CT Frequently Asked Questions
General FAQ
- What are the physician qualifications?
- Is ARRT certification in CT required for CT technologists?
- What clinical examinations will be required?
- Will there be an ACR CT Accreditation designated phantom? If so, when should one be ordered?
- What will be the requirements for phantom testing?
- How long does the accreditation process take?
- Is the 45-day time frame based on calendar days or working days?
- What is the purpose of the Scanner Attestation?
- Do you need a Medical Physicist Survey for each scanner?
- What happens if I fail?
- Where can I find information regarding reducing doses for pediatric and small adult patients?
- Can the clinical and phantom images be submitted in a digital format?
- The Instructions for Scanning the ACR phantom specify that we must use the Adult Abdomen Scan Field of View for scanning the ACR phantom. On our scanner, this causes errors in CT number and non-uniformities. Can we use a Scan Field of View more closely matched to the size of the phantom?
- How do I know what detector configuration (N x T) I am using if the scanner console doesn't explicitly report that information?
- Our multi-slice CT system uses a spiral Adult Abdomen protocol. The instructions for section 5 say to use the adult abdomen protocol and "perform an axial scan instead while keeping the remaining technical parameters unchanged." Our system CAN'T perform an axial scan with the same detector configuration that is used in our spiral adult abdomen acquisition protocol. What detector configuration should I use? How do I turn my spiral acquisition protocol into an axial protocol with the correct slice thickness (CT number calibration and slice thickness images, Film page 1, Boxes 4 - 12; Module 1)?
- The Toshiba Aquilion-16 can only perform an AXIAL scan in the 4-detector mode (N=4). How do I measure CDTI for a 16-detector clinical scan mode (N=16)? Is measuring CTDI with the 4-detector mode (N=4) acceptable?
Toshiba FAQ
1. What are the physician qualifications?
The physician qualifications are consistent with the ACR Standard for Performing and Interpreting Computed Tomography, in effect January 1, 2002.
2. Is ARRT certification in CT required for CT technologists?
They must be ARRT-certified. However, it is strongly recommended but not required, that CT technologists be ARRT-certified in CT.
3. What clinical examinations will be required?
Three examinations of the head/neck, chest and abdomen regions will be required from every scanner being accredited. Facilities will be able to choose which exams they will submit from the following lists. At least one of the exams chosen for each scanner will be a specialized exam, and if the scanner is also used for pediatric patients, then one of the exams must also be from a child between the ages of 0 and 15. Also, your facility's typical scanning protocols for the submitted images will be required for accreditation.
Head/Neck
- Head CT (such as for headaches and to exclude neoplasm)
- CT of the temporal bones*
- CT of the cervical spine for known or suspected fracture*
- Pediatric head CT (such as for headaches, seizures, or suspected mass)
- Pediatric sinus CT for infection
- Pediatric cervical spine CT*
- Pediatric temporal bones CT*
Chest
- Chest CT (such as for evaluation of known or suspected lung cancer or cough)
- CT for suspected pulmonary embolus*
- High Resolution CT of chest (HRCT) for evaluation of diffuse lung disease*
- CT for assessment of possible aortic dissection*
- Pediatric chest CT (such as for detection of metastatic disease, trauma, infection or cough)
- Pediatric High Resolution CT of chest (HRCT) for evaluation of diffuse lung disease*
Abdomen
- Abdomen CT (such as for detection of possible liver metastases or lymphoma)
- CT for known cirrhosis (R/O hepatoma)*
- CT for evaluation of known renal mass (including ROI measurements)*
- CT for evaluation of the patient with suspected pancreatic carcinoma*
- Pediatric abdomen CT (such as for blunt trauma, suspected appendicitis, or abdominal pain)
- Pediatric CT for adrenal/renal mass*
* Asterisks denote specialty exams.
4. Will there be an ACR CT Accreditation designated phantom? If so, when should one be ordered?
Yes, it will be available to purchase through Gammex RMI. You will receive a phantom order form in your testing materials package. Please make sure that you order your phantom as soon as possible to allow ample time for shipping and completion of all testing materials within the allotted time frame provided by the ACR.
