of image intensifiers based on the conversion factor. This is the
radio of the luminance of the output phosphor to the input exposure rate, measured in milliroentgen
per second. The candela is the unit of measurement for luminance, or light intensity. This method
is considered far more accurate and reproducible than the older method for calculating Brightness
Gain, which was measured by an estimation of flux rather than actual measured quantities.
Since the conversion factor is approximately 0.01 times the brightness gain, image intensifiers
typically have conversion factors of 50 to 200.
Most modern IIs are Multifield Image Intensifiers and possess Magnification Modes. Multifield
image intensifier tubes are designed to have a variable field of view. The larger the input
phosphor, the larger the field of view will be, which is useful for examinations where
a large area of the patient’s anatomy must be viewed. A lower GI series is a good example
for this. Because less panning is required, studies can be kept shorter potentially. When
magnification mode is selected, this simply shifts the focal point within the image intensifier
closer to the input side. As this happens, the electron field widens, or gets larger
at the output. Since the size of the output phosphor does not change, this just causes
a smaller portion of the patient anatomy to be visible within the field of view. This
helps with image resolution since the number of pixels available at the output is the same,
yet the area of anatomy inputted into the image intensifier is smaller, meaning there
are more pixels per area of anatomy. This also causes the image to be much dimmer. When
there is less input light, the minification gain is reduced. We’ll talk about Automatic
Brightness Control soon, which should compensate for this loss of brightness which causes a
lower signal to noise ratio. Unfortunately, mag mode requires increased mA, so skin dose
does increase for the patient. The radiologist must consider all these factors when opting
to operate the fluoroscope in magnification mode.