Control Of Scatter Radiation

CONTROL OF SCATTER RADIATION

CONTROL OF SCATTER RADIATION

Scatter radiation is primarily a product of Compton interaction. X-rays that travel in a different direction after exiting the patient body Number of scattered x-ray quanta striking the detector is often large than number of primary (unscattered) quanta Detector (screen) sensitivity to x-ray scatter has an impact on the image Scatter radiation produces fog on the x-ray film which tends to dull the image. It increases density but reduces contrast on the radiograph.

EFFECTS OF SCATTER RADIATION ON IMAGE CONTRAST

• Contrast is the degree of difference in brightness between areas of an image

If you could only capture transmitted, unscattered x-rays, the image would be very sharp The corresponding bone-soft tissue interface, would be very abrupt, and therefore the image contrast would be high

FACTORS AFFECTING SCATTER RADIATION

KILOVOLTAGE – Wavelength of the primary beam (increased kVp) As x-ray energy increases, the relative number of photons that undergo Compton interaction also increases. Thus increasing Kilovoltage, increases the production of scatter radiation.

However, the use of low kV technique would require an increase in mAs, thus increasing patient dose.

• FIELD SIZE – As field size increases, the production of scatter also increases

Area of object irradiation (increased x-ray field size As field size increases, intensity of scatter radiation also increases rapidly. Especially during fluoroscopy

FACTORS AFFECTING SCATTER RADIATION

• PATIENT THICKNESS: – The thicker the part to be examined the higher the production of scatter radiation

COMPRESSION BAND:- Since the greater the volume of the tissue irradiated the greater the amount of the scattered radiation produced.

• The volume of the patient can be reduced by the use of compression band
• Compression band is a wide cloth band about 20-35cm wide.

This reduces the volume irradiated and therefore scatter formed Compression devices therefore; improves spatial resolution by reducing patient thickness and bringing the object closer to the recorder medium . Compression also reduces patient dose and increase contrast resolution

FACTORS AFFECTING SCATTER RADIATION

• OBJECT’S DENSITY AND ATOMIC NUMBER:- The higher the atomic number and density the higher the scatter radiation

REDUCTION OF SCATTER RADIATION

REDUCTION OF SCATTER RADIATION FALLS INTO TWO GROUPS

• Reduction of scatter radiation formed
• Control (reduction) of scatter radiation reaching film

HOW TO REDUCE OF SCATTER RADIATION FORMED

• Use of displacement band to reduce thickness of the part under investigation

Lowest possible kVp consistent with adequate penetration of the part in question Use of beam restricting devices:- This limits the field size to smaller area thus the production of scatter radiation also decreases. Smallest possible field size e.g., Beam collimation

CONTROL (REDUCTION) OF SCATTER RADIATION REACHING FILM

USE OF GRIDS:- Grids absorbs scatter radiation before it reaches the film USE OF AIR-GAP TECHNIQUE:- Gap between the patient and the film decreases the amount of scatter radiation that will reach the film

HOW TO REDUCE OF SCATTER RADIATION FORMED

LIMITING OF THE PRIMARY BEAM

Beam restrictors are devices that limits the field size to a small area of interest.

The smaller the area of the patient exposed to radiation the smaller the volume of the tissue irradiated and the smaller the amount of scatter produced

TYPES OF BEAM-RESTRICTING DEVICES

Aperture Diaphragm

Cones or Cylinders :- Cones and diaphragm are metal devices which restrict the size of the x-ray beam Variable aperture collimator

HOW TO REDUCE OF SCATTER RADIATION FORMED

LIMITING OF THE PRIMARY BEAM

APERTURE DIAPHRAGM:- The simplest type of beam restrictor, and is made up of lead or lead-lined metal sheet attached to the x-ray tube head. One diaphragm is used for each articular size of film

HOW TO REDUCE OF SCATTER RADIATION FORMED

LIMITING OF THE PRIMARY BEAM

• RADIOGRAPHIC CONES AND CYLINDERS:-These are modifications of the aperture diaphragm. It has an extended metal structure which produces a circular image.

