CR-DR

PHOUOSTIMULABLE LUMINESCENCE / PHOSPHOR

• Many Compton and photoelectric x-ray interactions occur with outer-shell electrons, sending them into an excited, metastable state .When these electrons return to the ground state, visible light is emitted .

• Over time, these metastable electrons return to the ground state on their own.

• this return to the ground state can be accelerated or stimulated by exposing the phosphor to intense infrared light from a laser

• sufficiently sensitive that it can become fogged by background radiation.

BARIUM FLUORO BROMIDE

have atomic numbers of 56, 9, and 35, respectively, with K-shell electron binding energies of 37, 5, and 12 keV, respectively.

IMAGING PLATE

• The PSP screen is housed in a rugged cassette that appears

similar to a screen-film cassette.

• In this form as an image receptor, the PSP screen-film cassette is called an ________

• The ______ is handled in the same manner as a screen-film cassette; in fact, this is a principal advantage of CR.

• CR can be substituted for screen- film radiography and used with any x-ray imaging system.

• The PSP screen of the ______ is not loaded and unloaded in a darkroom. Rather, it is handled in the manner of a screen-film daylight loader.

Light Stimulation–Emission

• Expose

• Stimulate

• Read

• Erase

Thermoluminescent dosimetry (TLD) and optically stimulated luminescence (OSL)

TLD

Light is emitted when a _______ crystal is heated.

OSL

Light is emitted when an _______ crystal is illuminated.

Expose

• When an x-ray beam exposes a PSP, the energy transfer results in excitation of electrons into a metastable state. Approximately 50% of these electrons return to their ground state immediately, resulting in prompt emission of light.

• The remaining metastable electrons return to the ground state over time. This causes the latent image to fade and requires that the IP must be read soon after exposure

8 HRS

CR signal loss is objectionable after approximately _______

Stimulation

• The finely focused beam of infrared light with a beam diameter of 50 to 100 μm is directed at the PSP.

• As laser beam intensity increases, so does the intensity of the emitted signal

Light Emission-Read

• The laser beam causes metastable electrons to return to the ground state with the emission of a shorter wavelength light in the blue region of the visible spectrum

Some signal is lost as the result of;

• (1) scattering of the emitted light

• (2) the collection efficiency of the photodetector.

Erase

• The final stage in PSL signal production, the stimulation cycle of PSL signal acquisition does not completely transition all metastable electrons to the ground state. Some excited electrons remain.

• If residual latent image remained, ghosting could appear on subsequent use of the IP.

• Any residual latent image is removed by flooding the phosphor with very intense white light from a bank of specially designed fluorescent lamps.

• Imaging plates should be used soon after the ______ cycle has been completed

MONOCHROMATIC

The laser light used to stimulate the PSP is ______

solid state laser

• is the stimulating source of choice for CR

• produce longer wavelength light and therefore are less likely to interfere with emitted light

POLYCHROMATIC SPECTRUM

• The resulting emission has a ______

OPTICAL FILTERS

are necessary to allow only emitted light to reach the photodetector while blocking the intense stimulated ligh

CR READER

• represents the marriage of mechanical, optical, and computer modules.

Mechanical Features

The drive mechanisms of the CR reader move the imaging plate (IP) slowly along its long axis, while an oscillating beam deflection mirror causes the stimulating laser beam to sweep rapidly across the IP.

Optical Features

• Components of the ____ subsystem include the laser, beam-shaping optics, light- collecting optics, optical filters, and a photodetector.

• The laser is the source of stimulating light; however, it spreads as it travels to the rotating or oscillating reflector.

• Emitted light from the IP is channeled into a funnel like fiber-optic collection assembly and is directed at the photodetector, PMT, PD, or charge-coupled device (CCD).

• Before photodetection occurs, the light is filtered so that none of the long-wavelength stimulation light reaches the photodetector and swamps emitted light.

<100 μm

light beam is focused onto the reflector by a lens system that keeps the beam diameter small ______

beam-shaping optics

keeps the beam size, shape, speed, and intensity constant.

EMITTED LIGHT

SIGNAL

STIMULATING LIGHT

NOISE

SIGNAL TO NOISE RATIO

PROPER FILTERING IMPROVES _______

CCD (CHARGE COUPLE DEVICE)

• is a light-sensitive integrated circuit that captures images by converting photons to electrons

• A ____ sensor breaks the image elements into pixels.

