Radiation Dose in CT – Part 1

hello good morning my name is Mahadeva Mahesh I'm a medical physicist medical physics is a field of which bridges the gap between physics and medicine I'm a qualified medical physicist and I want to talk about physics topics starting with this radiation dose in CT today I'm going to talk about on the following topics how are CT dose measured what are City dose descriptors understanding City dose display and what are the typical radiation dose for typical City procedures let me start with explaining why the city dose is such an important aspect right now in this particular graph we are seeing here is the radiation exposure of the US population from variety of sources on the right hand side is a pie chart showing the radiation exposure to the US population as published in 2009 it's a snapshot data based on 2006 this data is an update on the previous published data back in 1982 if you look at the 1992 data the majority of the radiation exposed to US population was from background only about 15% was for medical exposure however the medical exposure pie chart has increased tremendously in the past 25 years among them one of the major contributors is City dose and that's where the emphasis about CT dose has reached a point what I want to also want to talk about the growth of CT procedures in the u.s. in this particular graph shown here is the number of CT procedures in the US in the past 15 years the number of CT procedures have grown linearly from 26 million procedures in 1998 all the way to close to 780 to say 80 million procedures as of 2015 as you can see the number of City procedures have increased both in the hospital and non hospital settings uniformly just to add an additional information the multi detector city was became commercially available in 1998 and that has led to a tremendous growth in City procedures so is the number of City benefits from this ma the new technology to understand Ct doses it's also imperative to understand what the type of radium procedures we do normally in clinic this is the data published in 2015 demonstrating the category of City procedures as you can see here the majority of the CT procedures are head chest abdomen in pelvic in fact nearly 76 percent of all the procedures done in the u.s. is head just abdomen and pelvic and there are other procedures - just beginning to show on the national data such as CT angiography cardiac CT and calcium scoring and so forth having this in the background I just want to discuss some basic physics of CT dose measurement when I say CT dose measurement um it is embedded - and start with how CT does is measure the first caveat is the CT dose is not measured directly on patients even though we express it he dose to patients all the measurement is done using standard phantoms medical physicists typically use standard phantoms measurings those on the phantom and then extrapolate in terms of explaining to CT dose to the patient measurements are then used to estimate the patient loss it is also important to see how the city dose the radiation dose distribution is different from typical radiography and fluoroscopy procedure and in CT for example in city in in in radiography in fluoroscopy the entrance windows is highest and the dose radiation dose decreases as they x-rays pass to the patient in fact the rule of thumb is on an average 15 percent of the doors gets out of the patient for an average-sized patient however this is distinctly different in City Sin City on the x-ray tube rotate around the patient the surface dose on the patient is the maximum in City compared to the dose in the center in fact many of us observe while while reading an obese patient the the image is quite noisy in the center of the image that is due to limited number of x-ray photons reaching that particular area physicists call it as photon starvation it is important to understand the distribution of CT and how it can translate in the DC dose measurement ideally the radiation dose delivered in city should be like a square if you are if you are using a 10 centimeter millimeter slice thickness then the block of anatomy exposure on the patient should be exactly 10 millimeter like a like a like a square or a rectangular block however in real world it's not exactly like that it is more like a okay like a bell curve in that sense the actual peak dose is only contributed to a small portion of this particular slice and there is a tail end to the whole thing because of this we also need to understand how the city dose distribution plays in different sizes of the patient that is taken into consideration when we are doing the CT dose measurement for example here and a typical dose distribution in City for a small size such as head pediatric and any object smaller size it is demonstrated that the surface dose is almost same as the central dose so for a physicist to do a measurement on a phantom he can do the measurement anywhere on the phantom measure it the surface are in the central dose it doesn't really make a difference because the surface dose is almost same as the central dose however when we are looking at a larger patient such as an abdominal dose or an abdominal size of the patient the dose distribution is different as demonstrated here on if on an abdominal phantom the surface dose is almost twice that of the central dose so for a CT dose this particular distribution is accounted in there in a formula what is called a city di weighted value weighting means basically giving a weight each on for the measurement at the surface and the measurement done at the center to kind of provide a uniform like a corrected value of the city dose done on the measurement of the Phantom which mimics what the city dose distribution really appears because of that the physicist introduce another term called CTD i wated which is basically a measurement on the Phantom and taking the measurement done on the surface and giving a variant value of 2/3 the value at the center at the edges and one-third the value at the center to accommodate what is called a city di weighted we also do lot of scans in a helical model when you do helical mode the concept of pitch arrives the pitch is defined very simply as follows it is a ratio of the table travel divided by the actually x-ray beam width so let's imagine a scenario or if the table is traveling at 40 millimeter per per gantry notation and the beam width is also 40 millimeter that translate to a pitch of 1 which means conceptually you are not leaving any anatomical gap on the other hand if the table travels quite fast and the beam width is still the same then there can be anatomical gap that can translate to a pitch of 1 greater than 1 why is this important because the radiation dose is proportional inversely proportional to the pitch value keeping all the other fat the same if we just change the pitch the radiation dose change accordingly in an inverse relationship that can be explained conceptually in this particular slide where a pitch greater than 1 means you are basically extending this helix leaving some gap in the anatomy that can translate to