IMAGE RECORDING DEVICES
Before the light image coming from the image intensifier is processed for the display system,
it is always transmitted through a bundle of optic fibers directly to a CCD or CMOS
camera.
The CCD, or charge-coupled device, was introduced in the 1980s, and the technology allowed for
the creation and marketing for home camcorders, as it replaced the TV camera tube.
The CCD is a small, flat plate approximately 1 cm in length and breadth for a home camcorder.
The type used in fluoroscopy are about 2.5 cm, coupled to the 2.5 cm output phosphor.
In fluoro, it’s coupled by a short bundle of optic fibers, as opposed to an optical
lens option, which would allow for distortion effects like vignetting.
The CCD’s sensitive surface is made of a crystalline silicon, a semiconductor.
When light photons exiting the image intensifier tube interact with the silicon layer of the
CCD, ionization of the molecules separates electrons from them, leaving positively-charged
“electron holes” behind.
A layer of microscopic electrodes beneath the silicon is given a negative charge, and
a dielectric layer above acts as a ground for the freed electrons.
The electrons then drift toward the dielectric layer, and the positively-charged holes drift
toward the electrodes.
This drift consists of a series of electrons moving up to fill each vacancy in the molecule
above, in sequence, so the vacancy itself (or the hole) appears to drift in the opposite
direction.
When a hole reaches an electrode, it pulls an electron from it, creating a positive charge
on the electrode, which can easily be measured by the circuit.
This charge is then collected within a storage capacitor.
Thin film transistors are used in direct conversion systems to form an active matrix array, which,
once scanned, sends an electronic signal to an amplifier to become digitized and stored,
and ultimately is sent to the display system.
The CCD has a high sensitivity, or DQE.
DQE stands for detective quantum efficiency.
If we get one electron for each light photon being emitted by the image intensifier, we
would have 100% quantum efficiency.
But as you already know, any time energy is transformed, we lose a certain percentage
of that efficiency.
The dynamic range of a CCD is about 3000:1 for fluoroscopy.
With direct conversion, the CCD produces high contrast and signal to noise ratio, with almost
no image lag, low blooming, and high resolution.
Its resolution is determined by its pixel count and physical dimensions.
Your textbook provides an example of the CCD being 2.5 cm in size with a matrix of 2048
x 2048 pixels, making each pixel about 14 microns in size.
Using progressive scanning, where each line of the raster pattern is read sequentially,
the CCD can acquire images at 60 frames per second, compared to older TV camera tubes
using interlaced scanning only capable of 30 FPS.
CMOSs, or complimentary metal-oxide semiconductors, were pioneered in the 1970s around the same
time CCDs were being developed.
The name refers to the use of two metal oxide semiconductor field effect transistors, or
MOSFETs with one p-type, or positive transistor and one n-type, or negative transistor.
They stack together to form an electronic logic-gate.
This became the dominant method for manufacturing integrated circuits for computers, but for
cameras, CMOS circuits were inferior to the CCD.
Both systems use the photoelectric effect to liberate electrons in a silicon layer and
collect them as an electrical charge in a layer of transistors, but the primary difference
is the method in which the charge is read from the chip.
In CCDs, the charge is sent across the chip as an analog signal, read at one corner of
the array where the fluctuations are separated into individual readings to represent pixels,
then are digitized.
In CMOS sensors, each individual detector element (or dexel) has its own transistor,
amplifier, noise-correction circuit and digitization circuit so that it outputs digital data that
moves down traditional wires to exit the sensor.
For the CCD, the signal uniformity is very high, while all of the added electronics for
the CMOS reduce the surface area for the light capture, resulting in a higher amount of image
noise.
Improvements in the 1990s to CMOS technology made it nearly equal to CCDs, and since the
CMOS has a higher speed than the CCD, it only consumes about one-hundredth the power, and
can be manufactured less expensively.