Computed Radiography (CR) and Photostimulable Luminescence
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What is the primary function of the photostimulable phosphors (PSP) in Computed Radiography?

  • To reflect infrared light for better imaging
  • To hold the X-ray energy as a latent image (correct)
  • To minimize static electricity buildup
  • To protect the imaging plate from damage

Which layer in a PSP screen is responsible for reflecting light towards the imaging plate?

  • Binder layer
  • Protective overcoat
  • Reflective layer (correct)
  • Antistatic layer

How do electrons in the phosphor crystals of a PSP screen return to their ground state?

  • When stimulated by focused infrared light (correct)
  • By cooling to room temperature
  • Through a chemical reaction
  • By exposure to X-ray energy

What is the purpose of the antistatic layer in a PSP screen?

<p>To prevent static electricity buildup (C)</p> Signup and view all the answers

What is a metastable state in the context of photostimulable phosphors?

<p>A temporary state where electrons can be easily released (C)</p> Signup and view all the answers

What is the purpose of lead backing on the Imaging Plate (IP) in Computed Radiography?

<p>To reduce x-ray backscatter (B)</p> Signup and view all the answers

Which factor primarily affects the emitted signal during the stimulation of the Storage Phosphor Screens (PSPs)?

<p>Intensity of the infrared laser beam (C)</p> Signup and view all the answers

What happens to the latent image created in a Storage Phosphor Screen (PSP) after approximately 8 hours?

<p>It fades and becomes indistinguishable (A)</p> Signup and view all the answers

During the stimulation process in Computed Radiography, what is the primary region of the visible spectrum where the stimulated emission is detected?

<p>Blue region (A)</p> Signup and view all the answers

What is the role of photodiodes (PDs) in Computed Radiography?

<p>To detect stimulated emission (A)</p> Signup and view all the answers

What describes the initial reaction of electrons when exposed to an x-ray beam in the PSP?

<p>Electrons are excited into a metastable state (A)</p> Signup and view all the answers

How does the diameter of the laser beam affect the Computed Radiography system?

<p>It affects the resolution of the system (A)</p> Signup and view all the answers

In the light stimulation process of Computed Radiography, what is a common cause of signal loss during detection?

<p>Light scattering (C)</p> Signup and view all the answers

What are Storage Phosphor Screens (PSPs) primarily composed of?

<p>Small particles that are electrostatically protected (C)</p> Signup and view all the answers

In Computed Radiography, which component functions similarly to a screen-film cassette?

<p>Imaging Plate (C)</p> Signup and view all the answers

What is the primary purpose of the precision drive mechanism in the CR reader?

<p>To ensure accurate movement of the image plate (B)</p> Signup and view all the answers

Why is laser blanking necessary during the retrace in a CR reader?

<p>To avoid errors in image capture (B)</p> Signup and view all the answers

What characteristic of solid-state lasers makes them advantageous in CR systems?

<p>They emit longer wavelength light (D)</p> Signup and view all the answers

What is the consequence of exceeding the error tolerance during scanning in a CR reader?

<p>Appearance of wavy edges in the image (D)</p> Signup and view all the answers

How do optical filters function in the optical subsystem of a CR reader?

<p>They allow only the emitted light to reach the photodetector (A)</p> Signup and view all the answers

What is a potential issue caused by incorrect grid alignment during scanning?

<p>Increased likelihood of aliasing artifacts (B)</p> Signup and view all the answers

What role do beam-shaping optics play in the CR reader’s optical subsystem?

<p>They focus the laser beam to maximize interaction with electrons (B)</p> Signup and view all the answers

What is the likely effect of having a polychromatic emission from the image plate?

<p>It decreases the amount of light reaching the photodetector (A)</p> Signup and view all the answers

What is the primary process involved in digitization within a CR system?

<p>Sampling and quantization of signals (D)</p> Signup and view all the answers

What characteristic of the CR imaging process differs from screen-film imaging?

<p>The presence of artifacts (B)</p> Signup and view all the answers

Which component is responsible for filtering light to improve signal quality in a CR reader?

<p>Optical filters (C)</p> Signup and view all the answers

How does the clock speed of a computer influence the CR scanning process?

<p>It manages both slow and fast scanning operations. (D)</p> Signup and view all the answers

Which of the following best describes the output of the photodetector in a CR system?

<p>A time-varying analog signal (C)</p> Signup and view all the answers

What describes the image receptor response function for CR compared to screen-film receptors?

<p>The CR function shows a characteristic curve. (A)</p> Signup and view all the answers

What is the effect of using a high-speed drive mechanism in a CR reader?

<p>It enhances the scanning efficiency. (B)</p> Signup and view all the answers

What happens to the emitted light from the imaging plate before it reaches the photodetector?

<p>It is filtered to improve resolution. (C)</p> Signup and view all the answers

What is the main source of noise in computed radiography images?

