Measuring Dose in CT

Getting back to dose - energy deposited in a given mass of tissue - we need to figure out how to measure it in CT. Remember that a CT is basically shooting a lot of x-rays at different angles along a length of the body. We cannot simply add up "each x-ray." In order to measure dose in CT, the CT Dose Index (CTDI) was invented. CTDI, often written as CTDI₁₀₀, is the amount of radiation delivered to one slice of the body over a long CT scan.

Why do we have to use an entire CT scan to measure the dose to one slice? Well, many x-rays scatter into the patient's body - so a good amount of dose to one slice is deposited in adjacent slices. Also, remember that the dose is not uniform from the surface to the inside of the body - the superficial tissues absorb more of the dose, and less radiation reaches the center of the body. Because the CT tube circles the body 360 degrees, this is symmetric* all the way around the body. (*Tube current modulation, such as Siemens CAREDose or GE Auto mA change the amount of radiation based on the direction of the tube so that it's not symmetric; this is discussed more below in the dose reduction section.)

How is CTDI measured? A large tube full of water is placed in the CT scanner and scanned. Because the radiation is not uniform from the edge to the center, measuring devices are placed just below the surface and at the center of the tube. (The "100" in CTDI₁₀₀ refers to how long the radiation measurement device is when the CTDI is measured for a given scanner.) The CTDI calculation assumes that the radiation decreases linearly from the outside to the center and is calculated as CTDI = (1/3) * radiation_center + (2/3) * radiation_periphery. It is then divided by the length of the scan to give a CTDI per slice. So CTDI represents the dose to the tissue in that slice; it depends on the size of the body (more on that later), the tube current and kV, and the pitch.

The CTDI we discussed above, CTDI₁₀₀, is also known as CTDI_w where w stands for weighted. In modern helical CT scanners, the scanner does not simply scan one slice after another. Instead, it scans the entire volume in a helical trajectory. Thus, there isn't really a true 'slice', as the z-position of the scanner is different at each angle. As discussed in the section on helical CT, the spacing between successive revolutions of the CT tube represents the pitch of the scan. (Remember, Pitch = Table movement after 360 degree rotation / Collimator width .) The wider the helix, the less dose the patient receives because the same portion of tissue is being irradiated at fewer angles.

Left: Helical CT with a low pitch. Right: Helical CT with a high pitch. Notice how the same slice is exposed to more radiation in the low pitch scan.

We can convert CTDI_w (=CTDI₁₀₀) to helical mode, and this is called the volume CTDI, or CTDI_vol.

CTDI_vol = CTDI_w / Pitch

As expected, the larger the pitch, the lower the dose. Obviously there are disadvantages of high pitch, which are discussed in detail in the Helical CT section. Many institutions and governing bodies use CTDI_vol as a quality-control measure. For example, the American College of Radiology has set 'reference levels' for adult head, adult abdomen, and pediatric abdomen CTs (75 mGy, 25 mGy, and 20 mGy, respectively).

Now that we have a dose for a single slice, how can we take into account radiation exposure for the entire scan? As a reminder, dose (represented here as CTDI_vol) is an average energy deposition per kilogram of tissue. The dose won't change if you decide to include the entire body in your abdomen and pelvis CT. However, there is a measure of total radiation deposition that we can use, and it is the dose-length product or DLP. As the name implies,

DLP = CTDI_vol * scan length

DLP is usually expressed in units of mGy*cm. Just as discussed in the section on fluoroscopy with regards to the dose-area product, the DLP may provide a better measure of the stochastic risk to the patient after a given CT. It represents not only the amount of radiation to a given tissue (dose) but also the amount of tissue exposed. The DLP can be converted to an effective dose for a 70 kg average patient using lookup tables (which were generated from simulation or phantom measurements). As an example, a paper by Huda and colleagues contains some conversion factors. It is important to remember, though, that these conversions are gross approximations and represent values generated by averaging some example scans. They do not take into account the patient's actual anatomy. For example, if the patient has pendulous breasts that are included in an abdomen/pelvis CT, the breast dose (and therefore, the effective dose) will be much different from that of a patient with small breasts entirely outside of the scan region. Additionally, these conversion factors are not accurate for obese or very thin (or pediatric) patients; see below.