5. What will be the requirements for phantom testing?
Phantom images and dose measurements will be required from every scanner being accredited. Using Computed Tomography Dose Index (CTDI) phantoms, a medical physicist must perform dose measurements on every scanner that you will be submitting for accreditation. Using these CTDI measurements, your physicist will be able to calculate various descriptors of dose for your adult head, pediatric abdomen (5 y.o.) and adult abdomen examinations.
6. How long does the accreditation process take?
Please allow approximately 4-6 months for your images to complete the review process and to receive your final results.
7. Is the 45-day time frame based on calendar days or working days?
The time frame is based on calendar days. After you apply for accreditation, you will receive all of the testing materials and labels. The due date is printed on the labels you receive.
8. What is the purpose of the Scanner Attestation?
The Scanner Attestation should be signed and sent in to the ACR only if you have a scanner that does not perform the exams from the three required categories (i.e. head/neck, chest, and abdomen regions). Please reference the clinical image section in the Program Overview.
9. Do you need a Medical Physicist Survey for each scanner?
Yes, a Medical Physicist Survey must be done yearly for each scanner being accredited. However, the site does not have to send the report to the ACR.
10. What happens if I fail?
You will only have to repeat the areas that are deficient and not have to repeat the whole entire process again. The fee will be $800/scanner for clinical or phantom images and $1600/scanner if you have to repeat both. You will have 60 days to submit the repeated images.
11. Where can I find information regarding reducing doses for pediatric and small adult patients?
The FDA sent out a public health notification on "Reducing Radiation Risk from Computed Tomography for Pediatric and Small Adult Patients" on November 2, 2001. This notification can be found on the FDA Web site at http://www.fda.gov/cdrh/safety/110201-ct/ct.html.
12. Can the clinical and phantom images be submitted in a digital format?
The clinical images can be can be submitted on a CD. The instructions for submitting the clinical images on CD will be in the testing materials package the site gets when they apply for accreditation.
13. The Instructions for Scanning the ACR phantom specify that we must use the Adult Abdomen Scan Field of View for scanning the ACR phantom. On our scanner, this causes errors in CT number and non-uniformities. Can we use a Scan Field of View more closely matched to the size of the phantom?
For the scans of the ACR phantom, a facility can use a scan field of view or patient size appropriate to the phantom. Even though the clinical abdomen protocol uses a large scan field of view, it is acceptable to use a smaller field of view appropriate for the size of the phantom. Please note that this only applies to scanning of the ACR phantom and not to CTDI phantom measurements; for those measurements, the scan field of view that would be used for the protocol being measured (head, ped abdomen, adult abdomen) must be used.
Scanner Equipment, CTDI, and other Physics Topics
1. How do I know what detector configuration (N x T) I am using if the scanner console doesn't explicitly report that information?
In multi-detector-row CT (MDCT), the reconstructed slice thickness is not always the same as the Tomographic thickness of one data channel (T as defined in the ACR accreditation documents). For example, a given helical acquisition protocol on a GE MDCT scanner specifies the acquisition parameters in terms of "Thick, Speed and Mode or Pitch." Here "Thick" refers to the reconstructed image thickness and not the underlying detector collimation (T). This definition of T is consistent with International Standards, but can be confusing because users also associate "T or t" with the image thickness. In MDCT, the two parameters are not the same and both are important to know.
For ACR accreditation reporting and measurements, particularly the CTDI measurements and calculations, it is imperative that the physicists know the underlying detector configuration for any given helical protocol. Below we provide a conversion table for GE LightSpeed QX/i and LightSpeed Plus systems (4-detector-row scanners) that allows the physicist to determine detector configuration if the console acquisition parameters (Thick/Speed/Pitch) are known.
In future software versions (LS2002), the values of N x T and the IEC pitch will be explicitly reported on the operator's console, making the table below unnecessary. However, LS2002 will not be available for the QX/i systems (the original 4-detector-row systems); thus users of those systems will want to continue to consult the table below.
The ACR CT Accreditation Physics Subcommittee is working on providing similar information for other manufacturer's system.
