CT Scanner Fears Stem from Errors not Dose Measures

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Growing scrutiny about the safety of CT scans has arisen in part out of widely publicized cases of excessive CT scanning, and of radiation over-exposure in CT scans, particularly CT perfusion scans for stroke, which have raised questions about CT dose index (CTDI) and CTDI volume measurements.

One headline case was that of the 22-year-old Philadelphia woman with massive internal injuries who underwent 49 CT scans over a 14-month period. Other reports of frequent and possibly unnecessary imaging, along with sporadic reports of medical imaging errors have raised doubts and fears among the public about the safety of medical imaging in general and CT scans in particular.

Dr. Aaron Sodickson, director of Emergency Radiology at Brigham and Women’s Hospital, Boston told The Hub that the concerns about safety in the public’s mind are far out of proportion to the actual risks for properly configured CT scanners.

“While on my way to an FDA meeting about this issue recently, my neighbor on the plane told me about her experience,” Sodickson said by phone. “She was found to have a mass in her neck, for which a CT scan had been recommended, but she refused out of fear that the scan would give her cancer. Based on her age and the typical radiation exposure of this type of scan, I estimated that her chance of getting cancer from the CT scan was less than 1 in 10,000, a miniscule risk compared to the potential risk that her neck mass posed. She decided that she would go ahead and have the scan. But it is seldom that we have the opportunity to directly allay patient’s fears by explaining the risks and benefits in this way.”

Naturally, governmental bodies are responding to media and public concerns with regulations and proposals to regulate. California passed regulations last year that now require hospitals and clinics to record radiation dose of CT scans and to report any overdose to the patient, the treating physician, and the California Department of Public Health.

Dr. Dianna Cody, chief of the Radiologic Physics Section in the Department of Imaging Physics at the University of Texas MD Anderson Cancer Center told The Hub that more regulation is on the way.

“The state of Texas is recommending protocol reviews at every institution,” Cody said in a phone interview, “and it is likely that other governmental regulations are coming, so it is imperative that radiologists make sure they are being careful and being proactive in educating the public on what is and is not likely to be helpful.”

The trouble is, calculating the patient dose from CT scans is not so simple and is poorly understood by many in the media and may not be fully understood by regulators. In a review article published in the May issue of Radiology, lead author Dr. Cynthia McCollough of the Mayo Clinic, Rochester, carefully explains that the most commonly used “dose” metric, the CTDI (CT Dose Index) is not the same thing as patient dose.

The article points out that CTDI was developed because the irradiation geometry of CT scanners was quite different from that of other x-ray modalities. In CT scanners, the x-ray tube irradiates only a narrow section of the body during a full rotation around the patient, but does so for multiple rotations along the length of the patient.

“The CTDI method sought to create an “index” to reflect the average dose to a cylindrical phantom in the central region of a series of scans,” McCollough’s group wrote. “The word “index” was specifically included in CTDI’s name to distinguish the quantity from the radiation dose absorbed by a patient.”

The CTDI technique uses a long ionization chamber to measure the radiation delivered inside one of two standard diameter acrylic cylindrical “phantoms.”  With this consistent approach, radiation output of CT scanners could be quantifed and reproduced by scanner manufacturers and tested by the FDA. Consequently, CTDI-based metrics became the reference standard for measuring, comparing, and communicating the radiation output of a CT system.

Cody says the way scanners measure dose is based on CTDI calculations done at the factory where vendors measure radiation very carefully on a representative sample of machines. That calculation is then programmed into the machines.

“We’ve found that most of the time those calculations are easily within 20 percent of the actual measured CTDI and often within 5 percent,” Cody says. “The newer machines are very stable, very reliable. That is probably one of the positive aspects of the advances in the technology. These scanners are a lot more stable than we had thought.”

These CTDI values are included in either a screen-capture “patient dose report” or a structured Digital Imaging and Communications in Medicine (DICOM) dose report, which reinforces the incorrect belief that CTDI is a measure of patient dose.

Sodickson says the CT Dose index is a good measure of the x-ray output by the machine, it is accurate and reliable, and radiologists can trust the values. What is important is to know what the CTDI is intended for and what it is not intended for.

CTDI is sometimes used as a surrogate for radiation doses to the internal organs of the patient.  The main limitation of this approach is that it does not take into account the size of the patient,” Sodickson says.  “If a large and a small patient were to have a scan with the same CTDI, the organ doses to the small patient would be much higher than those to the large patient. Without making corrections for the size of the patient, CTDI is thus not an accurate predictor of patient dose, nor was it intended to be.”

Instead, according to McCollough, CTDI was designed to be a standardized measure of the radiation output of a CT system, measured in a cylindrical acrylic phantom, which enables users to gauge the amount of emitted radiation and compare the radiation output between different scan protocols or scanners.

Digital radiography news There are three things going on in practice, Cody points out. There is the stability of the machine, how it is used, and how variable is the patient population served by the hospital using the machine.

