X-ray detection

What is indirect conversion?

- X-ray to light to electrical signal.

- A phosphor or scintillator makes the light; something else turns the light into a signal.

What uses indirect conversion?

Film, CR, most flat panels and conventional CT

What is direct conversion?

X-ray straight to electrical charge, with no light step.

What uses direct conversion?

Amorphous selenium flat panels, and the photon counting detectors in the newest CT scanners

What happens at each light step?

The light step is where sharpness is lost (light spreads) and where efficiency is won (a good scintillator stops more of the beam

What is the structure of a film and screen?

● A cassette holding film between two intensifying screens

What does the screen phosphor convert in film and screen?

The screen phosphor converts X-rays into light

What does the film do and what type of conversion is this?

the film records the light

- indirect

Where was exposure created in film and screen?

Exposure creates a latent image in the silver halide crystals

What happens to wet chemical processing in film and screen?

Wet chemical processing: develop, fix, wash, dry.

Name the 4 reasons the film went?

- One chance at exposure

- No processing after the fact

- Chemistry and time

- Storage and sharing

Why was there only once chance of exposure in film and screen, what was the issue?

A fixed characteristic curve with narrow latitude. Over or under expose and the film was repeated, more dose.

Describe the plate for computed radiography?

● A photostimulable phosphor, typically barium fluorohalide doped with europium, on a flexible backing inside a cassette

Explain what happens in the plate in computed radiography?

- X-rays excite electrons in the phosphor; a proportion are caught in traps and stay there

- That pattern of trapped electrons is the latent image.

Why are plates read promptly in computed radiography?

fades slowly over hours

What scans the plate in computed radiography?

A fine red laser scans the plate line by line

How does the reader read the plate in CR?

- A light guide carries the flashes to a photomultiplier tube, which turns them into an electrical signal

- The signal is digitised, one pixel at a time, into the image

How is the plate emptied in CR?

The plate is flooded with bright light to empty the remaining traps, then returned to the cassette

What are the 6 tips when working with CR?

1. Wipe the plate before use

2. Find damaged plates

3. Erase plates that have been sitting

4. Remove artefacts that get stuck

5. Clean the reader

6. Handle by the edges

What is a PMT tube?

A vacuum tube that turns a very small flash of light into a measurable electrical pulse

How does the PMT work?

1. Light strikes a photocathode and releases a few electrons

2. Those electrons are accelerated onto a series of dynodes, each at a higher voltage than the last

3. Each dynode releases several electrons for every one that arrives

4. After ten or so dynodes the handful has become millions: a gain of around a million

List the features of PMT tubes?

Very sensitive, fast, and fragile; needs high voltage and a vacuum

Where are PMT scanners still being used?

- CR readers

- Gamma cameras

- PET scanners in nuclear medicine

- Scintillation detectors in physics

- QA equipments

What are newer PET scanners using and what are they?

silicon photomultipliers: solid state, compact, and unaffected by magnetic fields, which is what made PET-MR possible.

What doesn't use PMT today and what is used instead?

- Flat panel detectors

- CT detector

- The light is read by a photodiode sitting directly under the scintillator

What differs between a CR and flat panel detector?

Either a scintillator (indirect) or a photoconductor (direct)

What are the three things CR and flat panel detectors share

- Pixel array underneath

- Row by row readout

- Calibration built in

How does the pixel array underneath CR and flat panel detectors work?

Millions of pixels on a glass sheet, each with a storage capacitor and a thin film transistor acting as a switch.

- Pixel pitch around 100 to 200 µm; smaller in mammography.

How does row by row readout CR and flat panel detectors work?

Rows are switched on one at a time and the charge in each pixel is read along the columns, digitised, and assembled into the image within seconds.

How does rcalibratoin built in CR and flat panel detectors work?

Dark signal and gain differences between pixels are measured and corrected before you ever see the image

How does indirect digital radiography work?

● Scintillator converts X-rays to visible light: caesium iodide grown as needles, or gadolinium oxysulphide as a powder

● Light travels down to an amorphous silicon photodiode in each pixel and becomes charge

What does caesium iodide act as indirect DR?

Caesium iodide needles act like optical fibres, so the light spreads less and the layer can be thicker

- More X-rays stopped without losing sharpness

Outline the pathway of indirect digital radiography?

X-ray → light → charge

What does the high defective quantum efficiency mean for indirect digital radiography?

so good images at low dose.

How does direct digital radiography work?

● A layer of amorphous selenium absorbs X-rays and releases charge directly

● A bias voltage across the layer pulls the charge straight down to the pixel electrode beneath it, with almost no sideways spread

What are the features of direct digital radiography and why does it dominate mammography?

● Very sharp: excellent spatial resolution

What is selenium and what does it do in direct DR?

- Selenium is a lighter element, so at general radiography energies it stops fewer photons than caesium iodide

- It is also sensitive to temperature and to damage

Outline the conversion route in direct DR?

X ray → charge

What 4 things did digital radiography change?

- Wide dynamic range

- Processing

- Exposure index

- Dose creep

How did digital radiography change wide dynamic range?

The detector responds usefully across a huge range of exposures. Under and over exposure no longer looks like a light or dark film.

How did digital radiography change processing?

- Windowing, edge enhancement and noise reduction happen before you see the image.

- The picture is a processed picture.

How did digital radiography change exposure index?

- the image no longer tells you the exposure, the system reports a number that does.

What 3 things show a fault in the detector?

- Dead pixels and dead lines

- Calibration drift

- Damage

How do dead pixels and dead lines show a fault in the detector?

A failed pixel, or a failed row or column line, gives no signal.

- Small numbers are mapped and interpolated over by the software

- A cluster or a full line is visible

How does calibration dift show a fault on the image?

Gain and offset maps go stale with temperature and age. The result is uneven brightness, faint stripes or a repeating pattern across the field.

How does damage show a detector fault?

A dropped wireless panel can crack the scintillator or the glass. Selenium panels are also vulnerable to heat and cold.

- The mark is permanent.

What type of detector is used in CT?

A CT detector is an indirect detector bent into an arc

What type of scintillator is used and the materials in CT?

A ceramic scintillator such as gadolinium oxysulphide or a garnet, with a photodiode behind each element

Describe the elements used in CT?

Thousands of elements across the arc, 64 to 320 rows deep, read out thousands of times per second as the gantry turns

What has the same conversion route as a CT scanner?

● indirect flat panel, engineered for speed and for very high dose rates

What is used between each element in CT?

Anti-scatter septa between elements, and a bowtie filter in front

Xenon was used in earlier CT, what are xenon detectors in CT?

Chambers of pressurised xenon gas

- X-rays ionised the gas and the charge was collected between plates

What were the benefits and limitations of xenon detectors in CT?

Cheap, stable and uniform, but they stopped far fewer photons than a solid scintillator, and could only be built as a single row

When was Multi-row helical CT possible?

Multi-row helical CT was only possible once xenon had gone.

What materials are used in photon-counting CT?

Cadmium telluride or cadmium zinc telluride

How do these materials convert X-rays into electrical charge in CT photon counting?

Converts each X-ray straight to charge but with enough stopping power for CT energies.

How does photon-counting CT measure individual photons and their energy?

Each photon is registered individually and its energy measured, instead of the total light being integrated.

What are the main benefits of photon-counting CT?

Sharper images, lower dose, and energy resolved data from every scan