Radiation Dosimetry: CT (Revised 3-25-2024)

Revision History

It is necessary for your qualified medical physicist to perform CTDI testing on every CT unit at your facility. Using these CTDI measurements, your physicist will be able to calculate various descriptors of dose for your adult head, pediatric head (1-year-old), pediatric abdomen (5-year-old, ~40 lbs.), and adult abdomen examinations (depending on the modules and patient types performed on that unit). These calculations will use the average technique factors provided by your facility. You can access the appropriate calculation spreadsheet on the CT Accreditation page. Please note: The CT Phantom Data/Dose forms have Single Energy (SE) and Dual Energy (DE) tabs. You only need to complete the tabs relevant to your submission type. 

Pass/Fail Criteria and Reference Levels 

The ACR has updated the dose reference levels for the phantom submission. Reference levels help identify situations where doses are above expected values and should be investigated. Units exceeding these reference levels (but remaining below the pass/fail criteria) will not fail accreditation. However, even if the unit is granted accreditation, the ACR strongly urges the facility to consult with its medical physicist to determine if it is possible to reduce the examination dose without sacrificing image quality. Corrective action for exceeding dose reference values will be checked by a survey team if your facility is chosen for a validation on site survey.

These pass/fail criteria and reference levels are based on a detailed analysis of the dose data and image quality collected during the first three years of the accreditation program. The requirements and recommendations consider the amount of radiation necessary for adequate image quality. Because multi-slice CT units are more prevalent since the accreditation program started, the dose criteria and levels incorporate a more appropriate dose descriptor (CTDI_vol). This will help facilities reduce unnecessary radiation dose to patients undergoing CT examinations while still maintaining sufficient radiation levels necessary for appropriate diagnoses. CTDI_vol is one of the pass/fail criteria for all protocols: adult head, adult abdomen, pediatric head and pediatric abdomen.

Required Materials 

• Phantom data form, with average facility protocols for each examination to be submitted in the phantom portion of the accreditation testing package.

• Calibrated CTDI (pencil) ionization chamber (typically 10 cm in length)

• Calibrated electrometer

• Acrylic (PMMA) cylindrical phantoms (figure 1, having cylindrical holes (at 1 cm from the edge, and one at the center):

  • Head CTDI phantom: 16 cm diameter

  • Body CTDI phantom: 32 cm diameter

• Dose calculation Excel spreadsheet, available under Testing and QC forms on the CT Accreditation page

Note: Air kerma may not be used for dose measurements. The dose forms that you will use as part of the online testing package are calculated using exposure readings, not air kerma. If your meter reads out air kerma, you must either change your meter settings or divide by 0.876 before entering the measurements into the data form.

Figure 1: CTDI phantom, pencil ionization chamber, and electrometer

Dosimetry Scanning Instructions 

1. Position the phantom appropriately at the isocenter of the scanner. Ensure that the phantom is correctly aligned in all three planes (sagittal, axial and coronal).

• For the adult head scans, position the 16 cm phantom in the head holder.

• For the adult abdomen scans, position the 32 cm phantom on the table top.

• For the pediatric head scans, position the 16 cm phantom on the table top.

• For the pediatric abdomen scans, position the 16 or 32 cm phantom on the table top.

2. Connect the pencil chamber to the electrometer and insert the pencil chamber into the central hole in the phantom. Ensure that all other holes (those at 3, 6, 9, and 12 o’clock positions) are filled with acrylic rods.

3. Using the appropriate protocol as entered in the phantom data form; acquire a single axial slice at the center of the phantom, with no table increment. DE CTDI dosimetry images can be acquired in either a single scan or two scans depending on the available energy selection modes for the axial scans. If the protocol is normally scanned helically, change to an axial scan, keeping the remaining technical parameters unchanged.

All CTDI dose information must be acquired using axial scans. 

In multislice CT, CTDI is a function of detector configuration. It is imperative that the detector configuration and total beam width used matches the site’s clinical protocol (NxT) as closely as possible, but not exceed the site's clinical protocol (N X T).  If N x T used for dosimetry does not exactly match the clinical value, be sure to modify the table increment used in the calculation to yield the same pitch value as used clinically.

4. Record the following in the dose forms for the appropriate examination:

5. Repeat the scan two more times and record the measurements from each scan in the dose form. If the data differ by more than 5%, check your equipment and rescan the data until the three measurements agree within 5%.