Are tapered metal structures which may be fitted to the x-ray tube at the beam’s exit port They are usually manufactured either of the brass or steel and are open at both ends; the end nearest the tube is the apex of the cone while the wide part is directed towards the film Radiographic cones comes in a variety of the sizes which result in different areas of radiation field

HOW TO REDUCE OF SCATTER RADIATION FORMED

3). RADIOGRAPHIC DIAPHRAGMS:- Simple tablet of heavy metal has central rectangular aperture of the x-ray beam and can be slotted into a fitting on the tube port

TWO TYPES:

PLATE DIAPHRAGM

LIGHT BEAM DIAPHRAGM

• PLATE DIAPHRAGM:- Simple metal plate with a hole in it

The diameter of the hole determine the field covered at a particular focal film distance

HOW TO REDUCE OF SCATTER RADIATION FORMED

LIGHT BEAM DIAPHRAGM:- Utilizes lead leaves usually multiplane to collimate the beam of radiation This consists of two pairs of movable leaves of metal usually situated in the x-ray beam Each set of pairs can be moved independently of each other, making it to produce a rectangular of any dimensions within the maximum limits of the housing Movement of leaves is controlled by leaves and knobs The field covered is indicated by light from a high intensity lamp which is focused on to radiolucent mirror ( silvered plastic or aluminium equivalent 0.5mm) which is positioned at an angle of 45 degree to the central ray Because lamp has a short life it operated by a press button, self cancelling switch which stays on for only limited period of time about 15 sec

HOW TO REDUCE OF SCATTER RADIATION FORMED

A scale indicates the dimensions of the field covered at different focal film distances The front of the LBD is made of clear plastic and either intersecting black lines or a black dot indicating the centre of the beam when the light shines on the patient Light beam diaphragm can be rotated through 90 degrees on its mounting to allow the rectangular field to be lined up to the cassette when it is used on the table top The underside of the LBD is fitted with channels to allow the fitting of the cones to provide a circular field Beam of radiation and beam of visible light are diverging therefore area indicated by the light on the patients skin is always smaller than the area covered by radiation on the film

CONTROL (REDUCTION) OF SCATTER REACHING FILM

AIR – GAP TECHNIQUE

• &

SECONDARY GRID

Cassette is placed some distance from the patient about 8-12inches Scatter radiation is not powerful enough to reach the film and thus it’s absorbed by the air and the result is better contrast on the film The focal film distance must be increased to reduce magnification caused by the large object film distance, the exposure also needs to be increased The X-ray beams originating from the X-ray source on the left are shown passing through a subject on the way to exposing the film on the right.

CONTROL (REDUCTION) OF SCATTER REACHING FILM

The beams are scattered but miss the film due to the air gap.

The amount by which the image of the chest is magnified on the X-ray film is dependent on how far the subject is from the film and how divergent the X-ray beams are.

Therefore, when using an air gap technique the X-ray source should be around 7 inches from the film.

CONTROL (REDUCTION) OF SCATTER REACHING FILM

SECONDARY GRID

CONTROL (REDUCTION) OF SCATTER REACHING FILM

SECONDARY GRID

SECONDARY GRID :- A device that absorbs scatter radiation before it reaches the film.

They are very effective device for reducing scatter radiation from reaching recording media A device made from carefully fabricated series of radiopaque material (lead) alternating with sections of radiolucent material (aluminum or plastic) An alternative and more commonly used method of reducing scatter is the placement of a lead grid between the patient and the film.

The grid is a series of sections of radiopaque material (grid strips) alternating with sections of radiolucent material (interspaces material)

CONTROL (REDUCTION) OF SCATTER REACHING FILM

SECONDARY GRID

The grid is designed to transmit only x-rays that are traveling in a straight line from the source to the recording medium The grid is made up of many strips of X-ray absorbing lead place closely together and angled in such a way that only the X-rays travelling in a particular direction, corresponding to that of a straight line between the X-ray tube and the film, are allowed through, scattered X-rays are absorbed by the lead strips.

Fine vibration of the grid ensures that the strips of lead in the grid do not cast noticeable shadows on the film It has protective covering front and back of thin aluminum or plastic and protective strips around the edges of the grid Using a grid enables the subject to be placed against the X-ray cassette so that the divergence of the X-ray beam is less significant, and the X-ray source can be closer to the cassette, around 2 cm

CONTROL (REDUCTION) OF SCATTER REACHING FILM

SECONDARY GRID

GRID STRIPS :- Should be very thin and have high photon absorption properties. Lead is most common material used.

INTERSPACE MATERIAL:- Holds the lead strips apart; it must be rigid in order to maintain thin strips precisely in position, but it should not absorb the primary beam Any interspacing material may absorb some part of the primary beam as well as being a filter for residual scattered rays; the relationship of the function to the other necessarily figures is the assessment of a grid’s efficiency.

• Used to maintain precise separation between the delicate lead strips

Aluminum or Plastic Fiber are commonly used Grid Casing = covered completely by thin aluminum to provide rigidity and to seal out moisture.

