Computed Tomography






Optimising radiation dose
 


With a dramatic increase in the use of CT over the past decade, a key issue in the industry now is how to reduce the radiation dose for paediatric patients and patients who may receive multiple scans. Dr Sabah Al-Lawati reports.

Computed tomography (CT) systems allow acquisition of high quality x-ray images. These capabilities have greatly expanded the utility of CT, and CT usage has correspondingly increased1. Indeed, a review of US national data from 1996 through 2007 of University of Michigan Health System has reported in Annals of Emergency Medicine that the use of CT scans in emergency rooms has increased 330% in the last 12 years.

With high quality CT imaging being performed more frequently, patients can benefit from a quicker and more accurate diagnosis and precise diagnostic information for planning therapeutic procedures. However, in spite of the tremendous contributions of CT to modern healthcare, some attention must also be given to the small health risk associated with the ionising radiation received during a CT exam.

In 2009-2010, the overexposure of more than 400 patients in the US to ionising radiation during brain perfusion CT scanning resulted in hair loss and skin redness2. Concerns over the dose of radiation these patients received led the US FDA to update their recommendations for CT scanning. While this type of overexposure results in visible changes, other effects can take years to manifest and may include cancer and genetic mutations.

One study suggested that as much as 0.4% of all current cancers in the United States may be attributable to the radiation from CT studies based on CT usage data from 1991–1996.1 This study and a similar series of previous articles received considerable attention from the media, causing the CT community to review the amount of radiation prescribed for CT scans, especially for paediatric patients, resulting in an aggressive effort to minimise CT doses.

Sex, age, weight, and the area of the body scanned all significantly impact total radiation exposure and resulting risk of cancer development. Those at greatest risk for developing radiation exposure-related cancer later in life are children and young women. The increased risk to women is primarily from later development of lung and breast cancer. The estimated risk for a 20-year-old man is considerably lower. In both sexes, the lifetime attributable risk for one scan decreases with advancing age, although women continue to remain more susceptible than men to the effects of ionising radiation, according to some studies.

Patients who are obese may also be at greater risk for later development of cancer because of the increased dose of radiation needed to achieve adequate image quality.

However, the benefits of CT examination cannot be underestimated. CT imaging exams allow for noninvasive diagnosis of disease and monitoring of therapy, and can support medical and surgical treatment planning. For many diseases, early detection, more effective diagnosis and improved monitoring of therapy through the use of imaging exams may contribute to reduced morbidity, additional treatment options and increased life expectancy.

By coordinating efforts of government, medical organisations and private manufacturers of CT imaging equipment, patient exposure to radiation can be minimised, leading to a reduction in related risks, while not compromising image quality and medical diagnostics.

Radiation exposure

Even though CT provides “low level” radiation exposure, each exposure increases cancer risk in a cumulative fashion. There is also clear evidence that age at time of exposure and sex play a significant role in later development of cancers due to ionising radiation. Unfortunately no clear guidelines are currently available.

Concerns have been raised regarding the wide variations observed among radiation doses associated with imaging exams. Radiation doses vary greatly based on centre and technician. The median effective radiation dose of an abdominal and pelvic CT (the most common CT examination performed) is widely reported to be 8-10 mSv. Yet a study by Smith-Bindman and colleagues found the median dose to be 66% higher than this figure, and the dose for a multiphase CT scan was nearly four-fold higher.5 In addition, the researchers found a 13-fold variation in dose level (from high to low) in each CT study type they investigated. This investigation involved four medical centres in the San Francisco Bay area and the study demonstrates a need for everyone involved, including patients, technicians, and ordering physicians, to think through the need for the examination and to ask questions, including whether the settings and exposures are correct for the patient’s age, body type and procedure, before the examination is performed.

The Manitoba study, recorded the radiation dose patients received from a CT scan at 13 Manitoba hospitals and compared the data to similar surveys done elsewhere. The study found that the amount of radiation patients receive from a CT scan can vary widely and concluded that the dose should be reduced to better protect against the risk of cancer.

It is noteworthy that while new CT devices include displays of dose metrics, some lack other safeguards, such as settings that optimise radiation dose or alerts when the radiation dose in a given exam exceeds a particular reference level or range. In addition, providing meaningful, real-time dose metrics for paediatric procedures can be particularly challenging.

Paediatric patients

The risk of cancer in children due to radiation exposure is about two to three times higher than adults because paediatric patients have a longer life expectancy and their organs are more sensitive to radiation damage.

