X-ray tube An X-ray tube is a vacuum tube that converts electrical input power into X-rays. The availability of this controllable source of X-rays created the field of radiography, the imaging of partly opaque objects with penetrating radiation. In contras ...

s, the heel effect or, more precisely, the

anode An anode usually is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, which is usually an electrode of the device through which conventional current leaves the devic ...

heel effect is a variation of the intensity of

X-rays An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

emitted by the

depending on the direction of emission along the anode-cathode axis. X-rays emitted toward the

are less intense than those emitted perpendicular to the cathode–anode

axis An axis (: axes) may refer to: Mathematics *A specific line (often a directed line) that plays an important role in some contexts. In particular: ** Coordinate axis of a coordinate system *** ''x''-axis, ''y''-axis, ''z''-axis, common names ...

or toward the

cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device such as a lead-acid battery. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. Conventional curren ...

. The effect stems from the absorption of X-ray

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...

s before they leave the anode in which they are produced. The probability of

absorption Absorption may refer to: Chemistry and biology *Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which su ...

depends on the distance the photons travel within the anode material, which in turn depends on the angle of emission relative to the anode surface.

Factors

Source-to-image distance (SID)

The distance from the anode (the source of X-rays) to the image receptor influences the apparent magnitude of the anode heel effect. At short SID, image receptor captures a wider range of X-ray intensities than the same size image receptor at larger source–image distances (SID).

Beam width and receptor size

Wide X-ray beams can be cropped to narrow X-ray beams by use of a beam restricting device (variable-aperture X-ray "

collimator A collimator is a device which narrows a beam of particles or waves. To narrow can mean either to cause the directions of motion to become more aligned in a specific direction (i.e., make collimated light or parallel rays), or to cause the spat ...

", fixed aperture, or cone). A beam that is wide along the cathode–anode axis contains a wider range of X-ray intensities than a narrow beam. In a wide beam, a large image receptor captures a wider range of X-ray intensities than a small receptor (at the same SID). Both of these factors influence the visibility of the anode heel effect. A smaller field size results in a less pronounced heel effect.

Anode angle

When the angle of the anode is large, the usable X-ray photons do not have to travel through as much of the anode material to exit the tube. This results in a much less apparent anode heel effect, though the effective focal spot size is increased.

Solutions

Almost all modern diagnostic

X-ray generator An X-ray machine is a device that uses X-rays for a variety of applications including medicine, X-ray fluorescence, electronic assembly inspection, and measurement of material thickness in manufacturing operations. In medical applications, X-ra ...

s exhibit heel effect. Graded intensity across the beam, generally a drawback, can be turned to advantage in some techniques by positioning the object or patient relative to the X-ray tube. For example, when imaging a

foot The foot (: feet) is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is an organ at the terminal part of the leg made up o ...

, which is thicker at the ankle end than the toes, the toes should be positioned toward the anode and the ankle toward the cathode. Arbitrary intensity gradients can also be produced by placing a beam compensator (a wedge of homogeneous material) across the X-ray beam before exposure. There have also been some efforts to eliminate the heel effect through automatic

software Software consists of computer programs that instruct the Execution (computing), execution of a computer. Software also includes design documents and specifications. The history of software is closely tied to the development of digital comput ...

adjustment of

pixel In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a Raster graphics, raster image, or the smallest addressable element in a dot matrix display device. In most digital display devices, p ...

values.

References

{{Reflist X-rays Medical physics