a reduction in radiation dose and also can also have other implications such as lowering the resolution and so forth and a pitch less than 1 typically means this overlapping of the anatomy that automatically translate to a higher pitch so this is wrapped in the city dose measurement as the following called a CTD I wall city a wall is basically a CTD I waited divided by the pitch so conceptually if the pitch value is less than 1 means lot of overlap the citadel wall will be higher on the other end of the pitch value is greater than 1 then the CTD R value will be lesser under CDD I'm ready up till now I've been talking about CTD I see TDI is a definition put out when in the early 70s what is called as computed tomography dose index it is just an index it is not indicating of the entire city scan it is a measurement done at the center of a phantom of a single slice and and that's why it's called a CT di in the index number and the unit of the CT di is Millie gray or Millie rad and so forth but in CT we don't just confined to one slice we do a number of slice so in order to estimate the dose to the patient we had to accommodate the actual anatomy exposed and that's represented in what is called as dose length product dose length product is basically representing a total dose in terms of the total Scanlan therefore a DLP is another dose descriptor introduced which basically comments the CTD eyewall multiplied by the Scanlan so in this scenario here in this patient the CT di wall is almost same in both patient however DLP is twice in the patient the bottom because the scan length is double the scan length of the top one in a way why this is important because dose length product kind of represents the biological risk to the patient larger the anatomy is an exposed great of the biological risk that's why the DLP concept is very important so wrapping the all the definition which I mentioned up to learn here is the one slide demonstrating what is CT dosimetry is basically on the left hand side is CT di 100 is basically the measurement done on a standard phantom using a hundred centimeter chamber that's called CT di hundred from there we we compute what is called as weighted value c td i weighted which is taking into account the distribution variation in the distribution based on the small or large patient if the scan involves then we introduce what it's going to see today a wall you multiply that with the scan length it's called dose length product from dose length product we arrive at a effect you dose value it is a risk estimation to the body and to arrive that is a lot of mathematical simulation and factors which is used which will be part of the discussion in City dose part or later so as of right now the two key dose descriptors in the city or the following CT computed tomography dose index which is expressed as CTD wall which is expressed in milligram and dose length product are dlp expressed in moliro centimeter infant as of now some states in the u.s. started to introduce regulation have already introduced regulation where the radiologists are physicians who are reading the Radio City images are supposed to record this information the patient chart for latter day to express some type of a risk and so forth so it's been right now this is the two main descriptors to remember for understanding the patient dose from there we can arrive at the effective dose and so forth having described briefly about the various aspect of CG dose measurement I just want also want to introduce the different varying factors which important which plays a role in the city dose and also image quality so here described or a variety of scan parameters which have an influencing both directly and indirectly on this image quality and also on the radiation dose in fact the discussion on various factors is a discussion by itself for another podcast so how to understand some of the city dose display in fact to make it easier now it is important how this information is readily available on the information for example um to understand City dose display there are two ways one is a pre scan display and a post scan display in a pre scan display as shown here basically when a technologies is setting a techniques or for before scanning a patient the display there is an information about the projected dose to the patient is projected here that's because since city dose is not measured directly on the patient is based on the Phantom this information is already built in as a lookup table if any of the parameters change such as tube voltage RF at mas that immediately reflects back to the change in the city da1 manufactures are becoming lot more sophisticated in displaying this information in fact even tag the type of phantom measured to display at that particular value where it is arriving at most commonly which are physicians or radiologists rates is the post can display if you look at the thousands of CT images for a patient one of the last image tag to the patient study is what is called as protocol page or a dos page and so forth and that provides a insight into the type of technique used and the type of the dose descriptors we were describing earlier on this one on this particular piece showing on the bottom right hand corner is a post scan display image for a particular one particular manufacturer shown here toes it provides details on scan series and scan parameters it also tells the DLP which is then can be used effectively by the medical physicist or radiologist to arrive at an effective dose estimation in fact going even one one step further now all the images do have what is called as DICOM standard structure Dada dose report in the DICOM header even lot more information is available regarding City dose but for simplicity looking at the post can display image one can arrive at the type of scan done for the patient along with the type of technique used along with the type of dose deliver to the patient from which we can arrive it's some type of an effort to those value but this part of the podcast I'm concluding here with showing some typical radiation dose for CT protocol these are published information such as average effective dose for a standard protocol such as head neck chest abdomen and pelvic from the date of this publication to now some of these procedures are delivering at them can be delivered at a much lower dose but decent the average value is displayed here to conclude this part of the podcast the radiation dose in City is not measured directly on the patient it is based on the phantom measurement the radiation dose estimates for CT exams are best expressed as CTD a wall and DLP it is important to remember that CT those index is not patient dose finally understanding scan parameters are factors that affect those in image quality in CT is critical thank you

Channel: CTisus

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Keywords: Computed Tomography, CT, Radiology, Volume, Rendering, MIP, Medicine, Health, free, medical, physics, lecture, dr mahesh, mahadevappa mahesh

Id: YaYSLlLA5Zs

Length: 17min 36sec (1056 seconds)

Published: Wed Nov 30 2016