<p>Scatter radiation (D)</p> Signup and view all the answers

What characteristic curve is associated with the value of each pixel in a computed radiography image?

<p>Linear (D)</p> Signup and view all the answers

What allows for potentially reduced radiation dose in computed radiography compared to screen-film radiography?

<p>Wider latitude of exposure (C)</p> Signup and view all the answers

Which of the following components in computed radiography is responsible for recording the latent image?

<p>Imaging plate (C)</p> Signup and view all the answers

How many gray levels does a 14-bit computed radiography image have?

<p>16,384 (C)</p> Signup and view all the answers

What is the role of metastable electrons in computed radiography?

<p>They emit light to create the visible image (A)</p> Signup and view all the answers

Which factor does NOT apply to control the imaging process in computed radiography?

<p>Energy levels of the laser (B)</p> Signup and view all the answers

Which of the following is a common cause of signal loss in computed radiography?

<p>Inadequate sampling (D)</p> Signup and view all the answers

What describes the effect of scatter radiation in computed radiography?

<p>It creates image noise (A)</p> Signup and view all the answers

What is the primary mechanism through which the latent image is made visible in computed radiography?

<p>Stimulating laser beam (D)</p> Signup and view all the answers

What is the primary function of the coupling element in digital radiography?

<p>To transfer the x-ray-generated signal (C)</p> Signup and view all the answers

Which of the following materials can be used as a capture element in digital radiography?

<p>Sodium iodide (B)</p> Signup and view all the answers

What advantage does scanned projection radiography (SPR) offer compared to other imaging techniques?

<p>Reduced scatter radiation (B)</p> Signup and view all the answers

Which characteristic is NOT associated with charge-coupled devices (CCDs)?

<p>Large physical size (D)</p> Signup and view all the answers

In the digital radiography process, what is the role of the collection element?

<p>To collect light photons from the coupling element (C)</p> Signup and view all the answers

What is an advantage of using charge-coupled devices (CCDs) in medical imaging?

<p>They have high sensitivity to low light levels. (D)</p> Signup and view all the answers

How do cesium iodide and amorphous silicon work together in digital radiography?

<p>CsI converts x-rays into light, which is then captured by a-Si. (C)</p> Signup and view all the answers

What does the fill factor in an active matrix array-thin film transistor (AMA-TFT) refer to?

<p>The percentage of the pixel sensitive to x-rays. (A)</p> Signup and view all the answers

What is the role of fiber optics in the CsI/CCD configuration?

<p>To transmit light efficiently from the scintillator to the CCD. (B)</p> Signup and view all the answers

What is a characteristic feature of the response of charge-coupled devices (CCDs)?

<p>Their response to x-ray exposure is linear. (A)</p> Signup and view all the answers

Which property of cesium iodide contributes to a lower radiation dose for patients?

<p>High photoelectric capture efficiency. (A)</p> Signup and view all the answers

What is the primary benefit of using tiled CCDs in digital radiography?

<p>They allow for the use of larger area x-ray beams. (D)</p> Signup and view all the answers

What does the term 'dynamic range' imply regarding CCDs?

<p>The range of x-ray exposure levels that can be detected. (D)</p> Signup and view all the answers

What is the primary function of amorphous selenium (a-Se) in digital radiography?

<p>To convert X-rays directly into electronic signals (B)</p> Signup and view all the answers

What does the term 'spatial resolution' refer to in digital radiography?

<p>The ability to distinguish small details (C)</p> Signup and view all the answers

Which of the following accurately describes scanned projection radiography (SPR)?

<p>It improves contrast resolution but may have limited spatial resolution (B)</p> Signup and view all the answers

What is the role of the active matrix array (AMA) in digital radiography?

<p>To serve as an array of thin-film transistors (TFTs) for signal collection (A)</p> Signup and view all the answers

How does the thickness of amorphous selenium (a-Se) impact its function in digital radiography?

<p>A thickness of 200 µm is optimal for direct ionization (A)</p> Signup and view all the answers

What element serves as a coupling element in direct digital radiography?

<p>Amorphous selenium (a-Se) (B)</p> Signup and view all the answers

Which of the following describes the consequence of using smaller pixels in digital radiography?

<p>Higher spatial resolution (A)</p> Signup and view all the answers

What is the main reason for collecting the created charge in the amorphous selenium (a-Se) layer?

<p>To trigger the readout process via TFTs (B)</p> Signup and view all the answers

What is the significance of the fill factor in an active matrix array?

<p>It determines the efficiency of photon collection (C)</p> Signup and view all the answers

Flashcards

Computed Radiography (CR)

A digital radiography technology that uses photostimulable phosphors (PSP) to capture and store X-ray energy.

Photostimulable Phosphors (PSP)

Materials that store X-ray energy as excited electrons, later released as light when stimulated by infrared light.

