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1 CT Angio = 600 chest x-rays
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1 CT Angio = 600 chest x-rays

so sayeth the latest JAMA. That's the average dose. But there is also a wide variability in the dose you get, depending on where you get it. The highest dose was 6 times the lowest dose discovered, among 50 international imaging sites.

The dose even varied among different models of the same machine.

The sites that did a lot of scans didn't necessarily know/employ the methods that gave a lesser dosage.

The AHA has issued a recent advisory on the subject, too.

This all would suggest to me that younger patients (<60? <50? <40?) might want to avoid the 64-slice CTA, if possible.
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367994_tn?1304957193
For consideration radiation dose is estimate, link between dose and cancer unclear.  Also, radiation cannot really be measured...only a fairly crude estimate.  CT-scanner radiation output can be measured but this is not necessarily the same as the amount of radiation that the patient absorbs and that may be the reason for the variation among CT scan sites.

Another issue is to clarify and assuming a lenear relationship the relationship between radiation dose and risk of cancer, which is the subject of ongoing debate. We're using a fairly conservative model and assuming a linear relationship from very high-dose levels—the kind of doses to which people were exposed in Hiroshima and Nagasaki—to the very low-dose levels we're talking about in medical imaging."

Assuming this linear relationship to be true, A median radiation dose used recently in CCTA—around 20 mSv—would potentially mean that one in 2000 exposed patients would develop a fatal cancer. Doses of radiation used in medical imaging are constantly being reduced due to new dose-saving technology. The next generation of CT scan will substantially reduce risk.  Younger individuals have higher risk from exposure.  For older individuals there is less risk, and it takes from 10 to 20 years to develop cancer.

I recently had a Coronary CT angiography and calcium score exam and it is considered the most accurate noninvasive test for the detection of coronary artery disease. Given the low doses of radiation involved, when there is suspicion but not clear evidence that a patient has a blockage in a coronary artery needing evaluation, it is clearly safer to perform a coronary CT angiogram than to perform an invasive coronary angiogram.”

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690060_tn?1247845341
you bring up the important point of relative risk vs absolute risk.

Still, there are some methods by which the dose can be reduced, and so a PT might be well advised to ask if those methods are being employed at the site where they are getting their scan. IIRC only 70% or so were doing so.

OTOH, apparently it's true that obese people have to get more radiation in order to get a clear picture.

So do those with tachycardia. I'd guess it has to do with blasting the radiation over a longer time in order to get an image when the heart is at rest.

Then again, the AHA advisory had mostly to do with population risk - and so using the CTA as a screening measure in low risk PTs is advised against. If we take your figure of 1/2000, and apply that to millions of low-risk people potentially being screened, then you'd get many cancer cases.

I'm just putting this out there as a casual summary for those who aren't going to go and read the actual articles.


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367994_tn?1304957193
Quote: "So do those with tachycardia. I'd guess it has to do with blasting the radiation over a longer time in order to get an image when the heart is at rest".

It is necessary to slow the heart to get a better image.  I don't believe it has anything to do with the radiation factor (subbsequent images will not produce a clear image with a fast beating heart) as a fast beating heart (ideal to get heart rate below 60) will not provide images that clearly delineate relevant objects.  The borders will be fuzzy...same as with echocardiogram when estimating EF and chamber dimensions.  Are you suggesting more images are CT scan are taken of a fast beating heart, and then compiled with the computer for the best image?  If the heart beat does not provide clear images, the test is considered inconclusive

Relative risk is the issue and the benefits thereof.  CT scan weighs more heavily in favor for older individuals...less vulnerable to radiation compared to younger (general consensus) as well as the cancer risk is a slow developing phenonomon (10-20 year).  

The population risk includes doctors who do not or may not adequately and carefully screen an individual's risk. There is no evidence the population of CT scans are with individuals of high or low risk.  I wouldn't be uncomfortable saying 50% or more are of high risk!

  
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159619_tn?1318997813
I have read the paper published by JAMA yesterday concerning this subject and it basically backs up Ken's statement. I think the key here is to wait for a true study outlining the actual relationship between medical doses of radiation and the risk of cancer before we can assume any ARR (absolute risk reduction) or relative risk, not enough data yet.
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690060_tn?1247845341
>>  I don't believe it has anything to do with the radiation factor

that is incorrect. There are radiation sparing strategies that cannot be used in people with fast or irregular heartbeats. (The x-ray tube voltage is lowered, controlled via info from ECG. This thread was from a while ago, but IIRC 'retrospective gating' was one such dose-reduction strategy.)

As far as population risk vs individual risk: although the CTA is many times more radiation, the amount of radiation is still fairly low. So the risk to any individual is low, but if millions of people get CTA, then some will inevitably develop adverse consequences.

It's like buying two lotto tickets instead of one. You double your chances, but so what? Your chances are still less than tiny. But when millions buy tickets, some will get a hit.

These articles were meant to discourage CTA as a screening tool, and that by definition is talking about population risk.

They made the same warnings about whole-body CT some years ago, when yuppies were doing that as a trend.
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367994_tn?1304957193
>>  I don't believe it has anything to do with the radiation factor... (I am and was referring to clear images, clear enough to make a diagnosis.  If the heart rate cannot be lowered to 62 or below, the test will be inconclusive (almost always).