• are still used for applications that demand precision and a high degree of sensitivity.

• Light-sensing element for most digital cameras

  Three principal advantageous imaging characteristics:

  • 1.sensitivity

  • 2.dynamic range

  • 3. size

  • _____ is a silicon-based semiconductor

Complementary metal-oxide semiconductor (CMOS)

• A photographic detector. None are in use except for intraoral dental imaging.

• sensors are easier and cheaper to manufacturer than CCD sensors

• They also use less energy and produce less heat.

• sensors have a reputation for being more susceptible to image noise, which can impact quality and resolution. But their quality has improved significantly in recent years, and _____ sensors now dominate the image sensor market.

spatial resolution, contrast resolution, noise, and artifacts.

• four principal characteristics of any medical image are

Screen-film image receptors-optical density (OD)

range from 0 to 3 because OD is a logarithmic function that represents three orders of magnitude, or 1000.

30 shades of gray

the screen-film image can display only approximately _______ on a viewbox

TOE SIDE

Most screen-film imaging techniques aim for radiation exposure on the ______ of the characteristic curve.

CR

• characterized byextremely wide latitude.

• Five decades of radiation exposure results in almost 100,000 gray levels, each of which can be evaluated visually by post processing.

SCATTER

The principal source of noise on a radiographic image is ______ radiation; this is the same whether screen-film or CR image receptors are used.

DR

Capture element may be:

• Sodium Iodide (NaI)

• Cesium Iodide (CsI)

• Gadolinium Oxysulfide (GdOS)

• Amorphous Selenium (a-Se).

• The coupling element is that which

transfers the x-ray–generated signal

to the collection element.

• Coupling element may be:

• a lens or fiber-optic assembly

• a contact layer, or a-Se.

• Collection element may be:

• a photodiode

• a charge-coupled device (CCD)

• a thin-film transistor (TFT)

CCD AND PHOTODIODE

light-sensitive devices that collect light photons.

TFT

is a charge sensitive device that collects electrons.

SCANNED PROJECTION RADIOGRAPHY

• During the 1980s and the early 1990s, SPR was developed for dedicated chest DR .

• The principal advantage of ___ was collimation to a fan x-ray with associated scatter radiation rejection and improvement in image contrast.

• In _____ the x-ray beam is collimated to a fan by prepatient collimators.

• Postpatient image-forming x-rays likewise are collimated to a fan that corresponds to the detector array—

• Scintillation phosphor, usually NaIor CsI—and is married to a linear array of CCDs through a fiber-optic light path.

Sensitivity

ability of the CCD to detect and respond to very low levels of visible light.

Dynamic range

the ability of the CCD to respond to a wide range of light intensity, from very dim to very bright. The dynamic range relative to that of a 400- speed screen-film radiographic image receptor.

CESIUM IODIDE/CHARGE COUPLED DEVICE

• The scintillation light from a CsI phosphor is efficiently transmitted through fiber-optic bundles to the CCD array.

• The result is high x-ray capture efficiency and good spatial resolution—up to 5 lp/mm.

• Charge-coupled devices (CCDs) can be tiled to receive the light from an area x-ray beam as it interacts with a scintillation phosphor such as cesium iodide

(CsI)

• CsI/CCD is an indirect DR process by which x-rays are converted first to light and then to electronic signal.

CESIUM IODIDE/AMORPHOUS SILICON

• An early application of DR involved the use of CsI to capture the x-ray, as well as transmission of the resulting scintillation light to a collection element.

• The collection element is silicon sandwiched as a TFT

• Digital radiographic images can be produced from the cesium iodide (CsI) phosphor light detected by the active matrix array (AMA) of silicon photodiodes

• CsI/a-Si is an indirect DR processby which x-rays are converted first to light and then to electronic signal.

AMORPHOUS SELENIUM

• The final DR modality is identified by some as direct DR because no scintillation phosphor is involved. The image-forming x-ray beam interacts directly with _______producing a charged pair

• The _______ is both the capture element and the coupling

element.

• X-rays incident on the ______ create electron hole pairs through direct ionization of selenium.