6. The spreadsheet will calculate the average measurement from scans in mR, and the CTDI at isocenter in phantom in mGy.

7. Move the pencil chamber from the center position to the 12 o’clock peripheral position. Ensure that an acrylic rod is then inserted into the vacated isocenter position.

8. Repeat steps 3 through 7 and record the value in mGy as the CTDI at 12 o’clock position.

9. The dose form will calculate the following:

10. The dose forms will auto-populate the following from the phantom data form: Dose Notification as described in XR-29, if applicable. Please note that completion of Dose Notification values is not required for ACR CT Accreditation. The Dose Notification values described by MITA XR-25 (and included in XR-29) are now requested in the phantom site scanning data form for informational purposes only. This is intended to raise your facility’s awareness and understanding of this feature as it may apply to your scanner and protocol. XR-29 compliance is not a requirement of CT Accreditation. Therefore, this is an optional field. Please visit the ACR NEMA XR-29 (MITA Smart Dose) Standard Frequently Asked Questions for further information on XR-29 and the CMS rule.

11. Repeat steps 1 through 9 for each examination protocol you are submitting for the unit. Follow the instructions for filling out the phantom data forms.

Special Considerations

Pediatric Body CTDI Measurement: Images of the 32 cm phantom acquired at small SFOVs for pediatric dose may be cut off. For example:  

• Scanner A uses the small body SFOV for the clinical pediatric body protocol. For this SFOV, the scanner reports CTDIvol using the 32 cm phantom (in accordance with IEC standards). However, when the phantom is scanned, the maximum DFOV is only 24 cm and the outer portion of the phantom is cut off, leaving only the central 24 cm (and central hole) of the phantom visualized. Because the site is making the measurement using the correct clinical protocol (including the correct bowtie filter as dictated by the SFOV), the measurement will be correct and will also match what the manufacturer is using to report CTDIvol. Not visualizing the full phantom is acceptable.    

• Scanner B uses the small body SFOV for the clinical pediatric body protocol. However, for this scanner and this SFOV, the scanner reports CTDIvol using the 16 cm phantom. Therefore, the site uses the 16 cm phantom for its measurement and the entire phantom should be visualized. Again, because the site is making the measurement using the correct clinical protocol (including the correct bowtie filter as dictated by the SFOV) and the phantom size that matches what the manufacturer is using to report CTDIvol, the measurement will be correct and will also match that of the manufacturer. In this case, the entire phantom should be visualized and is acceptable.

Protocols That Use Collimators Greater than 100mm: There are scanners that have protocols which use collimation greater than 100 mm, the length of the ion chamber. For helical protocols with clinical collimations greater than 100mm, e.g. 256 x 0.625mm, make the physical dose measurement in the same way you ordinarily would by centering the phantom/ion chamber and performing an axial rotation using the axial equivalent of the clinical helical technique and collimation. In this situation the X-ray beam will exceed the length of the chamber but this will be taken in account in the CTDI field on the dose form in your online testing package (and the generic excel form available on the ACR CT Accreditation Testing and QC forms webpage) to avoid an inaccurately low CTDIvol. For collimations that equal or exceed 100mm (or the ion chamber length), the CTDIvol should be determined by using 100mm (or the ion chamber length) in lieu of NxT in the calculation. The table speed (I) should be changed to yield the same pitch value used clinically. Example for a 100mm ion chamber: If the helical scan uses N = 256, T = 0.625mm, and pitch = 0.75, the following should be input into the CTDI calculation form: N = 1, T = 100, and I = N x T x pitch = 1 x 100 x 0.75 = 75 mm/rotation.  For single rotation protocols with clinical collimations greater than 100mm (or the ion chamber length), e.g. 320 x 0.5mm, make the physical dose measurement in the same way you ordinarily would by centering the phantom/ion chamber and performing an axial rotation using the actual clinical technique and collimation. In this situation the X-ray beam will exceed the length of the chamber but this will be taken in account in the CTDI field on the dose form in your online testing package (and the generic excel form available on the ACR CT Accreditation Testing and QC forms webpage) to avoid an inaccurately low CTDIvol. For collimations that equal or exceed 100mm (or the ion chamber length), the CTDIvol should be determined by using 100mm (or the ion chamber length) in lieu of NxT in the calculation. Example for a 100mm ion chamber: When filling out the CTDI calculation form, any N and T that give a product of 100 would be acceptable, i.e. N =1 and T = 100mm. The table increment should also be set 100mm (or the ion chamber length).

