Fluoroscope

Direct-Exposure FilmDirect-Exposure Film

Direct exposure film has a relatively low absorption efficiency Direct exposure film has a relatively low absorption efficiency for photons in the diagnostic range, however it is still used in for photons in the diagnostic range, however it is still used in many combinations of image receptor systems.many combinations of image receptor systems.

Direct exposure film material has special design of two Direct exposure film material has special design of two photographic emulsions with protective layers between to photographic emulsions with protective layers between to optimize the absorption efficiency optimize the absorption efficiency . .

Correct exposure is important to produce a reliable image on the Correct exposure is important to produce a reliable image on the film. Over- or underexposure will result in loss of contrast and film. Over- or underexposure will result in loss of contrast and therefore possibly in loss of diagnostic information.therefore possibly in loss of diagnostic information.

Many different types image receptors are used in modern Many different types image receptors are used in modern diagnostic radiology. They all have in common that they form an image by diagnostic radiology. They all have in common that they form an image by absorption of energy from the X-ray beam (after transmitting through the absorption of energy from the X-ray beam (after transmitting through the body). The main characteristics will be discussed on the example of the body). The main characteristics will be discussed on the example of the direct exposure film.direct exposure film.

The proper film exposure can be obtained from the The proper film exposure can be obtained from the so-called so-called characteristic curvecharacteristic curve of the film material.of the film material.

The blackening of the film after X-ray exposure is expressed in The blackening of the film after X-ray exposure is expressed in terms of its optical density:terms of its optical density:

D D = = loglog1010((II00//II))

where where II00 and and II is the light intensities before and after passing throughis the light intensities before and after passing through

the exposed film material. the exposed film material.

The objective of this section is to correlate the optical The objective of this section is to correlate the optical density density DD (amount of blackening) with the received X-ray, exposure. (amount of blackening) with the received X-ray, exposure. This can be obtained with a simple model for the absorption process. This can be obtained with a simple model for the absorption process.

A single emulsion of film material initially contains A single emulsion of film material initially contains GG silver-bromine silver-bromine grains per unit area, the average cross section of the grain is grains per unit area, the average cross section of the grain is bb.. After After irradiation with a flux of irradiation with a flux of NN X-ray photons per unit area a total number of X-ray photons per unit area a total number of gg grains per area are sensitized. grains per area are sensitized.

The number of sensitized grains per incoming X-ray photon is: The number of sensitized grains per incoming X-ray photon is:

with with X X as the received X-ray dose and as the received X-ray dose and kk a conversion constant. a conversion constant.

The sensitized grains develop into a silver speck with an The sensitized grains develop into a silver speck with an average cross section average cross section a a after the film development. If light hits one of after the film development. If light hits one of the silver specks in the developed film, it is completely absorbed (black the silver specks in the developed film, it is completely absorbed (black spot). spot).

bodybody

filmfilm

X-RayX-Raysourcesource

DarkDarkLightLight

To relate the number of sensitized grains to the optical density To relate the number of sensitized grains to the optical density D D the absorption of the light in the film material of thickness the absorption of the light in the film material of thickness t t can be can be described as: described as:

Using this relation the optical density can be calculated to: Using this relation the optical density can be calculated to:

This relation is known as This relation is known as Nuttings LawNuttings Law ! !

Maximum optical density for an area on the film is obtained Maximum optical density for an area on the film is obtained when all grains are sensitized: when all grains are sensitized: g = Gg = G

The correlation between the optical density The correlation between the optical density D D and the and the maximum number of sensitized grains results in a relation between maximum number of sensitized grains results in a relation between the optical density the optical density D D and the received dose and the received dose XX : :

This is displayed in the figure. This is displayed in the figure.

The curve relating the optical density The curve relating the optical density to the film exposure dose is called the to the film exposure dose is called the characteristic curvecharacteristic curve of the film material. of the film material.

In the center part of the curve the relation In the center part of the curve the relation between optical density and the logarithm of the between optical density and the logarithm of the dose is approximately linear:dose is approximately linear:

For a small contrast in dose For a small contrast in dose X= XX= X11-X-X22 the associated the associated

change in optical density is: change in optical density is:

The constant F is known as the film-gamma and ranges The constant F is known as the film-gamma and ranges between 2-3. It corresponds directly to the slope of the linear section between 2-3. It corresponds directly to the slope of the linear section of the characteristic curve. of the characteristic curve.