• The created charge is collected by a storage capacitor and remains there until the signal is read by the switching action of the TFT

• direct DR process by which x-rays

are converted to electronic signal

Digital Mammography Imaging Study Trial (DMIST)

• This investigation involved the imaging of nearly 50,000 women with screen-film mammography and DM interpreted from a properly designed viewing station

• The stated intention of ______ was to determine whether DM was as good as screen-film mammography.

• The suspicion was that it was not, because the spatial resolution of DM (5 lp/mm) was much lower than that of screen-film mammography (15 lp/mm).

• Contrast resolution is more important than spatial resolution for soft tissue radiography

Digital mammographic tomosynthesis (DMT)

• an area x-ray beam interacts with the digital mammographic image receptor, producing a digital mammogram.

• This digital mammogram is repeated several times at different angles

• Each image is available in digital form and can be reconstructed as a three-dimensional matrix of values, each representing a voxel

• A tomographic section can be reconstructed with enhanced image contrast at a patient radiation dose equal to that for screening mammography, less than 2 mGyt(200 mrad)

PRE-PROCESSING

• _____ of digital images is largely automatic

• ______ is designed to produce artifact-free digital images.In this regard, preprocessing provides electronic calibration to reduce pixel-to-pixel, row-to row, and column-to-column response differences.

•The processes of pixel interpolation, lag correction, and noise correction are automatically applied with most systems.

Gain and Offset image

• are automatic calibration images designed to make the response of the image receptor uniform.

• _______ images are generated every few months, and ____ images are generated many times each day.

FLAT FIELDING

These preprocessing calibration techniques are identified as

Averaging techniques

also are used to reduce noise and improve contrast.

POST-PROCESSING

• _______ is where digital imaging shines.

• In contrast to preprocessing, which is largely automatic, ______ requires intervention by the radiologic technologist and the radiologist.

• ______ of the digital radiographic image is performed to optimize the appearance of the image for the purpose of better detecting pathology

ANNOTATION

• ______- is the process of adding text to an image. In addition to patient identification, _____ is often helpful in informing the clinician about anatomy and diagnosis

• Labels the image

Window and Level

Expand the digital grayscale to visible

Image inversion

Make white-black and black-white

PACS

• when fully implemented, allows not only the

  acquisition but also the interpretation and storage of

  each medical image in digital form without resorting

  to film (hard copy).

  • The projected efficiencies of time and cost are

  enormous.

  • _____ improves image interpretation, processing,

  viewing, storage, and recall

SPATIAL RESOLUTION

• (resolution in space) is the ability of an imaging system to resolve and render on the image a small high-contrast object.

• is the ability to render small objects on the image

SPATIAL FREQUENCY

• In medical imaging, spatial resolution is described by the quantity

• expressed in line pair per millimeter

• does not refer to size but to the line pair.

FOCAL SPOT SIZE

The spatial resolution of projection radiography is determined by the geometry of the system, especially _____

PIXEL SIZE

Spatial resolution in digital imaging is limited by ______

MODULATION TRANSFER FUNCTION

• is a term borrowed from radio electronics that has been applied to the description of the ability of an imaging system to render objects of different sizes onto an image.

• can be viewed as the ratio of image to object as a function of spatial frequency.

• Image detail (spatial resolution) is determined by system ______

CONTRAST RESOLUTION

• the ability to distinguish many shades of gray from black to white

• All digital imaging systems have better contrast resolution than screen-film imaging.

• The principal descriptor for _____ is gray scale, also called dynamic range.

DYNAMIC RANGE

is the number of gray shades that an imaging system can reproduce.

POST PROCESSING

allows visualization of all shades of gray.

SIGNAL

• The _____ in a radiographic image is that portion of the image-forming x-rays that represents anatomy.

• In all radiographic imaging the number of such x-rays is huge.

• The _____ represents the difference between those x-rays transmitted to the image receptor and those absorbed photoelectrically

CONTRAST RESOLUTION

• Image noise limits_____

• limited by noise or SNR

Detective Quantum Efficiency

• The probability that an x-ray will interact with an image receptor is determined by the thickness of the capture layer and its atomic composition.

• The descriptor used for medical imaging is detective quantum efficiency ____.

• ____ is related to the absorption coefficient and to the spatial frequency of the image-forming x-ray beam

• _____ is a measure of x-ray absorption efficiency.