Digital Imaging Technology

Computed Radiography

a technology whereby a photostimulable phosphor is housed in a light tight cassette similar to a screen film system.  In this case, though, instead of a film, the phosphor holds the image

• fraction of absorbed energy is held in traps which can be stimulated with a laser at a later time

what common phosphor is used for CR

barium fluorohalide

process of reading out a CR image

1. The cassette is placed in the reading machine.

2. The CR plate is extracted and translated through a fan beam laser as shown above.

3. The red laser light stimulates blue luminescence which is collected by a fiber optic array and shuttled to photomultiplier tubes (PMT).

4. The PMT signals are digitized into pixel values.

5. The CR plate is exposed to a bright white light thereby erasing it.

6. The CR plate is placed back into the cassette and ready for reuse.

what does a CCD use to make an image and where is it commonly used

visible light

common with digital cameras

Charge coupled devices - how are they read out

1. As the detector is exposed to light, charge is built up in each dexel.  These are then read off in a bucket brigade fashion on one end

   1. The bucket brigade works by collecting the charge in the readout row and then shuffling all the charge buckets one step down toward the readout row and the process repeats until the whole array is read out

Complementary metal oxide semiconductor (CMOS) detectors

1. CMOS detectors, while very common in consumer electronics, have only found limited applications in medical imaging.

2. The CMOS architecture allows random access to any dexel in the array.

3. They can operate at low power levels but suffer from readout noise and an inability to create large arrays with a maximum dexel size of around 50 μm.  This results in a limited field of view of around 10 cm x 15 cm.

4. Each dexel contains a photosensitive region, a storage capacitor, and readout electronics including an amplifier.  This, by itself, results in a limited sensitive area unless additional modifications are made.

Thin Film transistor arrays material

1. made of amorphous silicon and consist of a detection area and associated electronics.

2. The electronics portion of each detector element are etched onto the surface using a lithographic process.

   1. More advanced manufacturing techniques allows placement of the electronics behind the sensitive area.  This allows fill factors approaching 100%.

Fill factor - definition and common values

1. The ratio of the photosensitive area to the area of the dexel is termed the fill factor.  Typical values fall in the range of 50-80% and vary with dexel size. With larger fill factors attributed to the larger dexels.

components of each dexel in a TFT

1. The TFT is basically a switch that has three connections:

   1. Gate - this is the on/off switch (closed during exposure).

   2. Source - this is attached to the storage capacitor.

   3. Drain - is connected to drain conductor (readout line).

2. The charge collecting electrode captures the charge from the incident x-ray energy.

3. The storage capacitor stores the charge collected by the collecting electrode until readout.

TFT readout

performed for each detector element.  Each dexel’s gate switch is triggered individually allowing the charge to flow from the source to the drain.  These charge readouts are sent to amplifiers outside of the active area of the panel which amplify the signal and send it on to be digitized.

Indirect TFT detection process

1. This technique uses a scintillator optically coupled to the amorphous silicon TFT array and is more common in practice than the direct detection method outlined below.

2. The x-rays strike the scintillator and are converted into visible light which then strikes the TFT.

3. Typically, CsI is the scintillator of choice due to its columnar growth structure shuttling the light due the detector without appreciable lateral spread similar to a fiber optic cable (light pipes).

4. The TFT array in use must be light sensitive in design.

direct detection TFT process

1. This technique uses a semiconductor capable of directly converting incoming x-rays to charge.

2. The most common material for this is amorphous selenium.

3. A voltage is placed across the amorphous selenium that sweeps the charge produced toward the amorphous silicon TFT array.

   1. The electric field lines can be designed in such a way so as to minimize the fill factor effect (i.e. the charge is shuttled away from the electronics).

   2. Even though amorphous selenium has a low atomic number and, therefore, relatively low x-ray interaction rates, the electric field ensures almost no lateral spread of electrons.  Therefore, this layer can be made relatively thick so as to increase efficiencies.

4. The TFT array in use must be charge sensitive in design.

5. Inefficient at higher kVps and often results in higher doses than the indirect detection technique.

Dark Field calibration

Uses no radiation, just acquires a blank image.  Think of this as a baseline. It is used to remove array variations (such as banding) from the final image.

flood field calibration

Uniformly irradiate the whole detector, used to map the sensitivity of each pixel and remove variations.

drift image calibration

Perform two dark field images with 15 seconds in between.  The first image zeroes the pixels (they are read off), for 15 seconds the pixels acquire a charge due to leakage.  Take another image and subtract. The resulting image is a map of leakage for each pixel (drift images can be used for pixel defect maps).

pixel defect map calibration

removes bad pixels from the final image by replacing them with the average of its neighboring pixels.