For low exposure or high exposure the characteristic curve For low exposure or high exposure the characteristic curve levels out, exposure differences do not translate into differences in levels out, exposure differences do not translate into differences in optical density (blackening). optical density (blackening).

A film with an optical density of A film with an optical density of D D 0.5 appears 0.5 appears overall light, a film with optical density overall light, a film with optical density D D 2 appears 2 appears overall black. overall black.

To achieve best contrast the film must be exposed hi such To achieve best contrast the film must be exposed hi such a way that the region of interest in the patient cause film doses a way that the region of interest in the patient cause film doses which are in the center part of the characteristic curve. which are in the center part of the characteristic curve.

Additionally the contrast can be affected by the energy Additionally the contrast can be affected by the energy absorption efficiency of the image receptor material which in absorption efficiency of the image receptor material which in general decreases with energy. general decreases with energy.

Efficiency and / or sensitivity of film materialEfficiency and / or sensitivity of film material

The sensitivity of film material depends on size and The sensitivity of film material depends on size and density of grains, emulsion thickness and X-ray absorption density of grains, emulsion thickness and X-ray absorption efficiency. efficiency.

The figure shows a typical X-ray absorption efficiency The figure shows a typical X-ray absorption efficiency for a double emulsion film as a function of energy. for a double emulsion film as a function of energy.

The efficiency drops rather rapidly The efficiency drops rather rapidly with increasing energy and is mainly with increasing energy and is mainly determined by the interaction probability of determined by the interaction probability of the photons with the film material the photons with the film material (attenuation coefficient (attenuation coefficient ) and the thickness ) and the thickness of the material of the material tt. .

An important goal is to maximize the efficiency of the image An important goal is to maximize the efficiency of the image receptor material. receptor material.

The noise in the image may limit the contrast. The noise in the image may limit the contrast.

The noise in the receptor image arises from several sources: The noise in the receptor image arises from several sources:

fluctuations in the number of absorbed X-ray photons per unit area fluctuations in the number of absorbed X-ray photons per unit area

fluctuations in the absorbed photon energy fluctuations in the absorbed photon energy

fluctuations in the number of silver halide per unit area of emulsion fluctuations in the number of silver halide per unit area of emulsion

The first and the last are the main sources for noise The first and the last are the main sources for noise (quantum mottle and random darkening). (quantum mottle and random darkening).

To calculate the effect of quantum mottle we replace: To calculate the effect of quantum mottle we replace:

with with A A as area, as area, as interaction efficiency, and as interaction efficiency, and NN as as number of incident photons. number of incident photons.

The resulting expression for the noise in the optical density of the image is: The resulting expression for the noise in the optical density of the image is:

The noise due to quantum mottle is proportional to the The noise due to quantum mottle is proportional to the slope slope of the characteristic curve of the receptor material! of the characteristic curve of the receptor material!

To determine the noise due to To determine the noise due to random darkeningrandom darkening the the influence of granularity fluctuation (influence of granularity fluctuation (g g A A))1/21/2 in the number of in the number of developed grains (developed grains (g g A A)) in area in area A A for the fluctuation in optical for the fluctuation in optical density density DDGG needs to be calculated: needs to be calculated:

after substituting for after substituting for g.g.

Fluctuations therefore depend directly on fluctuations in grain size!Fluctuations therefore depend directly on fluctuations in grain size!

The figure shows that the quantum mottle The figure shows that the quantum mottle corresponds directly to the film-gamma and the random corresponds directly to the film-gamma and the random darkening due to the granularity distribution directly to the darkening due to the granularity distribution directly to the characteristic curve. characteristic curve.

Alternative Image ReceptorsAlternative Image Receptors

Intensifying Screens Intensifying Screens in front of the photo emulsion convert X-rays in front of the photo emulsion convert X-rays into visible light. The film material is more sensitive to light photons into visible light. The film material is more sensitive to light photons than to X-ray photons. This increases the energy absorp tion than to X-ray photons. This increases the energy absorp tion efficiency e by more than one order of magnitude, but the efficiency e by more than one order of magnitude, but the resolution decreases due to additional noise components. resolution decreases due to additional noise components.