While there is no agreed upon cut-off age where the potential risk is zero, generally the younger the patient (i.e. under 30 years of age) the higher the risk due to potential accumulation of multiple CT examinations over a longer lifetime and increased radiation sensitivity of developing tissues.

Recently, there has been a call for a justifiable clinical indication to warrant a CT scan. Justification is a shared responsibility between requesting clinicians and radiologists. Hence, for medical exposures, the primary tasks of the imaging community are to work with ordering clinicians in order to direct patients to the most appropriate imaging modality for the required diagnostic task, and to ensure that all technical aspects of the examination are optimised, such that the required level of image quality can be obtained while keeping the doses as low as possible.

The American College of Radiology Appropriateness Criteria provides evidence-based guidelines to help physicians in recommending an appropriate imaging test.7 The European Commission guidelines and United Kingdom’s Royal College of Radiologists document titled “Referral guidelines for imaging” also provide a detailed overview of clinical indications for imaging examinations, including CT.8 Thus, a CT exam should be performed only when the radiation dose is deemed to be justified by the potential clinical benefit to the patient.

Siemens launches Low Dose Information Center on the Web

Siemens Healthcare is the first medical engineering company to offer a Low Dose Information Center on the Web. This English-language platform for continuing education and information around the topic of dose reduction is aimed at doctors and clinical personnel. The range of topics covered includes basic information on X-ray radiation, technical innovations relating to dose reduction and sample applications and dose protocols from dayto- day clinical activities. The content is broken down according to specialist areas and all important imaging methods which make use of X-rays or radioactive ionizing radiation.

Regular training and up-to-date information are important factors in the reduction of radiation exposure in radiology and nuclear medicine. Siemens' Low Dose Information Center provides training and information material on possible dose reduction for all important Siemens devices which produce clinical images using X-rays or radioactive radiation.

With its many technical innovations, Siemens Healthcare has already contributed to the reduction of radiation dose levels in diagnostic and interventional radiology. The company is now offering more than 40 low-dose applications for its imaging methods, the Combined Applications to Reduce Exposure (CARE). These include applications which enable cuts in dose levels coupled with consistent image quality. The CARE applications also prepare examination reports containing patient data, the respective protocol and dose values. Clinics can analyse this information according to certain criteria, and optimise their protocols and processes accordingly.

Siemens has also published a “Guide to Low Dose”. As well as basic information on the subject of radiation exposure, this handbook contains comprehensive notes on how dose levels for all relevant imaging methods can be cut, in order to protect patients and clinical staff.

At the start of 2010 Siemens set up the SIERRA (Siemens Radiation Reduction Alliance) initiative and established a panel of experts, which aims at driving forward dose reduction in computed tomography (CT). The members of the Low Dose Expert Panel are 15 internationally recognised specialists in radiology, cardiology and physics.

Low Dose Information Center www.siemens.com/low-dose

The Guide to Low Dose
www.siemens.com/dose-reductionleadership


General principles of ALARA

Keeping the radiation dose ‘As Low As Reasonably Achievable’ (ALARA) is the guiding principle for a medically indicated CT examination. The guiding principles for radiation protection in medicine are:

1. Justification: The exam must be medically indicated.

2. Optimisation: The exam must be performed using doses that are As Low As Reasonably Achievable, consistent with the diagnostic task.

3. Limitation: While dose levels to occupationally exposed individuals (i.e. the radiologist or technologist) are limited to levels recommended by consensus organisations, limits are not typical for medically-necessary exams or procedures.

As the growth in CT utilisation has increased, particularly in paediatric patients, and concern over the population dose from CT has been expressed in the scientific literature and media, the radiology community has worked to implement ALARA principles in CT imaging. The guiding principle for dose management in CT is that the right dose for a CT examination takes into account the specific patient and the specific diagnostic task. For large patients, this indeed means a dose increase is consistent with ALARA principles.

Experts recommend now that when a CT is indicated, only as much radiation as is necessary for evaluation should be used. For example, protocols should be based on patient size in the paediatric setting. Only the necessary region should be scanned, and repeat scans of the region during the CT examination (multiphase examinations) should be minimised in frequency. Additionally, particularly sensitive regions need to be considered, such as slightly lower doses to the chest area.

The American College of Radiology has developed a CT dose index database that facilitates direct reporting of CT radiation dose for each examination to this central database or registry, allowing facilities to compare dose levels nationally. This initiative will help standardise the dose reporting process, and may potentially alert a facility when a threshold dose has been exceeded.

Groups including the American College of Radiology (ACR), the American Association of Physicists in Medicine (AAPM), and NCRP have undertaken work to establish nationally recognised diagnostic reference levels for many imaging procedures.