Instructions for use of the conversion table:
First determine and record the site's clinical protocols for adult head, high resolution chest, adult abdomen and pediatric abdomen exams. Once these are described, then this conversion table can be used to determine the values of N and T that correspond to the given helical acquisition parameters. This information is valid only for the specific make and models of scanners listed.
NOTE: The shaded cells in the table below are instances where the reconstructed slice thickness (Thick) is not the same as the channel thickness T.
|
If Helical Protocol is: |
Then Measure in Axial Mode with: (for module 1 and CTDI) |
Calculate CTDIvol with: | ||||
|
Thick (mm) |
TableSpeed (mm/rot) |
Pitch |
N |
T(in mm) |
Table Speed(mm/rot) |
Pitch(IEC definition) |
| 1.25 | 3.75 | HQ | 4 (4i mode) | 1.25 mm | 3.75 | 0.75 |
| 1.25 | 7.5 | HS | 4 | 1.25 | 7.5 | 1.5 |
| 2.5 | 3.75 | HQ | 4 | 1.25 | 3.75 | 0.75 |
| 2.5 | 7.5 | HQ | 4 | 2.5 | 7.5 | 0.75 |
| 2.5 | 7.5 | HS | 4 | 1.25 | 7.5 | 1.5 |
| 2.5 | 15 | HS | 4 | 2.5 | 15 | 1.5 |
| 3.75 | 7.5 | HQ | 4 | 2.5 | 7.5 | 0.75 |
| 3.75 | 11.25 | HQ | 4 | 3.75 | 11.25 | 0.75 |
| 3.75 | 15 | HS | 4 | 2.5 | 15 | 1.5 |
| 5 | 7.5 | HQ | 4 | 2.5 | 7.5 | 0.75 |
| 5 | 11.25 | HQ | 4 | 3.75 | 11.25 | 0.75 |
| 5 | 15 | HQ | 4 | 5 | 15 | 0.75 |
| 5 | 15 | HS | 4 | 2.5 | 15 | 1.5 |
| 5 | 22.5 | HS | 4 | 3.75 | 22.5 | 1.5 |
| 5 | 30 | HS | 4 | 5 | 30 | 1.5 |
| 7.5 | 11.25 | HQ | 4 | 3.75 | 11.25 | 0.75 |
| 7.5 | 15 | HQ | 4 | 5 | 15 | 0.75 |
| 7.5 | 22.5 | HS | 4 | 3.75 | 22.5 | 1.5 |
| 7.5 | 30 | HS | 4 | 5 | 30 | 1.5 |
| 10 | 15 | HQ | 4 | 5 | 15 | 0.75 |
| 10 | 30 | HS | 4 | 5 | 30 | 1.5 |
2. Our multi-slice CT system uses a spiral Adult Abdomen protocol. The instructions for section 5 say to use the adult abdomen protocol and "perform an axial scan instead while keeping the remaining technical parameters unchanged." Our system CAN'T perform an axial scan with the same detector configuration that is used in our spiral adult abdomen acquisition protocol. What detector configuration should I use?
How do I turn my spiral acquisition protocol into an axial protocol with the correct slice thickness (CT number calibration and slice thickness images, Film page 1, Boxes 4 - 12; Module 1)?
In multi-detector-row CT (MDCT), the reconstructed slice thickness is not always the same as the Tomographic thickness of one data channel (T as defined in the ACR accreditation documents). For a given helical acquisition protocol, the user must determine the corresponding values of N and T, as defined in the accreditation documents (see FAQ number 1). The reconstructed image thickness is often not the same as the underlying detector collimation (T).
The following guidelines should be used to determine what axial scan parameters are to be used for the axial images required for Module 1.
- The kVp, mA, time per rotation, scan FOV, display FOV, reconstruction algorithm, reconstructed slice width, and dose reduction technique should all match those of the adult abdomen protocol described in Table 1, unless otherwise directed. The axial table increment should be set to zero so that the phantom stays in the correct position between exposures.
- For Film page 1: Boxes 5, 6, 7 and 8, use a reconstructed slice thickness of approximately 1, 3, 5 and 7 mm, respectively.
- For Film page 1:Boxes 9, 10, 11, and 12, use a kVp value other than that listed in Table 1.
