Philips EI vs Siemens EXI: older Philips EI falls as exposure rises, Siemens EXI rises
Key takeaways
- Philips EI as described in AAPM Report 116: higher number, less exposure.
- Siemens EXI as described there: double the dose, double the number.
- IEC EI is 100 times the air kerma in microgray at the receptor, under a set beam quality.
- DI = 10 × log10(indicated exposure ÷ target). +3 is about double and -3 about half. DI isn't in stops.
- ARRT's outlines say exposure indicator and don't list vendor names.
An exposure index estimates the radiation that reached the receptor. Each vendor built its own, so the same number can mean opposite things. “EI” on one system isn’t even the same scale as “EI” on another.
How do the vendor scales differ?
These come from AAPM Report 116 (2009), mostly from vendor-written appendices for named systems. Newer software can differ, so check your manual.
| System | Name | When exposure rises | Rule |
|---|---|---|---|
| Philips DigitalDiagnost | EI | Number falls | EI = 1000 ÷ air kerma in µGy |
| Fujifilm CR | S-number | Number falls | Exposure in mR is about 200 ÷ S |
| Konica Minolta CR | S-value | Number falls | n times the dose gives 1/n the value |
| Siemens AXIOM Aristos | EXI | Number rises | Double the dose, double the EXI |
| Kodak (now Carestream) | EI | Number rises | +300 is double the exposure |
| Agfa CR | lgM | Number rises | +0.301 is double the exposure |
GE’s DEI and Canon’s REX are also covered in the report. GE’s is a ratio to the expected exposure. Canon’s changes with the brightness and contrast settings.
What is the IEC exposure index?
IEC 62494-1 defines EI as 100 times the air kerma in microgray at the receptor, under a set beam quality (RQA-5). It rises with exposure. It is a standard, and we couldn’t confirm that every vendor must display it. A Philips room today may show this scale rather than the older one. Even then, a 2026 study found EI still varies with kVp, anatomy and manufacturer, so a target from one system doesn’t always carry over.
How do you read the deviation index?
DI = 10 × log₁₀(indicated exposure ÷ target exposure).
DI isn’t in stops. A DI of +2 is about 1.6 times the target, not 2 times. AAPM suggests a target range of -0.5 to +0.5, and your department sets its limits.
Watch the inverse scales. On the older Philips scale, a reading of 400 against a target of 200 means half the target exposure, so DI is about -3, not +3.
DI is a guide. AAPM says poor collimation, prostheses or unusual positioning can give a false value, and a good DI doesn’t prove good technique. Windowing doesn’t change the exposure.
What do you need for the exam?
The ARRT® outlines say “exposure indicator” and name no vendors. From March 1, 2027, the outline drops the term CR, though its quality control section still lists plate calibration and plate uniformity. We found nothing from ARRT saying vendor abbreviations are tested.
See the Image Production guide, the Digital Image Processing chapter and the Image Acquisition set.
Frequently asked questions
What is the difference between Philips EI and Siemens EXI?
In AAPM Report 116, Philips EI on the DigitalDiagnost is inversely proportional to detector air kerma, while Siemens EXI on the AXIOM Aristos is directly proportional to dose. A higher number means opposite things.
Is the IEC exposure index on every system?
We couldn't confirm that, or that vendors are required to show it. IEC 62494-1 is a standard. Check what your system displays and its manual.
What does a DI of +3 mean?
About twice the target exposure. A DI of -3 is about half. AAPM Report 116 suggests a target range of -0.5 to +0.5, and your department sets its own limits.
Do I need to memorize vendor names for the ARRT exam?
ARRT's outlines list exposure indicator with no vendor names, and ARRT doesn't publish its questions. Know the concept and the direction of each scale type.
Sources
Found an error? Email [email protected] with the page and the source.
- AAPM Report 116, An Exposure Indicator for Digital Radiography, July 2009, vendor appendices (PDF p.47, 49, 55, 65, 69, 77) Official
- Tsalafoutas, AlKhazzam and Kharita. Exposure index in digital radiography and its dependence on acquisition parameters, anatomy, and manufacturer. J Appl Clin Med Phys, 2026 Journal
- Digital radiography exposure indices: A review. J Med Radiat Sci, 2014 Journal
- ARRT Radiography Examination Content Specifications, 2022 and March 1, 2027, listed on ARRT's reference documents page Official
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