“Some may use their CT scanners on tiny babies and on all body sizes up to very large football players,” she says. “So there are huge extremes that need to be adjusted for within the scan parameters. As a result the CTDI numbers used by a hospital may be all over the place due to their patient population. That needs to be understood by people concerned with the radiation exposure resulting from CT scanners.”

Sodickson says most scanners have good tools for making the necessary adjustments to the x-ray tube output based on patient size in order to maintain adequate image quality. These tools appropriately decrease CTDI for small patients and increase it for large patients.

“Each CT manufacturer has different methods of adjusting the CT parameters to account for patient size,” he says. “and it is vital that those using the technology clearly understand how to properly use their tools. The much publicized problems with over exposure have largely resulted from these tools not being properly configured.”

Sodickson believes that reducing such errors comes down largely to education. He says there is a need to ensure that CT technologists, radiologists, and medical physicists all receive more focused education in the appropriate use of the tools provided with the scanners.

Cody agrees, but notes that even proper use of the tools, doesn’t resolve all the problems. She says you also have to take into account the imaging task. She says that because CTDI is based on the assumption that the patient is moving through the gantry in a steady rate and the exposure of any single “slice” of tissue receives a contributory dose from neighboring slices.

“That does not happen in CT perfusion, where the patient remains mostly in one place during the scan. Consequently, because of the factoring in of adjacent tissue dosage, CTDI results for perfusion CT overestimate the patient dose,” she says. “We found that multiplying the CTDI by 50 percent or 60 percent yielded a much more accurate estimate of the actual dose the patient received,” she said.

Sodickson and Cody agree that the risk to patients from CT scans are very low with a properly configured scanner operated by expert users who understand the tools. That is not to say that there are no risks. Sodickson says his chief concern is from the cumulative exposure of those he calls “frequent flyer” patients.

“These are patients who have a lot of recurrent CT scans, such as those with kidney stones, Crohns disease, or recurrent chest pain,” Sodickson says. “We really need to do a better job of weighing the cumulative risks and benefits in these patients. For most patients, however, with infrequent imaging exposures, there is a very low risk of harm.”

Cody says despite the very low risks of CT scans, patients still need to ask questions. “The one thing I would add is that for parents of kids who need to go to the ER. If a CT scan is recommended, they should ask, ‘Are you using child-specific parameters?’ If the technologist or radiologist gives you a deer-in-the-headlights look, then you should stamp your feet and make sure they use a child-specific scan.”

By Michael O’Leary, contributing writer, Health Imaging Hub

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CT Scanner Fears Grow Out of Errors Not Dose Measurements

 

Growing scrutiny about the safety of CT scans has arisen in part out of widely publicized cases of excessive CT scanning, and of radiation over-exposure in CT scans, particularly CT perfusion scans for stroke,  have raised questions about CT dose index (CTDI) and CTDI volume measurements.

 

One headline case was that of the 22-year-old Philadelphia woman with massive internal injuries who underwent 49 CT scans over a 14-month period. Other reports of frequent and possibly unnecessary imaging, along with sporadic reports of medical imaging errors have raised doubts and fears among the public about the safety of medical imaging in general and CT scans in particular.

 

Dr. Aaron Sodickson, director of Emergency Radiology at Brigham and Women’s Hospital, Boston told The Hub that the concerns about safety in the public’s mind are far out of proportion to the actual risks for properly configured CT scanners.

 

“While on my way to an FDA meeting about this issue recently, my neighbor on the plane told me about her experience,” Sodickson said by phone. “She was found to have a mass in her neck, for which a CT scan had been recommended, but she refused out of fear that the scan would give her cancer. Based on her age and the typical radiation exposure of this type of scan, I estimated that her chance of getting cancer from the CT scan was less than 1 in 10,000, a miniscule risk compared to the potential risk that her neck mass posed. She decided that she would go ahead and have the scan. But it is seldom that we have the opportunity to directly allay patient’s fears by explaining the risks and benefits in this way.”

 

Naturally, governmental bodies are responding to media and public concerns with regulations and proposals to regulate. California passed regulations last year that now require hospitals and clinics to record radiation dose of CT scans and to report any overdose to the patient, the treating physician, and the California Department of Public Health.

 

Dr. Dianna Cody, chief of the Radiologic Physics Section in the Department of Imaging Physics at the University of Texas MD Anderson Cancer Center told The Hub that more regulation is on the way.

 

“The state of Texas is recommending protocol reviews at every institution,” Cody said in a phone interview, “and it is likely that other governmental regulations are coming, so it is imperative that radiologists make sure they are being careful and being proactive in educating the public on what is and is not likely to be helpful.”

 

The trouble is, calculating the patient dose from CT scans is not so simple and is poorly understood by many in the media and may not be fully understood by regulators.

 

In a review article published in the May issue of Radiology, lead author Dr. Cynthia McCollough of the Mayo Clinic, Rochester, carefully explains that the most commonly used “dose” metric, the CTDI (CT Dose Index) is not the same thing as patient dose.