- For Film page 1: Boxes 5, 6, 7 and 8, use a reconstructed slice thickness of approximately 1, 3, 5 and 7 mm, respectively.
- Film Page 1, Boxes 4 and 9 -12:
Use the same values of N and T as specified for the adult abdomen protocol in Table 1.
- If an axial acquisition cannot be made using that selection of N and T, keep T the same as described in Table 1 and use the next smallest allowed value of N.
Example: Siemens Sensation 16 system with N = 16 and T = 1.5 mm and reconstructed helical scan width = 5 mm. Axial images cannot be acquired using N = 16. Use the same value of T (1.5 mm) but the next lowest allowed value of N, which would be 12. Thus the 12 x 1.5 mm detector configuration would be used for the axial version of the spiral adult abdomen protocol with N = 16 and T = 1.5 mm. This is similarly true for the 16 x 0.75 mm detector configuration (use an axial 12 x 0.75 mm detector configuration).
- If the selection of N and T from Table 1, or the resultant values of N and T from 2a (above), will not allow the use of the reconstructed scan width specified in Table 1, select the next closest reconstructed scan width available. If two equally close choices are available, choose the larger scan width.
Example: GE LightSpeed 4-slice system with N = 4 and T = 3.75 mm and helical reconstructed scan width = 5 mm. In axial 4 x 3.75 mm mode (3.75 mm @ 4i), a 5 mm wide scan cannot be reconstructed. The next closest allowed scan thicknesses are 2.5 and 7.5 mm. The larger values should be chosen, which would be 7.5 mm.
Example: Siemens Sensation 16 system with N = 16 and T = 1.5 mm and reconstructed helical scan width = 5 mm. From 2a (above) a 12 x 1.5mm or 12 x 0.75mm axial scan would be used. A 5 mm wide scan cannot be reconstructed from this configuration. The next closest allowed scan thickness, which is 4.5 mm, should be used.
- If an axial acquisition cannot be made using that selection of N and T, keep T the same as described in Table 1 and use the next smallest allowed value of N.
- Film Page 1, Box 5:
Use the same values of N and T as specified for the High Resolution Chest protocol in Table 1.
- If an axial acquisition cannot be made using that selection of N and T, keep T the same as described in Table 1 and use the next smallest allowed value of N.
- If the selection of N and T from Table 1, or the resultant values of N and T from 3a (above), will not allow the use of the reconstructed scan width specified in Table 1, select the next closest reconstructed scan width available. If two equally close choices are available, choose the larger scan width.
- If an axial acquisition cannot be made using that selection of N and T, keep T the same as described in Table 1 and use the next smallest allowed value of N.
- Film Page 1, Box 6:
Use largest allowed axial value of T ˜ 3 mm for a reconstructed scan width ˜ 3 mm. Choose the largest value of N allowed for an axial acquisition having T ˜ 3 mm and a reconstructed scan width ˜ 3 mm.
Example: GE LightSpeed 8- or 16-slice system. Choose T = 2.5 mm, scan width = 2.5 mm, and N = 8.
Example: Siemens Sensation 16 system. Choose T = 1.5 mm, scan width = 3 mm, and N = 12.
- Film Page 1, Box 7:
Use largest allowed axial value of T ˜ 5 mm for a reconstructed scan width ˜ 5 mm. Choose the largest value of N allowed for an axial acquisition having T ˜ 5 mm and a reconstructed scan width ˜ 5 mm.
Example: GE LightSpeed 8-slice system. Choose T = 5 mm, scan width = 5 mm, and N = 4.
Example: Siemens Sensation 16 system. Choose T = 1.5 mm, scan width = 5 mm, and N = 8.
- Film Page 1, Box 8:
Use largest allowed axial value of T ˜ 8 mm for a reconstructed scan width ˜ 8 mm. Choose the largest value of N allowed for an axial acquisition having T ˜ 8 mm and a reconstructed scan width ˜ 8 mm.
Example: GE LightSpeed 8-slice system. Choose T = 3.75 mm, scan width = 7.5 mm, and N = 4.
Example: Siemens Sensation 16 system. Choose T = 1.5 mm, scan width = 6 mm, and N = 12.