CONTROL (REDUCTION) OF SCATTER REACHING FILM

SECONDARY GRID

GRID RATIO :- Three important dimensions on a grid: The thickness of the grid strips, the width of the interspace material, and the height of the grid The grid ratio is the HEIGHT of the grid divided by the INTERSPACE WIDTH:

• Grid ratio = h

D

Grids with high grid ratio are more effective in cleaning up scatter radiation. However, it will require higher exposure factor and therefore high patient dose High-ratio grids are more effective in cleaning up scatter radiation than low-ratio grids The angle of deviation is smaller for high-ratio grids. (the photon must be traveling in a straighter line to make it through the grid) However, the higher the ratio the more radiation exposure necessary to get a sufficient number of x-rays through the grid to the recording medium The higher the ratio the straighter the photon must travel to reach the recording medium

CONTROL (REDUCTION) OF SCATTER REACHING FILM

SECONDARY GRID

GRID FREQUENCY :- The number of grid strips or grid lines per inch or per centimeter.

The higher the grid frequency , the higher the radiographic technique required and the greater the dose to the patient.

The higher the frequency the more strips and less interspaces material and the higher the grid ratio As grid frequency increases, patient does is increase because more scatter will be absorbed Some grids reduce the thickness of the strips to reduce the exposure to the patient, this over all reduces the grid clean up Grids have frequencies in the range of 25 to 45 lines per centimeter (60 to 110 lines per inch)

CONTROL (REDUCTION) OF SCATTER REACHING FILM

SECONDARY GRID

GRID LATTICE:- Is the number of lead (tungsten) strips per centimetre measured across the grid i.e. At right angles to the grid lines In the construction of a grid to a given lattice, the number of lines to the centimetre or inch is dependent on The thickness of each strip

The distance between individual strips GRID FACTOR:- The factor by which the exposure must be increased when using a grid as compared to the exposure without grid OR Simply the number of times exposure must be increased when using a particular grid as opposed to the exposure required without the grid This is because grid removes a small amount of primary beam and most of the scatter radiation reaching film. Therefore, exposure must be increased to produce radiograph of adequate density Grid factor = Exposure with a grid applied Exposure without a grid applied

CONTROL (REDUCTION) OF SCATTER REACHING FILM

SECONDARY GRID

GRID PERFORMANCE:- The principal function of a grid is to improve image contrast Contrast improvement factor (K) :- the ratio of the contrast of a radiograph made with a grid to the contrast of the radiograph made without a grid.

The higher the grid ratio & frequency the higher the Contrast Improvement Factor (k) ASSESSMENT OF A GRID’S QUALITY :- A grid is efficient if it absorbs the highest amount of scattered radiation and the lest amount of primary radiation The grid ratio is an important determinant of the fraction of scattered radiation removed by a grid The ration of the amount of primary radiation passing through a grid to the amount of scatter passing through the grid is called the selectivity of the grid selectivity = Transmitted primary

Transmitted scatter

CONTROL (REDUCTION) OF SCATTER REACHING FILM

SECONDARY GRID

MEASURING GRID PERFORMANCE

Contrast improvement factor :- ratio of the contrast of an x-ray film taken with the grid to the contrast of an x-ray film taken without a grid.

Bucky factor :- the ratio of the incident remnant radiation to the transmitted remnant radiation. It measure how much of an increase in technique factor will be required compared with non-grid exposure.

Selectivity: – the ratio of transmitted primary radiation to transmitted scatter radiation

CONTROL (REDUCTION) OF SCATTER REACHING FILM

• TYPES OF GRID :- According to construction
• SECONDARY GRID:- can either be Stationary or
• Moving grid
• TYPES OF GRID (Stationary) :- According to construction
• PARALLEL GRID – grid strips are parallel to each other
• simplest type of grid
• All the lead strips are parallel
• Only clean up scatter in one direction (along the axis of the grid)
• Easy to make, however can cause grid cutoff with short SID’s

POINTS TO REMEMBER WHEN USING PARALLEL GRID

Use a relatively long focus grid distance particularly when using a large field of radiation Limit the field size as far as possible Don’t angle the beam of radiation into the lead strips by either tilting the beam transversely or angling the grid transversely

CONTROL (REDUCTION) OF SCATTER REACHING FILM

• TYPES OF GRID :- According to construction

ADVANTAGES OF PARALLEL GRID

There is no critical focal grid distance to be considered and the central ray may be directed in any direction It can be used in conjunction with a moving grid to further enhance contrast

DISADVANTAGES

• Exposure must be increased significantly to offset the high absorption of primary beam