PSL (Photostimulable Luminescence)

The process where electrons in PSPs release stored light energy upon exposure to infrared light.

PSP layers

Multiple layers in the PSP screen including a base, binder, reflective layer, phosphor layer, antistatic layer & protective overcoat.

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Electron excitation

Process where X-rays excite electrons to higher energy levels within the phosphor crystals.

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What's the image receptor in CR?

It's a photostimulable phosphor (PSP) screen, which captures x-ray energy and stores it as excited electrons.

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What is an IP?

It's a cassette that houses the PSP screen and acts as the image receptor in Computed Radiography (CR).

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Why is lead backing important in an IP?

It reduces backscatter radiation, improving the contrast resolution of the final image.

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What happens to electrons when an x-ray hits a PSP?

They are excited to a higher energy level, some returning to their ground state immediately, emitting light, while others remain in a metastable state.

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What is a latent image?

The stored information within the PSP due to excited electrons, requiring stimulation to be read.

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What is stimulation in CR?

It involves scanning the PSP with an infrared laser beam, which causes the excited electrons to return to their ground state, releasing light.

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How does light stimulation affect the emitted signal?

The intensity of the laser beam directly affects the intensity of the emitted light signal.

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What is the role of a photodetector in CR?

It detects the emitted light from stimulated PSP, converting it into an electrical signal for image processing.

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Why are photodiodes preferred in CR?

They have high efficiency in detecting emitted light, making them ideal for capturing the subtle variations in the image.

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How are CR images processed?

The electrical signal from the photodetector is digitized and processed to create a digital image.

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CR Cassette Movement

The CR cassette is inserted into the reader, and the image plate (IR) is moved along its long axis by a precision drive mechanism for scanning.

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Fast Scan in CR

A deflection device (like a rotating polygon or oscillating mirror) moves the laser beam across the image plate, creating a fast scan across the IR.

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Retrace and Blanking in CR

The drive mechanisms are synchronized, so the laser beam is turned off during retrace to prevent errors and ensure clear images.

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Tolerance in CR

The error tolerance is crucial. Even a fraction of a pixel error can cause wavy image edges, so extremely precise movement is key.

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Interrogation of Electrons in CR

The CR reader accurately analyzes each excited (metastable) electron within the latent image on the plate.

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Polychromatic Emission in CR

The light emitted from the image plate is polychromatic (contains various colors) and quite faint compared to the stimulation light.

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Optical Filters in CR

Optical filters are used to block the stimulating light and let only the emitted light reach the photodetector, ensuring a clear signal.

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Beam Diameter in CR

The laser beam used for stimulation is very thin, typically a few millimeters, so it can precisely target specific areas on the image plate.

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CR Reader Drive Mechanism

The drive mechanism moves the imaging plate slowly and steadily along its length while a laser beam rapidly scans across the plate, capturing the stored image information.

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CR Optical Components

Light emitted from the imaging plate is collected by fiber-optic assembly, filtered to remove unwanted wavelengths, and directed to a photodetector.

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Photodetector in CR

The photodetector converts the light emitted from the stimulated imaging plate into an electrical signal.

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Digital Image Formation in CR

The analog signal from the photodetector is processed for amplitude, scale, and compression, then digitized through sampling and quantization.

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CR Image Characteristics

CR mimics screen-film imaging but utilizes a digital image receptor, resulting in differences in spatial resolution, contrast resolution, noise, and artifacts.

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CR Characteristic Curve

The response of the CR image receptor to different exposure levels, often represented by a curve, which is similar to but distinct from a screen-film curve.

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CR Computer Control

The computer controls the scanning process, amplifies and compresses the signal, and converts the analog signal to a digital format.

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CR Image Receptor Response Function

It describes the sensitivity of the image receptor to different levels of radiation exposure, revealing how the captured information varies.

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What is Computed Radiography (CR)?

A digital radiography technique that uses photostimulable phosphors (PSP) to capture and store X-ray energy, later read by a laser scanner to create a digital image.

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What are Photostimulable Phosphors (PSP)?

Materials that store X-ray energy as excited electrons, which are later released as light when exposed to infrared light.

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What is the latent image in CR?

The stored information within the PSP due to excited electrons, requiring stimulation to be read.

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How does stimulation work in CR?

The PSP is scanned with an infrared laser beam, which causes excited electrons to return to their ground state, releasing light.

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What are the main sources of noise in CR images?

Scatter radiation is the primary contributor, particularly noticeable at low radiation exposures.

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How does CR contribute to lower patient radiation dose?

CR uses lower radiation exposure techniques compared to screen-film radiography, potentially reducing patient dose.

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What is a key characteristic of CR image receptor response?

CR has a linear characteristic curve over a wide range of radiation exposures, providing a broad gray scale.

