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Tests
& Imaging > Imaging
> CT Imaging
Last Updated: 04/30/2008
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Intro
| What to Expect and How to Prepare | About Long-term Risks
PET/CT | Resources and Research News
 Computed
Tomography (CT) is an imaging procedure commonly used to determine
locations and sizes of tumors, so-called staging of disease. CT is often used to measure response to treatment,
especially in clinical trials.
X-ray images are taken from different angles and
then processed by a computer to create detailed pictures of the inside of the body.
Each section of the test involves lying still for about 10-30 minutes.
The images produced are in cross sections.
Search for ACR
accredited Diagnostic Imaging Centers
ACR accreditation
means: "Your hospital, clinic or health center has
voluntarily gone through a rigorous review process to be sure it
meets nationally accepted standards. The
personnel are well qualified, through education and certification,
to perform and interpret your medical images and administer your
radiation therapy treatments. The equipment is appropriate for the
test or treatment you will receive, and the facility meets or
exceeds quality assurance and safety guidelines."
What to expect and how to prepare
 | Bring water with you so that you can drink after
the test to more quickly eliminate the contrast material.
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| Depending on the area to be scanned, you may be
asked not to eat for approximately 4-6 hours prior to the
test.
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| Depending on the scan, IV contrast may be used.
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| The technologist begins by positioning the patient on the CT table.
You may be given pillows to help hold still and in the proper position during the scan.
A wedge is usually available (ask the tech) for under the knees to
help support the back. The table will move slowly into the CT scanner.
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| Drink water after the test
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| Ask for copies of the images |
Patient-to-patient
advice: "When you have these scans, remember to ask
the technician for copies of the films (for your records). They
don't mind printing an extra CD or print - as long as you tell them
upfront BEFORE they sent the "number of copies".
Everything is digitalized and it makes it so much easier to keep
your records. In the olden days, we would have the big, heavy CT
films or MRI films we carried around. So, the improvements are
welcomed, however, I have also learned that computers crash and some
digital films get lost, hence, my recommendation that you ask for
copies. If you are nervous about asking for this, call
them before you go and find out if you can bring a CD for them to
back up to. In this way, you will learn if your facility is
co-operative or not. And naturally, always ask for a copy of
the radiology report to be mailed to your home for your records.
Having CTs spaced on different days is very, very common.
Personally, I had to have them several days apart because I am very
sensitive to the contrast dye. I also have to be pre-medicated with
Benadryl and Prednisone. It is one of the reasons that I
"gave up" CTs for monitoring remission. You don't have
this type of control while in a clinical trial and you must comply
with the demands of the trial protocol. Those of us who have already
participated in a trial understand how confusing it can be when they
request something of you and you don't understand the why of it.
One way that you might look at the four areas of scan is
"insurance" - while it is not necessary to have, it is
good backup against unseen or undiagnosed areas of disease
involvement. Also, a head/neck scan now, if clear, will be your
baseline for the future. Hopefully, you will never have evidence of
disease from the neck up, but if you should have questions,
you could refer back to those scans.
One of my patients has a little "thumb" drive (USB 2.0
flash driver) or other named device she carries with her for her
scans. The local imaging center copies her scans onto her zip/flash
driver and she has it to go back to her home computer. I was with
her when she had her physician's appointment (who didn't have the
"films" yet) so Linda just popped her little flash drive
into his laptop and up came her scans. It worked out so well, but I
think it depends on your facility and their "spirit of
cooperation". Some imaging centers act like they are guarding
the secrets of the universe and YOU are not allowed to know the
secrets of the universe because you don't wear a white jacket!!!
"You must ask your doctor!!" ~ Jama Beasley (advisor
to PAL, and patient navigator)
Factors that can influence interpretation of results
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Size of lymph nodes can be influenced by inflammatory reactive
conditions.
"diffuse lymph node enlargement secondary to infectious or
granulomatous diseases cannot be discriminated from metastases or
systemic lymphoma." Oncologic Imaging: A Dilemma Still Waiting to Be Solved
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Scar tissue - CT cannot distinguish between scar tissue and
viable tumors.
Following
treatment "a residual mass persisting on CT after treatment poses a common clinical dilemma: it may indicate the presence of viable lymphoma, which requires further treatment, or it can be benign, consisting of only
fibrotic and necrotic tissues." PMID:
12644887
For this reason PET or Gallium scans may be used after treatment to help differentiate active disease from scar tissue.
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Small changes in the size of nodes could be accounted for by the angle of the equipment in relation to the patient.