CTDI Measurements Using a Detector Configuration that is Not Available in Axial Mode: Some manufacturers simply do not allow an axial 64 x 0.6 mm detector configuration (where the outer images might suffer from considerable cone beam artifacts). This can make it difficult to perform CTDI measurements when 64 x 0.6 mm collimation is used for helical scans. There are fundamentally two options. The first is to use user tools specifically developed to assist in making axial CTDI measurements using clinically relevant parameters and detector configurations that might not be available in an axial (sequential) scan mode. One example is that Siemens has developed a “customer CTDI” measurement tool. This is available on Definition scanner models with software version VA34, VA40, or VA44, and Emotion or Sensation scanners with software version VB40. Instructions for use of the mode are included in the online operator manual (Life Card). If this option is not available, then the user can perform the axial CTDI measurements using settings (including collimation) that are “as close as possible” to the clinical setting. The site should describe this situation as well as the settings chosen to perform the CTDI measurements.

These new settings should be reported in the dosimetry spreadsheet with a note that they are different from those used clinically (and reported in the clinical protocol table). Please note that if collimation is changed for dosimetry testing purposes, then the table increment value (I) should also be changed to yield the same pitch value used clinically. 

Example: Siemens Sensation 64 scanner
Adult Abdomen Protocol: 120 kVp, 200 Quality Reference mAs, 64x0.6 mm collimation (using z-flying focal spot), pitch 1.0
In protocol table, use values: N=32, T=0.6 mm, I = 19.2 mm/rotation
However, 32 x 0.6 mm is not allowed in sequential mode on this scanner, so for dosimetry testing, there are two choices:

• Option 1 –use the customer CTDI measurement tool if it is available on your scanner

• Option 2 - use settings that are “as close as possible” to the clinical setting, which in this case
  could be:
  N=30, T=0.6 mm
  With a Table feed (I) necessary to give same pitch (Pitch = 1.0), I=18 mm/rotation

Flying Focal Spots: NMax and N Values: Some scanners use a “flying-focal spot”.  For these scanners you must look at the settings to determine the actual number of detectors used.

Nmax is the maximum number of tomographic sections that can be acquired in a single rotation. N = the number of actual data channels used.

For example, a Siemens Somatom Sensation 64 scanner utilizes a flying focal spot and has an Nmax = 64. However, when a site uses the collimation setting identified on the scanner as “64x0.6” (for example, for the clinical adult abdomen protocol), then N = 32 and T = 0.6 mm because the 32 actual detectors are used and sampled twice via the flying focal spot.

Another example is the Siemens Definition Flash, which also utilizes a flying focal spot. For this scanner, Nmax=128 and the collimation setting on the scanner is identified as “128x0.6”, but the actual number of data channels is 64, so N=64 and T=0.6 in the clinical protocol table (in this scanner, 64 actual detector rows are used and sampled twice via the flying focal spot in a manner similar to the scanner above).

Other scanners use “flying focal spot” technology and should be handled similarly.

For dosimetry testing, it is the radiation beam width that needs to be recorded in the dosimetry spreadsheet and two possible scenarios may arise. Reference Example 1 below if the scanner allows the same detector configuration in both axial (sequential) mode as well as helical scan, then the value of N and T described above should be used. The table increment in the dosimeter spreadsheet must be adjusted to yield the proper clinical pitch as indicated in the phantom data form. Reference Example 2 below if the scanner does not allow the same detector configuration in helical and axial (sequential) modes, then please see the information above on CTDI measurements using detector configurations not available in axial mode.

Example 1: Siemens Sensation 64 scanner
Adult Abdomen Protocol: 120 kVp, 200 Quality Reference mAs, 64x0.6 mm collimation (using z-flying focal spot), pitch 1.0. In protocol table, use values: N=32, T=0.6 mm, I = 19.2 mm/rotation (however, 32 x 0.6 mm is not allowed in sequential mode on this scanner, so for dosimetry testing, please see the information above on CTDI measurements using detector configurations not available in axial mode).

Example 2: Siemens Definition Flash Scanner
Adult Abdomen Protocol: 120 kVp, 200 Quality Reference mAs, 128x0.6 mm collimation (which uses z-flying focal spot), pitch 1.0N=64, T=0.6 mm, I = 38.4 mm/rotation. In this case, 128x0.6 mm is allowed in sequential mode, so no need to change settings for dosimetry: N=64, T=0.6 mm, I=38.4 mm/rotation. 

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