With what frequency must radiographic equipment be checked for linearity and reproducibility?

Laura LeBlanc, 1204502

As defined by Venes & Taber (2005), “linearity in radiography is the production of a constant amount of radiation for different combinations of milliamperage and exposure time”. In the clinical setting, it is essential that all general x-ray units produce a proportional change in exposure as milliamperage (mA) varies. The assumption is that an increase in mAs, should produce proportional increases in radiation exposure. If the x-ray unit is not properly functioning, the unit must be serviced as retested before used again for diagnostic procedures. Linearity tests are performed to monitor patient dose and image quality. Variation in output intensity during diagnostic exposures can result in unnecessary dose to the patient due to repeats from poor quality images. While performing this test, the assumption is that every other factor, such as kVp and the timer, are working accurately. It is important to perform QC tests on the kVp as well as the timer before completing the linearity test as if there are variations in linearity, mAs may not be the culprit - inaccurate performance from the kVp meter and the x-ray timer can also produce the same results. According to H.A.R.P., the average mR/mAs should not differ by any more than 0.10 times their sum. Safety code 35 is in agreement with H.A.R.P.. 

For the first component of the lab, the linearity for mAs change was tested. Exposure output (in microroentgens) was recorded using the Cobia dosimeter and was later converted to milliroentgen to calculate mR/mAs. Using a fixed kV (70 kV) and starting at 8 mAs, two exposures were taken after approximately doubling the selected mAs until 63 mAs was reached. Exposures were taken at 80 mA on a small focal spot setting and at 160 mA at a large focal spot setting. A 75cm SID was used in order to reduce anode-heel effect. When we chose to double the mAs, holding other factors constant, we should obtain (approximately) double the exposure, which explains why the ratio of mR/mAs should remain constant at a set kVp. The results are as follows: PART 1 (Room 6)

Figure 1: Cobia X-ray Meter

The following formula is used to calculate linearity:

Calculations given the values from the chart above as well as the calculation for linearity are as follows:

Although they show slight variances in mR/mAs, none of the above data for any exposure demonstrate deviance outside of the accepted limits stated by SC 35 or the HARP Act; hence corrective action is not needed.


PART 2 (Room 6)

The second part of this lab tests for linearity between the reciprocity of mA and exposure time. At a fixed kV (70 kV), mA and exposure time were adjusted so that 1.6 mAs was selected for each exposure. This is a potential source of error as 16 mAs should have been used to properly assess linearity. Two exposures were made for three different mA stations, and the mR/mAs value was calculated. Table 2 below, summarizes the results for part 2 of the lab and the linearity calculations follow.

Although a larger variance is present in mR/mAs, none of the above data demonstrate deviance outside of the accepted limits stated by SC 35 or the HARP Act; hence corrective action is not needed.

This test was once again completed in Room 3. The results are provided below:

PART 1 (Room 3):

Calculations given the values from the chart above as well as the calculation for linearity are as follows:

Although a larger variance is present in mR/mAs, none of the above data demonstrate deviance outside of the accepted limits stated by SC 35 or the HARP Act; hence corrective action is not needed.

PART 2 (Room 3)

Calculations given the values from the chart above as well as the calculation for linearity are as follows:

None of the above calculations demonstrate deviance outside of the accepted limits stated by SC 35 or the H.A.R.P. Act; hence corrective action is not needed. If the calculations demonstrated that either of the rooms did indeed deviate from the limits, corrective action would definitely be necessary. An improperly functioning room could result in overexposure to the patient, which is failure to comply with the ALARA principle striven for. Another potential impact from failing to meet the standard would be that an improperly set mA could result in damage to the tube through tube overload. In conclusion, it is very important to ensure linearity is within the acceptable guidelines.

Bushong, S. (2008). Radiologic science for technologists: Physics, biology, and protection (9th ed.). St.                    
     Louis, Mo.: Mosby/Elsevier.Health Canada. (2008, January 1). Safety code 35: Safety procedures for the installation, use and control 
      of x-ray equipment in large medical radiological facilities. Retrieved January 22, 2015, from http://www.hc-sc.gc.ca.

Papp, J. (2011). Quality management in the imaging sciences. St. Louis, Mo.: Mosby Elsevier.

Service Ontario. (2011). Healing Arts Radiation Protection Act. Retrieved from http://www.e-

Production of radiologic images of acceptable diagnostic quality obtained with minimum radiation exposure to patients is a basic goal of the Bureau of Radiological Health (BRH). This manual describes in detail the establishment and operation of specific elements of a quality assurance program. It presents proven QA techniques that can be adapted by individual radiology facilities according to their needs and resources as part of the BRH Quality Assurance Publications series providing QA information for many uses of medical radiation including ultrasound, nuclear medicine and radiation therapy as well as diagnostic radiology.

Authors: Hendee, W. R.; Rossi, R. P. Publication Date: Wed Aug 01 00:00:00 EDT 1979 Research Org.: Bureau of Radiological Health, Rockville, MD (USA); Colorado Univ., Denver (USA). Medical Center OSTI Identifier: 5545617 Report Number(s): FDA-79-8094
TRN: 80-003489 Resource Type: Technical Report Country of Publication: United States Language: English Subject: 62 RADIOLOGY AND NUCLEAR MEDICINE; 99 GENERAL AND MISCELLANEOUS//MATHEMATICS, COMPUTING, AND INFORMATION SCIENCE; NUCLEAR MEDICINE; MANUALS; BIOMEDICAL RADIOGRAPHY; IONIZING RADIATIONS; QUALITY ASSURANCE; DIAGNOSTIC TECHNIQUES; DOCUMENT TYPES; MEDICINE; RADIATIONS; RADIOLOGY; 550601* - Medicine- Unsealed Radionuclides in Diagnostics; 570000 - Health & Safety