Angiography Approaches to Dose Reduction

Angiography Approaches to Dose Reduction
 
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Dose and Radiation Risk in Angiography

Several dose parameters are specific to angiography: the detector dose, the patient entrance dose, the dose rate and the DAP (dose area product), which will be covered in the following sections.


Dose and Image Quality

In the past, when angiography was performed using traditional photographic film technology, the general rule was the higher the dose, the better the image quality.

With today’s improvements in imaging technology, is there still a trade-off between improving quality and saving dose?

Yes

In general, low dose goes hand in hand with less visibility, while higher image quality requires, among other factors, a higher dose. To obtain a specific image quality, it is necessary to choose the “right” dose for the tissue being penetrated.

Therefore, the solution is to make best use of dose and equipment.

Good image quality can be expressed in terms of detectability. Rose found:1

(The physical contrast is the difference in X-ray absorption between rays of the beam running through the object of interest and the rays next to it.)

What does this mean for angiography? Lesion detectability is directly related to detector dose, given a certain physical contrast and diameter of a vessel. In general good contrast, low noise, and high spatial resolution are necessary for good image quality. As a result, even fine details are visible.

1Rose A. Vision, human and electronic. Plenum Press, New York, 1973.
 


Patient entrance dose and Safety During Fluoroscopy

Modern detector systems make it easy to obtain high-quality results simply by selecting the required image quality by choosing an appropriate protocol; the requested dose at the detector entrance will automatically be kept constant (as much as possible) by adjusting the tube output dose. This automatic dose control compensates for patients of different body sizes.

The dose is highest at the point where the beam enters the patient. The absorbed dose at this beam entrance is an important measure: It signifies the accumulated patient entrance dose over the length of the procedure, measured in mGy (1,000 mGy = 1 Gy). This accumulated patient entrance dose is relevant for determining the skin burn damage resulting from the intervention.

Detector systems display and report only an estimate of the patient entrance dose at the interventional reference point (IRP). Values at the actual exposure entry location can be different depending on the patient’s body shape and other geometric measures, such as table and C-arm position.

 

  1. The red point on the X-ray tube housing indicates the position of the focal spot.
  2. The source-image distance (SID) is the distance between the focal spot and the image receptor. On the Artis systems, this receptor is the flat detector.
  3. ISO Center refers to the isocenter of the C-arm; i.e. the central point around which the C-arm rotates.
  4. The IRP is 15 cm beneath the isocenter and is assumed to be the skin entrance point. The calculated estimates for the displayed dose values refer to the IRP.

The IRP is the measuring point for:

Dose area product (DAP) in μGy m2

Dose in mGy

Dose rate in mGy/min

 

Note: The IRP does not change with table height.

 

As a general rule, the closer the beam entrance to the tube, the higher the real patient entrance
dose and vice versa.

 

The Siemens Artis system has two built-in safety regulations for fluoroscopy:

  1. By default, the dose rate (air kerma rate) at a specific location (30 cm in front of the detector) is limited to a certain level (e.g., 10 R/min = 87 mGy/min for the U.S. and European countries). It is assumed that this point is identical to the patient’s skin
  2. entry point. It is possible to increase the dose rate by using the “Fluoro +” button (high-contrast button). An audible warning occurs.
  3. After every five minutes of fluoroscopy, a message pops up on the display and a sound is emitted to remind the user of the applied dose. If the operator does not acknowledge this signal, radiation exposure stops after the next five minutes of fluoroscopy.


Dose Rate and Dependence of Absorption on Patient Thickness

Mean patient entrance dose has been reported for interventions in the brain. Although head size varies little among individuals, body size varies greatly. An increase in patient thickness of about 3 cm results in twice the entrance dose for a constant detector entrance dose (Figure 4). This rule of thumb is based on the assumption that tissue absorbs radiation in a similar way to water and that a certain quality of beam is applied.

A similar effect occurs when the direction of projection is changed to an oblique position (Figure 5). Because the shape of the body is more oval than circular, the length of the X-ray beam is now longer, resulting in a higher entrance dose. True values may differ significantly since the body is not really a homogeneous ellipsoid but consists of bones, organs, etc.2

2Cusma JT et al. Real-time measurement of radiation exposure to patients during diagnostic coronary angiography and percutaneous interventional procedures. J Am Coll Cardiol. 1999 Feb;33(2):427-35.



Dose Area Product and Inverse Square Law

In air, X-rays travel in a straight line. Their intensity decreases as the distance from the X-ray tube focus increases along with the surface area of the beam. The dose D at the distance d from the focal spot F drops to 1/4 D at the distance 2d and to 1/9 of D at three times the distance (Figure 6). The inverse square law for radiation dose shows that at twice the distance from the focus, the dose D is reduced by a factor of four with respect to the quadrupled surface – it is spread across a four-fold area.



Dose Area Product (DAP)

The DAP of a certain exposed area of constant dose is defined as dose times area and is independent from the distance to the source.

 

An example for distances d1 = d and d2 = 2d and for associated doses D1 = D and D2 = 1/4 D and the irradiated
areas a1 = a and a2 = a d22/d12 = 4a proves:

 

DAP1 = D1 · a1 = D · a

 

DAP2 = D2 · a2 = 1/4 D · 4a = D · a = DAP1

 

This means that the Dose Area Product remains constant at different distances.

 

Inverse square law:
The inverse square law for radiation dose shows that at twice the distance from the focus, the dose D is reduced by a factor of four with respect to the quadrupled surface – it is spread across a four-fold area.



Effective Dose in Angiography

Determining the effective dose in angiography depends on several factors, primarily on the variability in organ sensitivity to radiation. Recall that bone marrow is far more sensitive to radiation than the liver (refer back to the section on “Equivalent and Effective Dose”). The degree to which organs are affected by radiation also depends on the angle of the beams. Because dose distribution in angiography is not “homogeneous” as it is for CT, these factors must be considered when estimating the damage caused by irradiation.

 

Converting patient entrance dose and DAP to effective dose is reliable only if the X-ray parameters and the location of the beam running through the body are known. In modern angiography, the role of the effective dose is not as significant as it is, for instance, in CT.

 

Remember: The effective dose includes the sensitivity to radiation of the different organs. It is the sum of the equivalent doses in all irradiated organs multiplied by the respective tissue weighting factors.


 



Important Parameters that Affect Dose in Angiography

Several parameters affect dose in angiography.

  • The footswitch on time controls how long the beam is on the body and thus how long the body is irradiated; less time means less radiation.
  • High frame rates are used to visualize fast motion without stroboscopic effects. However, the higher the frame rate, the more radiation. Therefore it is best to keep the frame rate as low as possible.
  • SID: according to the quadratic law and a constant requested dose at the detector,a greater distance between the source and the imager increases the patient entrance dose. Raising SID from 105 cm (=SID 1) to 120 cm (=SID 2) increases patient entrance dose (i.e. the dose at the IRP) by approximately 30%.1

 

1If C-arm angles, table position, patient, and requested dose at the detector do not change.

Figure 7 illustrates the setup including the lower (SID = 105 cm) and the upper (SID = 120 cm) position of the detector.