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What is meant by 'polychromatic' light emission in CR?

The light emitted from the imaging plate contains various colors or wavelengths, rather than a single color.

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What is the purpose of optical filters in a CR reader?

Optical filters block the stimulating light and allow only the emitted light to reach the photodetector, ensuring a clear signal.

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Digital Radiography (DR)

A method of producing radiographs using digital image receptors instead of traditional film. It involves capture, coupling, and collection of X-ray information.

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Capture Element in DR

The component that absorbs X-ray energy, examples include photostimulable phosphors (PSP) in CR, or materials like sodium iodide (NaI), cesium iodide (CsI) in other DR modes.

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Coupling Element in DR

The element that transfers the captured X-ray signal to the collection element. This can be a lens, fiber-optic assembly, contact layer, or amorphous selenium (a-Se).

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Collection Element in DR

A light-sensitive device that converts the X-ray signal into a digital image. Examples include photodiodes, CCDs, or TFTs.

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Scanned Projection Radiography (SPR)

A type of digital radiography commonly used in CT imaging. The patient moves through a stationary X-ray tube and detector array. Provides a high-contrast image due to focused X-ray beam.

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CCD Sensitivity

CCDs are very sensitive to low light levels, making them ideal for capturing faint signals in applications like medical imaging and astronomy.

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CCD Dynamic Range

CCDs can capture a wide range of light intensities, from very dim to very bright, without becoming saturated.

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CsI/CCD Indirect Detection

X-rays are converted into light by a cesium iodide (CsI) scintillator, which is then detected by the CCD array.

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Tiled CCDs and Area Beams

Tiled CCDs allow for the use of large area x-ray beams, shortening exposure times for patients.

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CsI/a-Si Direct Conversion

This method uses cesium iodide (CsI) to directly capture x-rays and convert them into light, which is then sensed by a silicon (a-Si) array.

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AMA-TFT Individual Pixels

Each pixel in an active matrix array - thin film transistor (AMA-TFT) detector has its own light-sensitive element, capacitor, and transistor.

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Fill Factor in AMA-TFT

The fill factor in AMA-TFT refers to the percentage of the pixel's surface that is sensitive to x-rays, typically around 80%.

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Why CsI/a-Si is Used?

The high photoelectric capture efficiency of CsI, combined with its high atomic numbers, leads to lower patient radiation dose.

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Direct DR

A type of digital radiography that uses amorphous selenium (a-Se) as the image receptor, converting X-rays directly into electronic signals without a scintillator.

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Amorphous Selenium (a-Se)

A material used in direct DR systems. It absorbs X-rays and generates electron-hole pairs, creating an electronic signal.

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What is the role of TFTs in DR?

Thin-film transistors (TFTs) act as switches for individual pixels in a digital image receptor. They control the flow of electrons, allowing for accurate image readout and pixel activation.

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What is spatial resolution in DR?

The ability to distinguish small details in a digital image. It is limited by the size of the pixels in the image receptor.

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What is dynamic range?

The difference between the smallest and largest detectable signal levels in a medical image. It's like the range of shades of gray an image can show.

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What are the main advantages of SPR over tiled CCDs in digital radiography?

SPR offers improved contrast resolution due to the focused X-ray beam but it has limited spatial resolution compared to tiled CCDs.

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What is meant by 'limited spatial resolution'?

Limited spatial resolution means that the image receptor cannot distinguish very small details clearly. It's like seeing the world through a blurry lens.

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What is sensitivity in digital radiography?

The ability of the image receptor to detect and convert a weak X-ray signal into a usable image. Higher sensitivity means that the detector can see finer details, even with a low X-ray dose.

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Study Notes

Computed Radiography (CR)

  • CR is a form of digital radiography
  • Unlike screen-film radiography, CR uses a scintillator that emits light in response to x-ray interaction
  • In CR, electrons are trapped temporarily in a metastable state in the image receptor
  • The responses to x-ray interaction in CR are electrons temporarily trapped in a higher energy, metastable state.

Photostimulable Luminescence (PSL)

  • Some materials (e.g., barium fluorohalide) emit light initially and also emit light later when exposed to differing light sources, a process called PSL
  • The light-emitting component is called an activator, and it plays a similar role to the sensitivity center of a film emulsion.
  • Without the activator, there is no latent image.

Photostimulable Phosphor (PSP) Screen

  • PSP (barium fluorohalide) is similar in structure to a radiographic intensifying screen, and the cross-section is shown in Figure 12.4
  • The PSP crystal is layered, encompassing a phosphor layer, binder, and protective overcoat
  • Phosphor layer thickness is between 100-250 mm

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Description

This quiz covers the principles of computed radiography (CR) and photostimulable luminescence (PSL). Learn how these technologies utilize scintillators, electrons, and activators to capture and emit images effectively. Test your understanding of the innovations in digital radiography.

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