I believe the equipment controlling parameters for ct scan 64 slice are amps not voltage (volts remain static at AC 120).... Data from a scanner (64 slice) is detector collimation of 64, 0.625 mm, total z-axis coverage of 40 mm per rotation, gantry rotation speed of 0.35 s, tube voltage of 120 kV, a pitch of 0.20–0.26, and ECG modulated tube current
ranging from 88 to 228 mAs.

The 2 techniques (SPECT Ct scan) the results are not interchangeable regarding LV volumes, myocardial mass LVEF and LV functional parameters as determined by
64-slice CT agree over a wide range of clinically relevant values with gated SPECT.

You haven't made a case regarding radiation vs. fast heart rate for clarity of images from a CT 64 slice.  Radiation is coming less of an issue with 64 slice and less so with 128 slice.




ANS: that is incorrect. There are radiation sparing strategies that cannot be used in people with fast or irregular heartbeats. (The x-ray tube voltage is lowered, controlled via info from ECG. This thread was from a while ago, but IIRC 'retrospective gating' was one such dose-reduction strategy.)

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690060_tn?1247845341
let's stay with the main point, rather than getting sidetracked with minutiae

there are references easily available, such as "Not every patient is a candidate for the dose-modulation radiation technique, though. Patients with elevated heart rates or abnormal rhythms are excluded because the CT scan might not capture these irregular beats."

http://www.theheart.org/article/959575.do

That is from a recent article about triple ruleout CTA. There was very similar and clear language at theheart.org back at the time of the original postings in this thread, if you'd like to look it up.

And of course they're talking about clear images. What doctor would ever order a test expecting to get useless unclear images?

As I'd said, people with tachycardia therefore would have to get a higher dose. They are not eligible for dose-saving strategies. Seems simple enough.

But if you'd like to dispute that with the cardiologists/radiologists who author the studies and the people who write the summary articles, then theheart.org does have a section for comments.

[And yes, I did misspeak and say "voltage" instead of current, but why get bogged down by focusing on side issues? Especially since voltage reduction is in fact also sometimes used: "Lower tube voltage, less radiation exposure", conveniently from that same page.]
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367994_tn?1304957193
Quote:  It's like buying two lotto tickets instead of one. You double your chances, but so what? Your chances are still less than tiny. But when millions buy tickets, some will get a hit.

Response: You appear confused!  You are comparing random selection and fair value with risk/ benefit analysis with the overtone medical uncertainty principles.  As I have stated, the population risk is irrelevant to the individual risk as there is no control of risk factors. An individual and the doctor can assess the patient risk/benefit.  Statistical analysis for the population is definitely skewed.  It may be relevant to government agencies, insurance, etc. and it may indicate a problem in screening process, etc.

From your source: "Investigators showed that the use of tube-current modulation cuts the amount of radiation in half, compared with triple rule-out coronary CTA without tube-current modulation"....  You have confused volts with current and reading your post there is a loss of credibility.  Total voltage divided by resistance (measurement in ohms) provides the current...to vary resistance provides the active current.  Volts relates to potential!  It is not a matter of misspeaking, it is lack of understanding!.

QUOTE: And of course they're talking about clear images. What doctor would ever order a test expecting to get useless unclear images?

Ans:You woould be surprised?!

QUOTE: As I'd said, people with tachycardia therefore would have to get a higher dose. They are not eligible for dose-saving strategies. Seems simple enough.

Ans: Think outside the box.  On-one-hand you state the doctor wouldn't order a test if the outcome was questionable,  Then another quote you seem to understand the implication of tachycardia, but a higher does of radiation would be necessary.  I am consistant saying heart should be below 62 to get quality images...and that is the issue in the context of my answer.  I'm not familiar with the feasibillty of the new lower dose radiation application, and where does it say radiation is a factor in the imaging process.  How long did it take to find that? :)

Quote:"Patients with elevated heart rates or abnormal rhythms are excluded because the CT scan might not capture these irregular beats."

Ans: I'm not familiar with the feasibillty of the new lower dose radiation application, and where does it say radiation is a factor in the imaging process (clarity).  How long did it take to find that? :)  I have had a CT scan 64 slice, and I know the reason for lower heart rate during the test is for clearer images....also fast heart rate can effect the results of an echo.  An EKG should be able to capture irregular beats and rhythm problems, and the EKG is tied into CT scan so your quote is counter intuitive!




  
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690060_tn?1247845341
>> An EKG should be able to capture irregular beats and rhythm problems, and the EKG is tied into CT scan so your quote is counter intuitive!

I'd say that's your problem: it's not remotely about intuition. You're taking your belief that you understand it all, and are deliberately ignoring simple facts.

I'd originally stated that people with tachycardia get a higher dose of radiation. You first disputed that, and now evade it. Are you trying to somehow "win" a debate? I don't see a health group as a place to try to "win" debates.

It's plain and simple, and your endless use of jargon doesn't change that: people with tachycardia get a higher dose of radiation. Here it is again, in another article: "And many dose-saving techniques cannot be used in patients with very fast or irregular heart rates".

http://www.theheart.org/article/939087.do

It couldn't be simpler. But if you somehow feel a need to win something by obscuring the topic, then you go right ahead and get the last post in... while you fancy that you have ultimate and superior understanding and anybody who posts a simple fact is "confused" etc. You can even claim that the sun is blue if you want to.

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