Grid lines

CONTROL (REDUCTION) OF SCATTER REACHING FILM

TYPES OF GRID :- According to construction

• CROSSED HATCHED GRID :- Are grids that are made by putting together two linear grids with lead strips perpendicular to one another
• Made by placing two parallel grid on top of each other
• Have lead strips running along the long and short axes of the grid

Have twice the grid ratio as linear grids However, central Ray (CR) vs. grid placement is critical. The CR must align with the center of the grid and the grid and CR must be exactly parallel or grid cutoff will occur

CONTROL (REDUCTION) OF SCATTER REACHING FILM

TYPES OF GRID :- According to construction

• FOCUSED GRID:- Grid strips are angulated so that they lie on imaginary radius lines from the center of a circle.
• Designed to minimize grid cutoff
• Lead strips are aligned with the divergence of the x-ray beam
• Each focused grid must be identified with the appropriate SID

Wrong SID = Grid cutoff

CONTROL (REDUCTION) OF SCATTER REACHING FILM

• TYPES OF GRID :- According to construction
• GRIDDED CASSETTE :- The grid is fitted as an integral part of the cassette
• Uses aluminium as an interspacing material and as a protective covering

A number of different ratios are available as focused or parallel grids Aluminium interspacing material absorb more primary beam then organic material e.g., Paper, wood, plastic but their study construction renders them particularly useful for theatre and mobile radiography

CONTROL (REDUCTION) OF SCATTER REACHING FILM

TYPES OF GRID :- According to construction

• METAL FILTERS:- FILTERS :- A sheet of metal e.g. Lead foil (0.1mm) may be placed on top of the cassette to absorb scattered radiation

The metal sheet does tend to absorb more scatter than primary beam there are two reasons for that:

Because the scatter is travelling through the metal at a greater angle than the primary beam it must perforce travel through a greater thickness of metal Because the scatter has less energy i.e., Longer wavelength than the primary beam ,it is more readily absorbed

SECONDARY GRID

MALPOSITIONS OF THE GRID (GRID ERROS)

MALPOSITIONS OF THE GRID :- If a parallel grid is misaligned with the central x-ray beam the radiograph suffers from the effect of geometrical cut –off

THIS HAPPEN IN TWO WAYS:-

Tilting the grid so that the lead elements are no longer parallel to beam, this results in loss of density along a central band and at one side of the film Incorrect centring of the x-ray beam which produces underexposure of the radiograph at the edge away from the tube’s displacement

SECONDARY GRID

ERRORS IN THE USE OF GRID

GEOMETRICAL –CUT OFF:- This is the proportion of the primary beam which the lead strips of a grid absorb It is expressed as a percentage; for instance, 12.5% this being the amount of primary radiation lost as result of the geometry of the strips The thickness of the strips, their spacing and their height each has an influence upon geometrical cut off Effect of geometrical cut off can result in underexposure of the radiograph in certain circumstances Distance to cutoff = SID

• Grid ratio
• With decreasing SID more potential for grid cutoff

IR size will also influence grid cutoff

SECONDARY GRID

ERRORS IN THE USE OF GRID

THE GEOMETRICAL CUT-OFF CAN BE MINIMIZED BY :-

• Increase in the anode film distance
• Keeping the grid ratio low which deteriorates efficiency
• Constructing the grid so that it is thinner at the edges than at the centre

Using only the centre of the field

SECONDARY GRID

ERRORS IN THE USE OF GRID (FOCUSED GRIDS)

UPSIDE DOWN:- Focused grid positioned upside-down Result will be normal density in the middle of the radiograph with decreased density on the sides Focused grid must be placed with labeled tube side facing x-ray tube

SECONDARY GRID

ERRORS IN THE USE OF GRID (FOCUSED GRIDS)

• OFF –LEVEL:- When focused grid is not used at the proper focal distance ; central axis of the beam is not perpendicular to the grid.

Result will be image forming rays absorbed all across the radiographic field, with cutoff decreased density) visible over the entire radiograph Grid must be perpendicular to the central ray

SECONDARY GRID

ERRORS IN THE USE OF GRID (FOCUSED GRIDS)

• LATERAL DECENTERING
• Central ray does not strike the grid in the centre
• Cutoff visible, more to one side of the radiograph
• GRID –FOCUS DECENTERING:- When focused grid is positioned off center.
• Violation of the grid radius when using a focused grid

Normal density in the middle of the radiograph with cutoff visible on the sides

SECONDARY GRID MOVEMENTS

All stationary grids will give you grid lines on your radiograph. Thinner Pb(Lead ) strips will give you less noticeable lines. However, thinner strips have less Pb (lead) content not “cleaning up” as well Grid Lines are made when primary x-rays are absorbed in the grid strips.

