Digital Photospot Images

Figure D shows examples of Digital Angiography (DA) images acquired at the six dose settings (0.24 through 12) that are available on this Siemens angiography unit. These digital angiograms are examples of digital photo spot images, and are generally acquired at relatively high dose rates. Data pertaining to the image acquisition techniques for the images depicted in Figures D and E are summarized in the Table below.

Figure D. Examples of Digital Angiograms obtained at dose levels ranging from 0.24 to 12; note that the image in the right bottom corner used approximately 50 times more radiation than the image in the upper left corner. Upper left = 0.24, upper middle = 0.48, upper right = 1.2, lower left = 2.4, lower middle = 4.8, lower right = 12.0.

Table 1. Summary of the dose settings, x-ray tube voltage, and entrance skin air kerma values corresponding to the images depicted in Figures D and E.


Dose setting

X-ray tube voltage

Entrance air kerma (mGy)

0.24

70

0.044

0.48

70

0.083

1.2

70

0.20

2.4

70

0.39

4.8

70

1.3

12

75

2.3

The data shown in the Table indicate that the dose setting is approximately proportional to the amount of radiation that is emitted by the x-ray tube. Changing the dose setting from 0.24 to 12 will change patient doses by approximately a factor of 50. Accordingly, that it is very important for operators to understand the role of these settings, and to use these in an appropriate manner when scanning patients.

It is interesting to note that the system generally attempts to maintain an x-ray tube voltage of 70 kV, which will optimize the visibility of structures that contain iodinated contrast material. The reason for this is that iodine has a k-shell binding energy of 33.2 keV, and x-ray attenuation is highest just above this photon energy. Lower x-ray tube voltages would suffer from poorer patient penetration whereas higher x-ray tube voltages increase the average photon energy which reduces attenuation by iodine (absorption of x-rays by iodine is proportional to 1/E3).  At the highest dose setting of 12 the voltage is increased to permit sufficient x-rays to be produced.

There are limits to increasing x-ray tube output by increasing the mA (power limitations) as well as exposure time (long exposure times result in increased patient motion) which are described below.

Tube current (mA). The power loading of an x-ray tube is the product of the tube current (mA) and the x-ray tube voltage (kV), and is measured in watt or kwatt. The image at dose setting of 4.8 in Figure D was obtained at a tube current of 600 mA, and the power loading would be 42,000 watt (i.e., 42 kW). This may be compared with a typical maximum power capacity of an x-ray tube in interventional radiology of 80 or 100 kW.

Exposure time. The exposure time in radiography needs to be kept short to minimize any unnecessary patient motion. At the dose setting of 4.8 in Figure D, the exposure time was ~30 ms, which is typical of DA and DSA images.

In all of radiography, the choice of optimal techniques must take into account the detection task (e.g., presence of iodinated contrast), power limitations of the x-ray generator, heat dissipation in the anode, exposure time, and the corresponding patient dose. In the example depicted here, at the highest dose setting of 12 the manufacturer has elected to increase the x-ray tube voltage from 70 to 75 kV. Increasing this x-ray tube voltage will reduce image contrast, but will also help to reduce patient dose and total exposure time. Radiologists need to be aware of the complex interplay of techniques on patient dose and image quality, as well as the technical capabilities of the imaging equipment that they use.

Note that the dose per frame in Digital Angiography is approximately the same as in Digital Subtraction Angiography for a given dose setting. For example, selection of the highest dose setting of 12 will result in skin doses of the order of 2 to 3 mGy in both DA and DSA imaging. However, it is important to note that in DSA imaging, the vasculature alone is visualized, and the random noise (mottle) as depicted in Figure C is readily visible. By contrast, images in Figures D and E show that random noise is much more difficult to see. As a result, noise (mottle) is generally less of an issue for images containing anatomy, because vessel visibility is affected by anatomical structures in the vicinity of the vasculature of interest. Consequently, digital spot images are normally acquired at lower doses than those associated with DSA (see above).

Figure E shows enlarged images of DA images of the pelvic phantom that illustrates how vessel visibility changes with changes in radiation dose level. Changing the dose in DA images (Figures D and E) is less important than in DSA images (Figures B and C). It is therefore very important for radiologists to identify the amount of radiation that is required for a satisfactory diagnosis, and help keep patient doses As Low As Reasonably Achievable (ALARA). Application of the ALARA principle needs to be performed separately in both DA and DSA imaging, since the diagnostic tasks and image appearances are generally different. Optimal dose levels in DA imaging are should be lower than those used for DSA imaging.

Figure E. Regions of interests of Digital Angiograms (DA) of a pelvic phantom with added tubing containing diluted iodinated contrast medium obtained at six different dose settings, illustrating the changes of quantum mottle on overall visibility of simulated blood vessels.