NOTE: The Physics subcommittee of the ACR CT Accreditation Committee fully recognizes that the above schema is complex. The changes in CT scanner technology since the introduction of multi-slice CT systems in late 1998 have made it difficult to design an accreditation program that is valid for the wide range of systems currently in use throughout the United States (including entry-level single-slice spiral systems, state-of-the-art multi-slice systems, and still existent non-spiral systems). When the CT accreditation program was being developed and the image quality phantom being designed, four-slice spiral systems were only beginning to be installed. In light of the subsequent dramatic changes in CT technology and the changing nature of the installed base of CT systems, the need to redefine and clarify some aspects of the accreditation program is unavoidable. Please contact the ACR if you find a situation on your equipment that is not addressed by either the application materials or an FAQ, and we will make every effort to provide clarification and adapt the program consistent with the needs of the CT community.
3. The Toshiba Aquilion-16 can only perform an AXIAL scan in the 4-detector mode (N=4). How do I measure CDTI for a 16-detector clinical scan mode (N=16)? Is measuring CTDI with the 4-detector mode (N=4) acceptable?
(a) If a site is using a 16 x 1 mm helical protocol (N=16, T=1), then for CTDI measurements they could perform an AXIAL scan in the 4 x 4 mm detector configuration (N=4, T=4) and get the same radiation beam width as for the 16 x 1 mm configuration. The measured CTDI values for the N = 4, T = 4 mode will apply to both the 4 x 4 mm and 16 x 1 mm modes (the product of N and T equals 16 in both cases).
Similarly, if a site is using a 16 x 2 mm helical protocol (N=16, T=2), then for CTDI measurements they could perform an AXIAL scan in the 4 x 8 mm detector configuration (N=4, T=8) and get the same radiation beam width as for the 16 x 2 mm configuration. The measured CTDI values for the N = 4, T = 8 mode will apply to both the 4 x 8 mm and 16 x 2 mm modes (the product of N and T equals 32 in both cases).
(b) If the above solutions are not adequate for the particular detector configuration used clinically, the site can make the CTDI measurements using the service mode (there all radiation beam widths are selectable for an AXIAL scan).
4. My facility did not pass accreditation. May we appeal the decision? If so, what's involved?
A. Yes. Facilities that receive a deficiency or a failure may appeal the determination in writing within 15 days of the date of the final report. You must send the original images for all of the submitted cases in the category that did not pass along with a letter describing your reason for appealing. Only those images reviewed for the original determination (and having the original labels) will be considered during the appeal evaluation. These will be forwarded to an arbitrator (a reviewer who did not participate in the initial review) with a copy of the previous reviews and the appeal letter written by the facility. No other images will be sent to the reviewer for consideration in the evaluation. The arbitrator's determination will be final.
5. We recently appealed an adverse accreditation decision. When should we receive the results of the appeal?
A. You should receive the appeal results within 30 to 45 days of the date all required appeal materials were received by the ACR.
6. We did not pass accreditation because our technologists selected and submitted the wrong images. May we appeal the decision and submit new cases?
A. Although you may appeal the decision, you may not submit new cases. During accreditation review, the ACR reviewers assume that the submitted cases were reviewed by the modality's supervising physician (as specified in the Testing Instructions) and are examples of your best work. Consequently, during an appeal, you may only submit the original images with the original ACR labels.
7. We did not pass accreditation because our technologist did not submit all required images and provided insufficient information with the images that were submitted. May we appeal the decision and submit the rest of the required information?
A. You may appeal the decision; however, you may only submit the original images with the original ACR labels. Please call the Diagnostic Modality Accreditation Information Line at (800) 770-0145 for further guidance on your specific situation.
Toshiba FAQ
1. We have a Toshiba scanner and the Hounsfield numbers required on Module 1 (film page 1, box 4) are out of range when we use a large scan field of view. Can we use a smaller scan field of view?
For film page 1, box 4 CT # accuracy, a facility can use a scan field of view or patient size appropriate to the phantom. Even though the clinical abdomen protocol uses a large scan field of view, it is acceptable to use a smaller field of view appropriate for the size of the phantom for the CT # accuracy phantom image.