In some cases, ordering physicians may lack or be unaware of recommended criteria to guide their decisions about whether or not a particular imaging procedure is medically necessary. As a result, they may order imaging procedures without sufficient justification and unnecessarily expose patients to radiation. Various professional organisations, including ACR and the American College of Cardiology (ACC), have developed and are working to disseminate imaging referral criteria, called “appropriateness criteria” or “appropriate use criteria,” associated with a number of medical conditions.

Optimising radiation dose has been discussed in chest CT, the third most common CT examination. In a recent study published in Journal of the American College of Radiology, investigators from Massachusetts General Hospital, Harvard Medical School, in Boston, MA, and Johns Hopkins University in Baltimore, MD, reviewed practical strategies for reducing radiation dose associated with chest CT examinations, based on patient age, size, clinical indications and follow-up imaging.

Researchers at Mayo Clinic have been actively working to reduce radiation dosages used to acquire CT images. At the 52nd Annual Meeting of the American Association of Physicists in Medicine, data were presented by Mayo Clinic on a Gradient Adaptive Bilateral Filter, which demonstrated a 20-fold dose reduction in an in vivo animal perfusion CT.

Furthermore, the Mayo Clinic has recently implemented a new routine head CT protocol that cuts radiation dose by nearly 50%. While the American College of Radiology allows its accredited facilities to use head CT doses up to approximately 75 mGy, Mayo Clinic’s newly introduced patient protocol uses a dose of only 38 mGy.

Adaptive Statistical Iterative Reconstruction

One technique which may reduce the radiation dose associated with abdominal CT scans by up to 60% is the adaptive statistical iterative reconstruction (ASIR) technique. ASIR allows improvement of the image quality while reducing the radiation dose. A software program, based on ASIR, has been utilised by radiologists at the Mayo Clinic. The team studied image quality using both standard and reduced radiation doses in a “phantom” or dummy colon in 18 patients. The software program enhanced the quality of CT images while allowing for significant reductions in the radiation dose needed for colon scans. The details were published by Dr Daniel Johnson of the Mayo Clinic in American Journal of Roentgenology in 2010.

Recently, the National Electrical Manufacturers Association (NEMA) in the US has published a new technical standard (XR 25) for CT scanners. The scanners, which comply with the new standard allow users to adjust the scanners, avoiding unnecessarily high exposures.

GE launches Veo – a model-based system to reduce CT dose and maintain clarity

GE Healthcare announced in September 510(k) clearance in the US of its revolutionary Computed Tomography (CT) technology, called Veo, which may help physicians deliver accurate diagnoses by enabling excellent CT image clarity at dramatically lower dose. Veo represents the CT industry’s first Model-Based Iterative Reconstruction (MBIR) technique, and is available on GE Discovery CT750 HD systems in Europe, Canada and regions of Asia.

Dr William Shuman, Professor and Vice Chairman, Department of Radiology at the University of Washington, commented: “The radiological community continues to demand maximum CT image clarity at optimised dose levels to help best serve our patients. A superb image quality, submillisievert (mSv) CT exam has in many ways been seen as CT’s holy grail – making powerful new diagnostic tools like Veo all the more vital as we seek to minimise patient dose while maximising diagnostic confidence. The Veo reconstruction option may also improve the image quality of low dose non-diagnostic CT images (Filtered Back Projection), such that they become diagnostic.”

Veo has been shown to offer profound image clarity at yet unseen low dose levels. Current Veo users in Europe report successful chest CTs done with an equivalent amount of medical radiation dose as a chest x-ray, or less than one-tenth of one mSv.

Professor Johan de Mey, Chair of the Radiology Department at University Hospital in Brussels, Belgium, explained: “With a clinical chest CT at 0.05 mSv, we produced images where we could see and analyse pathology. With Veo, we can conduct lower dose CT scans in children, too, and this is particularly important in groups that require continued follow-up, such as those with cystic fibrosis or lymphoma.”

While complementing the robust imaging capabilities of GE’s advanced Adaptive Statistical Iterative Reconstruction (ASiR) technique, Veo represents a significant technological leap forward. Veo is a first-of-its kind doseoptimising technology, the most profound since ASiR – which, itself, was the world’s first iterative reconstruction technology and is now employed on more than 1,000 GE CT systems globally.

GE Healthcare is coordinating a multi-center study to investigate further improvements – including substantially lower dose levels – which Veo may offer across a range of applications.