Image intensifiers convert X-ray photons to electrons by photo Image intensifiers convert X-ray photons to electrons by photo electric effect on a photocathode. The electrons are focused electric effect on a photocathode. The electrons are focused onto by electrical fields on a fluorescent screen where they form onto by electrical fields on a fluorescent screen where they form an in tensified image which can be recorded on film or viewed an in tensified image which can be recorded on film or viewed with a TV camera. with a TV camera.

Xeroradiography is a dry non-silver photographic system, which Xeroradiography is a dry non-silver photographic system, which produces images on paper (Xerox copies). It is slower than produces images on paper (Xerox copies). It is slower than standard film receptors (which may result in higher doses) but has standard film receptors (which may result in higher doses) but has better resolution and better energy absorption efficiency e better resolution and better energy absorption efficiency e

lonography lonography replaces the photographic film by a replaces the photographic film by a position sensitive ion chamber. The X-rays induce by position sensitive ion chamber. The X-rays induce by ionization of the gas electron clouds which can be detected by ionization of the gas electron clouds which can be detected by an electrode with good spatial resolution. an electrode with good spatial resolution.

X-Ray Transmission Computed TomographyX-Ray Transmission Computed Tomography

Several problems exist with conventional radiography techniques: Several problems exist with conventional radiography techniques:

inability to distinguish soft body tissue because of limited contrastinability to distinguish soft body tissue because of limited contrast(see example blood-muscle); this can be fixed by the use of liquid(see example blood-muscle); this can be fixed by the use of liquidcontrast medium which has to be injected. contrast medium which has to be injected.

inability to resolve spatially structures along the X-ray propagationinability to resolve spatially structures along the X-ray propagationaxis resulting in loss of depth information (flat picture), becauseaxis resulting in loss of depth information (flat picture), becausethe three-dimensional body is projected on to a two-dimensionalthe three-dimensional body is projected on to a two-dimensionalreceptor. receptor.

Computed tomography (CT) techniques allows sectional imaging . Computed tomography (CT) techniques allows sectional imaging .

It is based on the principle that an image of an unknown object can It is based on the principle that an image of an unknown object can be obtained if one has an infinite number of projections through the object. be obtained if one has an infinite number of projections through the object.

Two scans through the body gives an image in x- and y-direction Two scans through the body gives an image in x- and y-direction (side view and front view). With increasing number of scans over a 360° (side view and front view). With increasing number of scans over a 360° angle range provides a set of images which allow to construct the three angle range provides a set of images which allow to construct the three dimensional structure of the body. dimensional structure of the body.

For X-ray tomography a planar slice of the body is defined For X-ray tomography a planar slice of the body is defined and X-rays are passed along this plane in all directions. To produce a and X-rays are passed along this plane in all directions. To produce a tomographic image typically 100 to 1000 scans are required. tomographic image typically 100 to 1000 scans are required.

To store the multitude of images and process the data requires computer. To store the multitude of images and process the data requires computer.

The two-dimensional The two-dimensional image corresponds to a three image corresponds to a three dimensional section of the dimensional section of the patient with the third dimension patient with the third dimension being the slice thickness which being the slice thickness which is typically a few millimeter is typically a few millimeter thick.thick.

The resulting spatial resolution is The resulting spatial resolution is 1 mm, a density discrimination 1 mm, a density discrimination (contrast) of better than 1% can be obtained with this technique. (contrast) of better than 1% can be obtained with this technique.

Present CT machines have an rotatable X-ray source. Present CT machines have an rotatable X-ray source. This allows to scan the patient who is located along the This allows to scan the patient who is located along the rotational axis from all sides and angles. The image receptor rotational axis from all sides and angles. The image receptor system is designed as a stationary ring of detectors around the system is designed as a stationary ring of detectors around the patient which receive the portions of the fanned X-ray beam. patient which receive the portions of the fanned X-ray beam.

Each data point acquired by the detector array is a Each data point acquired by the detector array is a transmission measurement through the patient along a given line transmission measurement through the patient along a given line x x between source and detector pixel. between source and detector pixel.

