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CTDIvol vs DLP – a simple explanation

What do they represent? CTDIvol is based on measurements obtained when scanning either a 16cm or 32 cm phantom.   Essentially, it represents scanner output.  DLP is derived from CTDIvol, but incorporates a scan length component.  Both function as reasonable proxies for absorbed dose but do not represent the actual patient dose.  In other words, if your CTDIvol and/or DLP is twice as high as it could be, then the doses you are imparting will be about twice as high as they could be.

Can CTDIvol and DLP results tell me two different things?     Yes.  CTDIvol represents the output when scanning a phantom, while DLP takes into account the scan length.  We’ve seen instances where CTIDvol is considered well within a “normal” range but DLP was unexpectedly high.  We found the scan settings were appropriate for the study, but the exam length  was longer than what others were using.

For example, a Chest CT could be started too high into the neck and end too far into the abdomen.  If this is the case CTDIvol (basic scanner settings) could be just fine, but because scans extended more than necessary above and/or below the requested area, the DLP could easily be too high.


The following information, obtained from, is a more technical discussion of CTDIvol and DLP for those interested.

The following is taken from an article posted on

CT Dose Index Volume (CTDIvol) The CTDIvol can be calculated as: CTDIvol = [(N x T)/I] x CTDIw where CTDIw = weighted or average CTDI given across the field of view N = number of simultaneous axial scans er x-ray source rotation T = thickness of one axial scan (mm) I = table increment per axial scan (mm)

In helical CT the ratio of the I to (N x T) is the pitch; therefore in helical mode:  CTDIvol = (1/pitch) x CTDIw

CTDIvol (or CTDI volume) represents the dose for a specific scan protocol which takes into account gaps and overlaps between the radiation dose profile from consecutive rotations of the x-ray source. Therefore CTDIw represents the average radiation dose over the x and y direction whereas CTDIvol represents the average radiation dose over the x, y and z directions.

Dose Length Product The dose length product (DLP) is the measure of ionizing radiation exposure during the entire acquisition of images.

Therefore, DLP (mGy-cm) = CTDIvol (mGy) x irradiated length (cm) (irradiated length is usually longer than imaged length in helical scanning)

CTDIw and CTDIvol are independent of scan length for determining the total energy absorbed whereas DLP is proportional to scan length.

Need help getting more from your participation in the ACR’s Dose Index Registry® ?  Let Dose Registry Support Services tailor a program designed specifically to help your department succeed.  Contact Dose Registry Support Services to see how we can help.

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End of Quarter: Time to Perform Your DIR Data Quality Checks

With the end of the quarter approaching the ACR’s NRDR sent a reminder to all participants to perform several quality checks so your report will be as accurate as possible.  These checks include making sure:

  1. The total volume of exams recorded in the DIR for your facility is reasonably close to the volume of studies you’ve performed;
  2. That any currently unmapped studies are mapped to an appropriate RPID; and,
  3. That your review your currently mapped studies to ensure they are assigned to the correct RPID.

I copied the NRDR’s email below in case you missed, or didn’t receive, it.   Please review and check your data.

Confirming Exam Name Mapping Accuracy

Checking your currently mapped study’s to ensure they are mapped correctly can be challenging and difficult.  For example,  the original facility we support began using the DIR in 2013 and currently has over 330 study names mapped to about 70 different RPIDs.  Suffice it to say some of the original studies were assigned incorrectly along the way.  The challenge they had was how to identify the incorrectly mapped studies? Looking at the list one at a time, and identifying studies that were incorrectly mapped, is tedious and time-consuming.

That is where Dose Registry Support Services was able to help – we developed a method of exporting and organizing all mapped studies in such a way that made it easy to grouped studies by RPID, which made it easier to identify mis-mapped studies.

Check back here soon.  Sometime in the next few days I plan to post step-by-step instructions describing the method we developed to easily identify incorrectly mapped studies.  We’ve used that approach for years and find it very useful.  I will likely post the announcement in a new blog post, but will put the instructions in an article on one of’s web pages.

As always, we are here to help.

