From the Guidelines
A lumbosacral CT scan typically delivers a radiation dose of approximately 6-10 millisieverts (mSv), though this can vary depending on the specific protocol, equipment, and patient factors 1. This dose is higher than a standard X-ray (which delivers about 1.5 mSv for the lumbar spine) but lower than some other CT examinations. The radiation exposure occurs because CT scanners use X-rays to create detailed cross-sectional images of the spine, allowing visualization of bone, soft tissue, and neural structures.
Key Factors Influencing Radiation Dose
- Scan length: Should be kept to a minimum to encompass only the anatomy of interest and not expose structures that are not relevant to the examination’s purpose 1.
- X-ray beam intensity: Determined by both the x-ray tube potential (in units of kV) and the x-ray tube current (in units of mA) 1.
- Rotation time: The time required for the gantry to perform 1 rotation is a selectable parameter, and exposure increases linearly with rotation time 1.
- X-ray beam filtration: Filters placed beneath the x-ray tube are used to selectively attenuate low-energy x-rays that do not significantly contribute to the image but do contribute to radiation dose 1.
Modern CT Scanners and Dose Reduction
Modern CT scanners often incorporate dose reduction techniques such as iterative reconstruction algorithms and automatic exposure control to minimize radiation while maintaining diagnostic image quality 2. For context, the average person receives about 3 mSv annually from natural background radiation. While the radiation dose from a lumbosacral CT is not insignificant, the diagnostic benefits typically outweigh the theoretical risks when the scan is medically necessary for evaluating conditions like herniated discs, spinal stenosis, fractures, or tumors.
Alternative Imaging Options
Alternative imaging options with no radiation, such as MRI, may be considered when appropriate, particularly for patients requiring multiple scans or those with higher radiation sensitivity like pregnant women or young patients 3. However, the choice of imaging modality should be based on the specific clinical question and the patient's individual needs, weighing the benefits of diagnostic accuracy against the potential risks of radiation exposure.
From the Research
Radiation Dose of Lumbosacral CT Scan
- The radiation dose of a Lumbosacral (lower back and sacrum) Computed Tomography (CT) scan can be reduced using various techniques, such as tailoring the scan to the patient, minimizing scan length, and using tube current modulation 4.
- A study on low-dose CT of the lumbar spine found that the effective dose was around 1.0-1.1 mSv, which is comparable to or even lower than the dose from radiography 5.
- Another study found that the use of body mass index (BMI) and automatic exposure control (AEC) can reduce the radiation dose of CT volume scans of the lumbar spine without compromising image quality 6.
- The radiation dose for specific CT scan protocols can vary, with reported doses ranging from 1.8 mGy for thoracic bone visualization to 10.6 mGy for lumbar soft-tissue visualization 7.
Factors Affecting Radiation Dose
- Patient size and body mass index (BMI) can affect the radiation dose, with larger patients requiring higher doses to achieve adequate image quality 6.
- The use of automatic exposure control (AEC) can help reduce the radiation dose by adjusting the tube current and voltage based on the patient's size and anatomy 6.
- The type of CT scanner and scan protocol used can also impact the radiation dose, with newer scanners and protocols often providing lower doses while maintaining image quality 8.
Image Quality and Diagnostic Information
- Low-dose CT scans of the lumbar spine can provide superior image quality and diagnostic information compared to radiography, with better visualization of anatomical structures and pathological changes 5.
- The use of low-dose CT scans can also reduce the time required to review images and make diagnoses, with reported review times of around 204 seconds for low-dose CT compared to 152 seconds for radiography 5.