Each data point therefore follows the basic equation: Each data point therefore follows the basic equation:

The attenuation coefficient The attenuation coefficient p p represents the sum of all attenuation represents the sum of all attenuation coefficients along the line coefficients along the line xx: :

From the image information the projection From the image information the projection PP((xx)) of the image is calculated:of the image is calculated:

Because the attenuation Because the attenuation coefficient coefficient corresponds directly to corresponds directly to the density of the body tissue along the density of the body tissue along the projection axis the projection axis x, x, the projection the projection corresponds to the density.corresponds to the density.

From a complete scan along different axes (From a complete scan along different axes (xx, , ) the cross ) the cross sectional density along the slice can be constructed using Fourier sectional density along the slice can be constructed using Fourier analysis methods. analysis methods.

From the image information the projection From the image information the projection PP((xx)) of the image is calculated :of the image is calculated :

The projection is directly proportional to The projection is directly proportional to the summed attenuation coefficient and to the summed attenuation coefficient and to the length the length x x through the body. through the body.

Because the attenuation coefficient Because the attenuation coefficient corresponds directly to the density of corresponds directly to the density of the body tissue along the projection axis the body tissue along the projection axis x,x, the projection corresponds to the the projection corresponds to the density. density.

From a complete scan along different axes (From a complete scan along different axes (xx, , ) the cross ) the cross sectional density along the slice can be constructed using Fourier sectional density along the slice can be constructed using Fourier analysis methods. analysis methods.

The resulting CT image is a two-dimensional matrix of numbers The resulting CT image is a two-dimensional matrix of numbers with each number corresponding to a spatial location in image and patient with each number corresponding to a spatial location in image and patient along the plane of the slice. The matrix is constructed from 512x512 pixels. along the plane of the slice. The matrix is constructed from 512x512 pixels.

Each pixel has a value (up to 4096=12 bits) which corresponds to the Each pixel has a value (up to 4096=12 bits) which corresponds to the level of gray (darkness, attenuation). This number is called CT number and level of gray (darkness, attenuation). This number is called CT number and contains the physical information ( attenuation contains the physical information ( attenuation density ) about the density ) about the corresponding body section. corresponding body section.

As the attenuation coefficient ranges between As the attenuation coefficient ranges between =0 (air) and =0 (air) and =1 (metal =1 (metal inlet) the attenuation is scaled to a maximum of 4096. The CT therefore inlet) the attenuation is scaled to a maximum of 4096. The CT therefore corresponds to a two-dimensional map of attenuation across the body slice. corresponds to a two-dimensional map of attenuation across the body slice.

The CT number The CT number CT CT is normalized to the attenuation of water: is normalized to the attenuation of water:

If the attenuation coefficient for a given pixel is equal to water, the CT If the attenuation coefficient for a given pixel is equal to water, the CT number=0, soft tissue material has CT number in the range of -100 to +100, number=0, soft tissue material has CT number in the range of -100 to +100, dense tissues like bones have high CT numbers from 300 to 3000. dense tissues like bones have high CT numbers from 300 to 3000.

CT numbers are rescaled attenuation coefficients!CT numbers are rescaled attenuation coefficients!

Typical Radiation Doses in Radiographic and in CT ExaminationsTypical Radiation Doses in Radiographic and in CT Examinations

The radiation dose received in CT is considerably The radiation dose received in CT is considerably higher than that of conventional screen radiography. higher than that of conventional screen radiography.

To compare the two types of X-ray exposures the received To compare the two types of X-ray exposures the received dose dose D D of X-ray radiation is converted to integral dose of X-ray radiation is converted to integral dose DI, DI, which which corresponds to the total amount of energy deposited in the body corresponds to the total amount of energy deposited in the body tissue of mass m: tissue of mass m:

In radiographic examinations the dose is not distributed evenly butIn radiographic examinations the dose is not distributed evenly butdrops from the entrance dose at the skin drops from the entrance dose at the skin DD00 towards deeper layers.towards deeper layers.

For the skull For the skull 0.33 cm0.33 cm11, , A A 256 cm256 cm22, the typical entrance , the typical entrance dose is Ddose is D00 4.8 mGy. This yields for the integral dose: 4.8 mGy. This yields for the integral dose:

The integral dose in a seven slice head CT is about The integral dose in a seven slice head CT is about twenty times higher than the integral dose of a single X-ray twenty times higher than the integral dose of a single X-ray exposure. exposure.

The integral dose is described by: The integral dose is described by:

DIDI = 3.7 = 3.7JJ