DIR DR Measures

Exposure Index (EI)

Data Element

Clinical Performance Measure

• Exposure Index (TID 113845)

• Study Description (0033, 1004)

• Acquisition Protocol (TID 125203)

• Image View (TID 111031)

• Median, 25^th, and 75^th percentile values of EI distribution

Understanding the Measure

• In DIR-DR, EI represents the exposure index concept defined in IEC standard 624942-1. 

• It is an indicator of the air kerma at the image receptor, which in turn influences image noise and therefore impacts image quality.

• EI is not a metric of patient dose or risk.

Target Exposure Index (EI_T)

Data Element

Clinical Performance Measure

• Target Exposure Index (TID 113846)

• Study Description (0033, 1004)

• Acquisition Protocol (TID 125203)

• Image View (TID 111031)

• Median, 25^th, and 75^th percentile values of EI_T distribution 

• EI_T mode (for registry statistics only)

Understanding the Measure

• EI_T is the expected value of the EI when exposing the x-ray image receptor properly. 

• EI_T may depend on the type of image receptor, the type of examination, the diagnostic question and other factors. It can be set by the manufacturer or by the imaging facility.

• Evaluating the appropriateness of the EI_T values for various examinations is an important component of a facility’s ongoing QC program.

• In general, adjusting EI_T requires the review of clinical image quality in consultation with a radiologist. Its adjustment should not be made based on statistical results alone. For example, one may attempt to adjust EI_T to match the median EI observed in the facility data. However, the median EI may simply reflects a general trend of overexposure or underexposure. Using it to set a new EI_T would only serve to validate the existing suboptimal practice.

• Currently, EI_T mode is reported in the Registry Statistics by Procedure dashboard only. 

Deviation Index (DI)

Data Element

Clinical Performance Measure

• Deviation Index (TID 113847)

• Study Description (0033, 1004)

• Acquisition Protocol (TID 125203)

• Image View (TID 111031)

• Median, 25^th, and 75^th percentile values of DI distribution 

• Weighted DI standard deviation for a facility (Wgt DI Std)

Understanding the Measure

• Deviation index is defined as

• It quantifies how much the measured EI deviates from the EI_T.

• A DI of +3 indicates the EI is approximately 100% higher than intended (a doubling).

• A DI of –3 indicates the EI is approximately 50% lower than intended (a halving).

• When the median DI differs substantially from zero, it could be due to 

  • suboptimal EI_T setting, 

  • suboptimal programmed techniques, 

  • insufficient adherence to the programmed techniques, or 

  • miscalibration of the automatic exposure control (AEC) device.

• Because DI typically follows a normal distribution, its standard deviation serves as a measure of the extent of practice variation. 

• Considering that an imaging facility may have multiple DR stations, a weighted DI standard deviation (Wgt DI Std) is defined for the entire facility to account for the difference in procedure volume among different DR stations: where N_i is the number of events contributed by DR station i.

  • Wgt DI Std is a measure of the degree of practice variation at the facility. Efforts to standardize techniques and practice should lead to a reduction of Wgt DI Std over time.

Kerma-Area Product (KAP)

Data Element

Clinical Performance Measure

• Dose Area Product (TID 122130)

• Study Description (0033, 1004)

• Acquisition Protocol (TID 125203)

• Image View (TID 111031)

• Median, 25^th, and 75^th percentile values of KAP distribution

Understanding the Measure

• KAP quantifies the total amount of radiation energy delivered to a patient by considering both the intensity of the radiation (kerma) and the area over which it is applied.

• In the DIR-DR reports, it is expressed in the unit of mGy·cm².

• It provides a measure that correlates with the overall stochastic radiation risk to the patient.