Diagnostic Radiology II

X-ray Tubes

Anode angle

• Anode angle defined as the angle of the target surface with respect to the central ray in the x-ray field

• Anode angles in diagnostic x-ray tubes range from 7 to 20 degrees, with 12- to 15-degree angles most common

Focal spot size

• Actual focal spot size is the area on the anode that is struck by electrons– Primarily determined by length of cathode

filament and width of focusing cup slot

• Effective focal spot size is the length and width of the focal spot projected down the central ray in the x-ray field

Focal spot size (cont.)

• Effective focal spot width is equal to the actual focal spot width

• Effective focal length = actual focal length sin

• Foreshortening of the focal spot length, as viewed down the central ray, is called the line focus principle

Anode angles

• Optimal anode angle depends on the clinical imaging application

• Small anode angle desirable for small field-of-view image receptors (cineangiographic and neuroangiographic equipment, where field coverage is limited by the image intensifier diameter)

• Large anode angles necessary for general radiographic work to achieve large field area coverage at short focal spot-to-image distances

Variation with position

• Effective focal spot length varies with the position in the image plane, in the anode-cathode direction

• In the width dimension, the focal spot size does not change appreciably with position in the image plane

• Nominal size specified at the central ray of the beam

Measuring focal spot size

• Pinhole camera

• Slit camera

• Star pattern

• Resolution bar pattern

Pinhole camera

• Very small circular aperture (10 to 30 m diameter) in a disk made of a thin, highly attenuating metal such as lead, tungsten, or gold

• Image of focal spot recorded with the pinhole camera positioned on the central axis between the x-ray source and the detector

Slit camera

• The slit camera consists of a plate made of a highly attenuating metal (often tungsten) with a thin slit (typically 10 m wide)

• Measuring the width of the distribution on the image yields one dimension of the focal spot

• Second radiograph, taken with the slit perpendicular to the first, yields the other dimension of the focal spot

Star pattern

• Star pattern test tool contains a radial pattern of lead spokes of diminishing width and spacing on a thin plastic disk

• Imaging the star pattern at a known magnification and measuring the distance between the outermost blur patterns on the image provides an estimate of the resolving power of the focal spot in the directions perpendicular to and parallel to the anode-cathode axis

Resolution bar pattern

• Similar to the star pattern tool, but containing radioopaque bars of varying widths and spacings

• Bar pattern images demonstrate the effective resolution parallel and perpendicular to the anode-cathode axis for a given magnification geometry

Heel effect

• The heel effect refers to a reduction in the x-ray beam intensity toward the anode side of the x-ray field

• For a given field size, the heel effect is less prominent with a longer source-to-image distance (SID)

• X-ray tube best positioned with the cathode over the thicker parts of the patient to balance the transmitted x-ray photons incident on the image receptor

Tube housing

• X-ray tube housing supports, insulates and protects the x-ray tube insert

• Oil in the housing provides heat conduction and electrical insulation

• Lead shielding inside the housing attenuates the x-rays that are emitted in all directions

• Regulations limit the maximum leakage exposure rate (0.1% @ 1m of the exposure rate at the same distance on the central axis of the useful beam)

Grid-biased tubes

• Focusing cup is electrically insulated from the cathode filament

• Approximately –2,000 V applied to focusing cup shuts off the tube current; turning off the grid bias allows tube current to flow and x-rays to be produced

• Used in applications such as pulsed fluoroscopy and cineangiocardiography

Filtration

• In general diagnostic radiology, added filters attenuate the low-energy x-rays in the spectrum that have virtually no chance of penetrating the patient and reaching the x-ray detector

• Patient radiation dose is reduced• Aluminum is the most commonly added filter

material• Regulations prescribe minimum filtration (e.g., 2.5

mm Al for machines designed to operate at potentials above 70 kVp)

Compensators

• Compensators are added filtration with a shape intended to change the spatial pattern of the x-ray intensity incident on the patient, so as to deliver a more uniform x-ray exposure to the detector

• Placed close to the x-ray tube port or just external to the collimator assembly

Collimators

• Collimators adjust the size and shape of the x-ray field emerging from the tube port

• Adjustable parallel-opposed lead shutters define the x-ray field

• Positive beam limitation collimators automatically limit the field size to the useful area of the detector