 

The article points out that CTDI was developed because the irradiation geometry of CT scanners was quite different from that of other x-ray modalities. In CT scanners, the x-ray tube irradiates only a narrow section of the body during a full rotation around the patient, but does so for multiple rotations along the length of the patient.

 

“The CTDI method sought to create an “index” to reflect the average dose to a cylindrical phantom in the central region of a series of scans,” McCollough’s group wrote. “The word “index” was specifically included in CTDI’s name to distinguish the quantity from the radiation dose absorbed by a patient.”

 

The CTDI technique uses a long ionization chamber to measure the radiation delivered inside one of two standard diameter acrylic cylindrical “phantoms.”  With this consistent approach, radiation output of CT scanners could be quantifed and reproduced by scanner manufacturers and tested by the FDA. Consequently, CTDI-based metrics became the reference standard for measuring, comparing, and communicating the radiation output of a CT system.

 

Cody says the way scanners measure dose is based on CTDI calculations done at the factory where vendors measure radiation very carefully on a representative sample of machines. That calculation is then programmed into the machines.

 

“We’ve found that most of the time those calculations are easily within 20 percent of the actual measured CTDI and often within 5 percent,” Cody says. “The newer machines are very stable, very reliable. That is probably one of the positive aspects of the advances in the technology. These scanners are a lot more stable than we had thought.”

 

These CTDI values are included in either a screen-capture “patient dose report” or a structured Digital Imaging and Communications in Medicine (DICOM) dose report, which reinforces the incorrect belief that CTDI is a measure of patient dose.

 

 

Sodickson says the CT Dose index is a good measure of the x-ray output by the machine, it is accurate and reliable, and radiologists can trust the values.  What is important is to know what the CTDI is intended for and what it is not intended for.

 

“CTDI is sometimes used as a surrogate for radiation doses to the internal organs of the patient.  The main limitation of this approach is that it does not take into account the size of the patient,” Sodickson says.  “If a large and a small patient were to have a scan with the same CTDI, the organ doses to the small patient would be much higher than those to the large patient. Without making corrections for the size of the patient, CTDI is thus not an accurate predictor of patient dose, nor was it intended to be.”

 

Instead, according to McCollough, CTDI was designed to be a standardized measure of the radiation output of a CT system, measured in a cylindrical acrylic phantom, which enables users to gauge the amount of emitted radiation and compare the radiation output between different scan protocols or scanners.

 

There are three things going on in practice, Cody points out. There is the stability of the machine, how it is used, and how variable is the patient population served by the hospital using the machine.

 

“Some may use their CT scanners on tiny babies and on all body sizes up to very large football players,” she says. “So there are huge extremes that need to be adjusted for within the scan parameters. As a result the CTDI numbers used by a hospital may be all over the place due to their patient population. That needs to be understood by people concerned with the radiation exposure resulting from CT scanners.”

 

Sodickson says most scanners have good tools for making the necessary adjustments to the x-ray tube output based on patient size in order to maintain adequate image quality. These tools appropriately decrease CTDI for small patients and increase it for large patients.

 

“Each CT manufacturer has different methods of adjusting the CT parameters to account for patient size,” he says. “and it is vital that those using the technology clearly understand how to properly use their tools. The much publicized problems with over exposure have largely resulted from these tools not being properly configured.”

 

Sodickson believes that reducing such errors comes down largely to education. He says there is a need to ensure that CT technologists, radiologists, and medical physicists all receive more focused education in the appropriate use of the tools provided with the scanners.

 

Cody agrees, but notes that even proper use of the tools, doesn’t resolve all the problems. She says you also have to take into account the imaging task. She says that because CTDI is based on the assumption that the patient is moving through the gantry in a steady rate and the exposure of any single “slice” of tissue receives a contributory dose from neighboring slices.

 

“That does not happen in CT perfusion, where the patient remains mostly in one place during the scan. Consequently, because of the factoring in of adjacent tissue dosage, CTDI results for perfusion CT overestimate the patient dose,” she says. “We found that multiplying the CTDI by 50 percent or 60 percent yielded a much more accurate estimate of the actual dose the patient received,” she said.

 

Sodickson and Cody agree that the risk to patients from CT scans are very low with a properly configured scanner operated by expert users who understand the tools.

 

That is not to say that there are no risks. Sodickson says his chief concern is from the cumulative exposure of those he calls “frequent flyer” patients.

 

“These are patients who have a lot of recurrent CT scans, such as those with kidney stones, Crohns disease, or recurrent chest pain,” Sodickson says. “We really need to do a better job of weighing the cumulative risks and benefits in these patients. For most patients, however, with infrequent imaging exposures, there is a very low risk of harm.”

 

Cody says despite the very low risks of CT scans, patients still need to ask questions.

 

“The one thing I would add is that for parents of kids who need to go to the ER. If a CT scan is recommended, they should ask, ‘Are you using child-specific parameters?’ If the technologist or radiologist gives you a deer-in-the-headlights look, then you should stamp your feet and make sure they use a child-specific scan.”


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