Attenuation artifact occurs when there is a poor signal to noise ratio resulting from excessive attenuation. It is often seen when imaging overweight individuals in whom the x rays have difficulty passing through the body to the detector. The image will appear grainy and contrast opacification will be reduced, making it difficult to accurately diagnose coronary artery lesions. Attenuation artifact may be avoided by not scanning significantly overweight patients, by increasing the tube current and voltage, by increasing the iodine concentration of the contrast, by increasing the contrast volume, and by increasing the contrast injection rate. Figure 5.1 is an example of attenuation artifact.

X ray beams are composed of individual photons with a range of energies. The greater the energy, the “harder” the beam. As the x ray beam passes through the body, it becomes “harder” because the mean energy of the beam increases as lower energy photons are attenuated. This effect results in two types of artifacts. The first is the “cupping” artifact or the dark “cup” shaped artifact adjacent to a high density object like calcium or metal. In addition, dark streaks are seen between dense objects in the image. Beam hardening artifact may occur from dense objects in the right atrium or right ventricle such as pacemaker or defibrillator leads, from bypass clips, sternal wires, stents (may be confused for in stent stenosis), artificial heart valves and dense calcium. It may also be caused by the spine which may result in less contrast in the posterior myocardium than in the lateral myocardium.

Manufacturers attempt to minimize beam hardening in the following ways. First, special filtration is used. A flat piece of attenuating (metallic) material is used to “pre-harden” the beam by filtering the low energy photons before they pass through the patient. An additional “bow tie” filter is used to further harden the edges of the beam that pass through the thinner parts of the patient. Second, calibration correction is used. Manufacturers will calibrate their scanners using phantoms that range in size. This allows the detectors to be calibrated with compensation tailored for the beam hardening effects of different parts of the patient. This helps to eliminate cupping artifacts. Third, beam hardening software is applied, which utilizes an iterative correction algorithm to bony regions. You, the CT angiographer, may also apply saline flushes after contrast administration to avoid high density contrast in the right atrium, which may be confused with right atrial masses. Figures 5.2 and 5.3 represent examples of beam hardening artifact.