MOVING GRID MECHANISM

• Movement of the grid during the exposure blur out the grid lines.
• Dr. Hollis Potter in 1920 conceptualized the idea of moving grid.
• Other names are “Potter-Bucky Diaphragm”, “Bucky diaphragm”, and “Bucky grid”.
• BUCKY –The person invented the grid

POTTER- The person introduced grid movements

Potter Bucky Grid

POINTS ON USING MOVING GRID

The moving grid has fewer lead strips per cm then a stationary grid The Pottery-Bucky may be released either mechanically or electronically but in either of the case the time of the movement must be longer than the time of the exposure. The grid should start to move before the exposure starts.

The movement of the grid must be smooth so that the cassette and patient do not more, thus causing unsharpness in the radiographs The Bucky mechanism is activated by a switch on before the exposure is made

Potter Bucky Grid

DISADVANTAGES OF MOVING GRID:-

Due to grid movement during exposure, there are some degree of off-centring Vibration of the cassette due to movement give unavoidable in geometric blurring and enlargement of the image due to increase in object film distance, needed to accommodate the moving grid and it’s associated mechanism

BUCKY ASSEMBLY

THE TOTAL BUCKY ASSEMBLY INCORPORATES

• A frame which holds the grid and allows grids to be readily interchanged
• The grid itself usually 46 x 43 cm in the case of a table

The grid mechanism

Situated underneath all the other items is a Robust steel tray in which can be placed any size of the cassette and which includes a locking devices for the cassette.

The grid is placed in a holding mechanism that begins moving just before the x-ray exposure and continues moving after the exposure ends

A BUCKY DIAPHRAGM IS FOUND IN THE FOLLOWING CLASSES OF X-RAY EQUIPMENT

• standard Bucky table
• A tilting table for fluoroscopy and general radiography
• The serial changer of such a fluoroscopic table
• Tomographic units Skull table
• A vertical Bucky

A universal Bucky

TYPES OF GRID MOVEMENT IN GENERAL ARE:-

• RECIPROCATING MOVEMENT
• OSCILLATING (VIBRATING) MOVEMENT
• CATAPULT MOVEMENT

RECIPROCATING MOVEMENT:- Grid moves fast in one direction and slowly in other direction, maintaining this sequence throughout the exposure.

Pressing the exposure switch on the unit energises an electromagnet which pulls the grid rapidly across the film and simultaneously tension a spring The exposure is started when the contact are closed shortly after the grid begins to move Just before it reaches the end of its travel the grid breaks a contact which stops current flowing in the electromagnetic and the grid is pulled across the film by the spring in the opposite direction.

This time movement is slowed by the oil in the hydraulic system Just before it reaches the end of its travel the grid operates a contact which energizes the electromagnetic once again This sequences continues, fast in one direction and slow in the other until the end of exposure A short exposure takes place whilst the grid is moving fast and a longer in slow return movement while longer exposure take place whilst the grid is moving alternately fast and slow

TYPES OF GRID MOVEMENT IN GENERAL ARE:-

OSCILLATING (VIBRATING) MOVEMENT

• OSCILLATING (VIBRATING) MOVEMENT:-Grid moves back and forth across the film during exposure

Supported by four leaf springs, it makes a backwards and forwards movement over the film When the exposure switch is put in prep an electromagnet is energized and it pulls the grid across to the limit of its travel Shortly after the grid begins its movement it operates a relay which starts the exposure The grid continues to vibrate during the exposure, but vibration decrease slower until finally it comes to rest after 15-30 seconds

TYPES OF GRID MOVEMENT IN GENERAL ARE:-

CATAPULT MOVEMENT

• CATAPULT MOVEMENT:- Grid makes one movement across film. Initial speed is very high , then it moves more and more slowly

The grid is driven by an electric motor There is no reversal of grid movement and the constantly changing rate of movement ensures that synchronism does not occur

FACTORS AFFECTING GRID SELECTION

Patient Dose

• Type of Exam
• Detail required
• Part thickness
• Desired technique (kVp)
• Equipment availability

GRID MAINTENANCE

• Application of grid tunnel
• Keep grid dry and clean

Treat grid gentle

SECONDARY GRID

PROBLEMS

GRID PROBLEMS

• Increased object image distance especially with moving grids
• The biggest problem with grids is misalignment

GRID PROBLEMS RESULT IN

Underexposed image or Underexposed edges of image