Radiation dose is directly related to the quality of our images in CT because the higher the dose the greater amount of signal reaching the detector and therefore the lower amount of noise we’ll see on the image.
Radiation dose in diagnostic imaging is described by three metrics:
- Absorbed dose
- Equivalent dose
- Effective dose
Absorb dose is a description of the absolute amount of radiation absorbed by a patient measured in milligrams.
Equivalent dose takes the absorbed dose multiplied by a weighting factor specific to the type of radiation being used in the case of x radiation this weighting factor is conveniently one. Equivalent dose is measured in millisieverts.
CT Dose Index (CTDI)
Effective dose takes into account both the radiation type and the tissue type being irradiated. This means it’s the equivalent dose multiplied by the tissue weighting factor which varies depending on the relative radiosensitivity of the tissue type being irradiated. There are several other expressions of radiation dose which are specific to CT imaging. CTDI stands for CT dose index. Most commonly in CT dosimetry we scan a 100 millimeter range and take measurements in an acrylic phantom based on that 100 millimeter range. The absorbed dose in the middle of this range is called the CTDI 100. However, the dose in the center doesn’t tell the full story in CT because we’re irradiating the patient from a 360 degree arc meaning the periphery of the patient receives more dose than the center of the patient. The weighted CTDI is a measurement that takes this into account by using dose measurements both at the center and the periphery of the phantom and then weighting the central measurement at one third times the CTDI 100 and weighting the peripheral measurement at two thirds times the ctdi100. Adding those two to give us the weighted CTDI. so we’ve discussed weighting radiation based on the radiation type the tissue type and now also based on the distribution of dose within the patient in the CT scanner. The next factor to take into consideration is the pitch of the scan. The CTDI volume is a measurement that takes the helical pitch into consideration. higher pitch means the lower dose due to under sampling and conversely lower pitch means higher dose due to oversampling. The CTDI volume is calculated by dividing the weighted CTDI by the pitch factor.
Dose-Length Product (DLP)
These are all measured in milligrams. As well as assessing the overall dose passing through a patient during the scan we also need to consider the z-axis length of the scan which is assessed in the dose-length product or DLP. This is simply the CTDI volume multiplied by the length of the scan in centimeters so the units are milligray centimeters. Radiation dose is directly related to the quality of our images in CT because the higher the dose the greater amount of signal reaching the detector and therefore the lower amount of noise we’ll see on the image.
Dose and Image Quality
In order to decrease the radiation dose in CT we may have to make some sacrifices in image quality. Ideally we want to decrease noise and improve our image quality in as many ways as possible without increasing the radiation dose. Iterative reconstruction is one technique we can use to decrease our noise and improve image quality. In doing this we can decrease our dose and settle for more noise in our original data set which will then be offset by these iterative reconstruction algorithms.
These techniques have enabled significant dose reductions in the last decade compared to older filtered back projection reconstruction techniques which left a much greater amount of noise in the images. So by reducing the noise in our images on the software side we’ve been able to reduce the amount of radiation required to obtain those images. Another simpler way to reduce noise on the software side is to simply build thicker slices from our data set. Because each thick slice is comprised of a greater volume of data than a thin slice would be noise in that data is suppressed when those thin slices are averaged into a thicker slice. The downside of this is a loss of spatial resolution. The radiation dose should be tailored to produce an acceptably low amount of noise in whichever slice thickness the images will be viewed in. we did speak about this in more depth on the post on image quality. When it comes to actually reducing the dose being emitted there are a variety of technical factors we can manipulate in order to do this, reducing the tube current which is the MA or the tube voltage which is the KV or the rotation time in seconds will all reduce the radiation dose to the patient. Increasing the helical pitch will also decrease the dose to the patient as they’re moving faster through the scanner relative to the rotation of the gantry. changing each of these parameters may be appropriate depending on a variety of things but it’s important to note that generally the ma will not be changed directly by the technologist. Instead most scanners are equipped with automatic ma modulation. Automated MA is calculated based on the scout images acquired before a scan. The scanner software will assess the scouts for areas of higher and lower density and will provide an appropriate MA as that z-axis location on the patient passes through the gantry for example on a chest abdomen and pelvis scan the MA will be higher through the upper chest to penetrate through the shoulders on both sides of the chest. Then it will drop down through the thorax because it’s mostly passing through air and lung tissue then it will increase again when it gets to the abdomen and then further as it gets through to the pelvis. This technique isn’t used for all body areas but will almost always be used for scans through the trunk where dose requirements can vary significantly throughout the scan.
When using this technique, a technologist will generally be able to set a certain level of desired image quality often based on the standard deviation or the amount of acceptable noise in the image and then the scanner will calculate the ma to aim for that target noise level. As well as adapting our dose throughout the length of the z-axis we can also use a few dose reduction techniques to reduce the entrance dose just at the anterior surface of the patient where radiosensitive organs like the breast thyroid and eyes are. In some cases, bismuth shielding can be placed over these areas to absorb low energy photons entering them while still allowing normal dose distribution to the remaining tissues. It’s important to add these shields after acquiring the scout images because otherwise they will be detected in the scout images and the software will simply increase the dose to compensate for them. Modern scanners are integrating different ways to achieve the same thing without the bismuth shielding by simply reducing the MA when the tube is in front of the patient so a smaller proportion of the overall dose to the patient will enter from the anterior surface. This is coupled with regular modulation of MA that decreases in the anteroposterior direction and increases in the lateral direction.
Filtration
filtration plays an important role in reducing radiation dose. Filters absorb low energy photons leaving the tube and in CT we use a bowtie filter as we discussed before.
the bowtie filter absorbs a greater number of photons at the periphery of the patient to reduce dose and ensure a more uniform distribution of photons reaching the detector.
Because the bowtie filter is designed to have a central point in the iso center of the gantry it’s really important to position the patient right in the iso center of the gantry. Meaning that they’re in the middle of the table with ways and also in the middle of the gantry in the up down direction. if not properly iso centered in the gantry the dose will not be distributed uniformly and certain areas of the patient may be starved of necessary dose while other areas receive excessive dose.
The final consideration for dose reduction in CT is out of field lead shielding and this is something which historically was used a lot but the evidence and consensus recommendations have shifted away from its use in recent years. Using a lead wrap around the pelvis when scanning the chest for example has not been shown to reduce the radiation dose to the pelvis. This is because the only radiation this would actually protect from is leakage radiation from the tube which is negligible on modern scanners. Most of the radiation reaching the pelvis on a chest scan is internally scattered radiation which obviously cannot be blocked by wrapping the pelvis in lead unless maybe you have one of those magic boxes that magicians use to cut people in half on stage but the current guidelines don’t support this method.