Michael Bohl
Dose Registry Support Services

Need help getting more from your participation in the ACR’s Dose Index Registry® ?  Let Dose Registry Support Services tailor a program designed specifically to help your department succeed.  Contact Dose Registry Support Services to see how we can help.



Dear DIR Participant,

 We are preparing for the next aggregate report within the following week. Before we do so we would like for you to perform a Data Quality review of your own data. Please review the information below, follow the directions, and if you find errors in your data inform us right away. Failing to conduct a Data Quality review may result in a less accurate quarterly aggregate feedback report.


We highly encourage you to review your submitted data no less than every two months to monitor your data. We refer to this as a ‘data quality check.’ You may receive email notices from NRDR to remind you to perform a ‘data quality check’ but if you do not receive a notice, you are still held responsible to perform this critical step in preparation for your aggregate Feedback Report. To determine if we are receiving your data please log into the NRDR portal using your log-in credentials and password. After you have logged in, go to the DIR on the menu (left-hand side of the screen) and click to open the DIR Menu.

Comparing Volume of Exams Received vs. Volume of Exams Sent

  1. Under the DIR Report subheading, there are many reporting tools. To determine the last time your facility submitted exam data to the NRDR, use the ‘Summary of Data Submitted’ and ignore the date range fields, and click ‘Submit.’ The result of the search will display if we have received exam data from your facility recently.
  1. Clear the query to conduct a new one. Using ‘Summary of Data Submitted’ include one of the date ranges provided below to coordinate with the most recent reporting period. Make a note of the number of exams received during this reporting period.
    January 1 – March 31 (quarter Q1)
    January 1 – June 31 (semi-annual Q1Q2)
    July 1 – September 30 (quarterly Q3)
    October 1 – December 31 (semi-annual Q3Q4)

Compare the NRDR number of exams received to your volume of exams sent (your PACS may be able to help identify the number of exams sent).

  1. To know the volume of exams received per month, use the ‘Summary of Data Submitted’ and change the date range to capture one month at a time. Compare the NRDR monthly total of exams received against your monthly total number of exams submitted.

If the difference between the NRDR numbers and your total number of exams submitted is greater than 5% error, then please contact us at

Determine if Each CT Scanner is Sending Exam Data

  1. Under the DIR Report subheading, there are many report tools that can provide you with this data. An easy report tool to work with is ‘Dose Information by Exam.’ Click it to open the report page.
  2. Enter a date range of at least 3 months so that you can review what your CT scanners have been sending since the last aggregate report (which is issued quarterly). When the page populates go to the top of the screen and click ‘Export to Excel.’ Sort on the columns that contain information about your scanner, such as, Institution name, Scanner model and Scanner ID. In this manner, you can review the date that each of your scanners sent exam data to the NRDR. If any data is missing or if an entire CT scanner’s exam data is not appearing in the report, contact or call 703-390-9858 to troubleshoot data transmission.
  3. Also check the ‘Study Description’ column to affirm the names of your exams are being captured. If missing Study Descriptions for your exam names are greater than 5% error rate (per scanner) please use the email and phone number above to contact us.

Using the Standardize Dose Index Report Tool

The purpose of the Standardized DIR Report tool is to provide a user friendly reporting tool which can be searched by values not available in the other DIR reporting tools, a few of which have been mentioned above.

Map Exam Study Descriptions to a Radlex ID (RPID)

Exam names that have not been mapped to a RPID will not be included in the aggregate report. Please follow the instructions in the Exam Name Mapping User Guide to navigate through this task. If you have a Master-Child facility registration, you must perform your exam name mapping at the Master facility. Any mapping in the Child facility will be over written by the RPIDs selected in the Master facility every 24-hours. There are only a few Master-Child facility registrations that have ‘lifted’ restrictions, and are able to map at the Child facility. For the majority of Master-Child registrations, this is not the case. As a precaution, we suggest that you map at the Master facility level to avoid loosing your RPID mappings at a Child facility that may not have ‘lifted’ restrictions in place.

Please complete your quality data check and RPID exam name mapping within two weeks from the date of this email.

 Many best regards,

The NRDR Team