Private initiatives

Private companies, involved in the manufacture of equipment that is used for CT, have also demonstrated a vested interest in public health safety with regard to radiation dose. Siemens has become the first manufacturer of medical engineering products to publish a “Guide to Low Dose”". This guide consists of basic information on the subject of radiation exposure, as well as comprehensive notes on how dose levels for all relevant imaging methods can be reduced.

Toshiba has also been actively involved in introducing automated, easy CT dosesaving tools for the past few years, based on the ALARA principle. The Adaptive Iterative Dose Reduction (AIDR) system they have developed can reduce radiation dose while maintaining a good image quality, according to the company. AIDR can remove up to 50% of image noise resulting in dose reduction of up to 75%.

Health risks

It is extremely important that all healthcare providers be aware of the risks associated with CT scans, especially in the paediatric, obese, and female populations. In order to reduce cancer risk, patients should be educated about the potential risks associated with all radiation technologies, but particularly with CT imaging.

Siemens have developed an information resource for patients waiting for X-ray or nuclear medicine examinations – www.medicalradiation.com. The website serves as a forum for scientific journalists and experts from Siemens Healthcare who provide basic details about medical radiation to patients and interested members of the public. The website is a resource for the fundamentals of the physics involved and provides an overview of the imaging procedures as well as notes on minimising exposure to radiation.

Conclusion

CT examinations present both benefits and risks. Although the advent of CT has led to improvements in the diagnosis and treatment of numerous medical conditions, it may also expose patients to ionising radiation, which may elevate a person’s lifetime risk of developing cancer. In general, the dose of CT is cumulative i.e. accumulated over a lifetime.

A balanced public health (government, medical centres/hospitals and private sector) approach is required to maintain the benefits of CT imaging exams while minimising any risks.

Managing the risks of CT procedures depends on two principles of radiation protection:

- Appropriate justification for ordering and performing each procedure – these types of imaging exams should be conducted only when medically justified.

- Careful optimisation of the radiation dose used during each procedure – optimal radiation doses must be used i.e. no more than necessary to produce a high-quality image.

Radiologists and technicians should reduce the dose of ionising radiation or tailor the examinations to obtain optimal diagnostic images while minimising exposure. The number of CT studies to which a patient is subjected should be minimised by talking with the radiologist about alternatives to the CT exam or by using clinical judgment and physical examination skills to determine appropriate treatment without use of CT. Finally, keeping track of the number of exposures to ionising radiation that patients receive may help to reduce their overall lifetime exposure and therefore minimise their lifetime risk; however, no clear mechanism for accomplishing this is currently available.

References

1. Brenner DJ, Hall EJ. N Engl J Med. 2007 Nov 29;357(22):2277-84.

2. Safety investigation of CT brain perfusion scans: update 11/9/2010. http://www.fda.gov/MedicalDevices/Safety/AlertsandNotices/ucm185898.htm
UpdatedNovember 9, 2010. Accessed December 1, 2011.

3. http://ec.europa.eu/energy/nuclear/radiation_protection/doc/publication/125.pdf

4. Fazel R et al. Exposure to low-dose ionizing radiation from medical imaging procedures. N Engl J Med. 2009 Aug 27;361(9):849-57

5. Smith-Bindman R, Lipson J, Marcus R, et al. Radiation dose associated with common computed tomography examinations and the associated lifetime attributable risk of cancer. Arch Intern Med. 2009;169(22):2078-2086

6. Huang B, et al. Whole-body PET/CT scanning: estimation of radiation dose and cancer risk. Radiology. 2009 Apr;251(1):166-74.

7. American College of Radiology. The American College of Radiology Appropriateness Criteria. 2008.

8. The Royal College of Radiologists. Making the best use of clinical radiology services: referral guidelines. 6. London: 2007

9. Diagnostic reference levels recommended by ACR are available online at http://www.acr.org/SecondaryMainMenuCategories/quality_safety/guidelines/med_phys.aspx Reference values recommended by AAPM are published in Gray JE, et al., “Reference Values for Diagnostic Radiology: Application and Impact,” Radiology, May 2005, Vol. 235, No. 2, pp. 354-358. Information about NCRP’s efforts to develop diagnostic reference levels is available online at http://www.ncrponline.org/Current_Prog/SC_4-3

10.ACR’s Appropriateness Criteria are available online at http://www.acr.org/SecondaryMainMenuCategories/quality_safety/app_criteria.aspx

ACC’s Appropriate Use Criteria (AUC) are available online at http://www.cardiosource.org/Science-And-Quality/Quality-Programs/Imaging-in- FOCUS/ACC-Appropriate-Use-Criteria.aspx

 Date of upload: 21st Jan 2012

 

                                  
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