"In a multiplanar reconstruction CT program (CT/MPR), the altered gantry angle corresponding to the patient's position during scanning for multiplanar reformatted CT may cause distortion of the image. The aim of this study was to quantitatively assess the distortion in reformatted central panoramic and cross-sectional images owing to the change of gantry angle. "
birjournals.org
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Recommended Resources
PET/CT
"The highly sensitive PET scan detects the
metabolic signal of actively growing cancer cells in the body and the
CT scan provides a detailed picture of the internal anatomy that
reveals the location, size and shape of abnormal cancerous growths.
Alone, each imaging test has particular benefits
and limitations but when the results of PET and CT scans are
"fused" together, the combined image provides complete
information on cancer location and metabolism.
The bottom line is that you can have both scans -
PET and CT - done at the same time." petscaninfo.com
Also See
| Comparing CT and MR Images Lymphomation.org
Side-by-side comparison of MR and CT images
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| Comparing CT with MRI Lymphomation.org
Abstracts and resources on the subject.
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About Long Term Risks of CT
Imaging
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"Radiation effects are latent, not showing up until many years later. They are also considered stochastic, that is a term used by the radiological community which really means probabilistic. They will never occur for most individuals, but although the risk is not "zero", it effectively is for most individuals."
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"Many cancers show up 5, 10, and 50 years later (as the atomic bomb survivors are showing). Leukemia and thyroid cancers show up within a few years, and usually drop off. It varies for different cancers. Also, the older you are, the more likely you will die of something else.
Also- radiation from imaging studies is so small, that I can
say with probabilistic certainty that anyone past mid-life will
die from something else.
However- it is prudent to carefully plan such imaging studies so that only the necessary information is collected, and that the amount of radiation the patient receives is, in fact, as low as reasonably achievable, the ALARA principle
(As Low As Reasonably Achievable)."1
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Radiation dose from imaging diagnostics and treatment is
cumulative ...
... but low dose radiation is not equivalent to high exposure (ie, an h-bombs) because the body apparently adjusts and heals when doses are divided up for treatments - known because so called fractionated doses can be given in higher
cumulative doses as treatment.
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Treatment doses are higher than diagnostic, and the effective dose is finely tuned and well documented in therapeutic radiation.
"Secondary cancers associated with radiation therapy are more probable, since these radiation doses are thousands, or tens of thousands greater than the radiation doses associated with imaging.
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Diagnostic x-rays use low dose ionizing radiation, but dosing is less precise, more variable.
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Different organs have different sensitivities to ionizing radiation. Marrow, breasts, gonads, etc. (Marrow is sensitive).
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| Dose terminology (Rem, Gray, mSv, ...) has confounded even the experts and makes comparing exposures very difficult ...
... but now we have so-called effective dose (E), which allows for easier comparisons of exposures and risks . The measure of effective dose is mSv. ( Effective dose in millisieverts (mSv).)
Some examples of E
= Natural Environmental background in 1 year ~ 3 mSv
= Chest x-ray ~ to .02 mSv (2.4 days of natural environmental radiation (NER))
= CT abdomen ~10 mSv (3.3 years of NER)
(~ means approximately equal to)
The current occupational cumulative regulatory limits are
 | Individual worker in general
population
1 mSv , or 50 chest xrays, or 4 months NER
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Worker
50 mSv, or 2500 chest xrays, or 16.7 yrs NER
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Emergency worker
500 mSv, or 25,000 chest xrays, or 167 yrs NER
The relative dose of radiation from CT is much larger than other imaging tests, but the risks are still very small that CT imaging will lead to a secondary cancer.
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| However, the risks from many CT images is not
insignificant, particularly if you are young and require frequent monitoring.
Some factors that influence risk
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The lower doses in diagnostic imaging *reduces* the risk of secondary cancers (compared to high exposures (h-bombs etc)) by 2-3 times.
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Radiation risk is age
dependent
= Adolescents and young adult have ~ 2-3 times greater cancer risk per unit dose,
= Adults over 50 have ~ 1/4th the relative risk per unit dose*
Most lymphoma patients are over 50.
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The radiation dose in medical imaging can vary by as much as 100 times
Some factors
= patient size (being larger increases the E)
= equipment design, operator settings, length of exam ...
= amount of radiopharmaceutical administered
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How to minimize your risk?
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Keep records of all x-ray exposures so you can give objective information to your doctor about your cummulative dose.
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Ask for a breast shield. See Breast Shield
Cuts CT Scan Radiation www.realage.com
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Inform your doctor about regulatory limits for ionizing radiation, and your current estimated effective dose.
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Ask the imaging providers to give information about the *effective dose* in mSv that you will receive.
... And if the ALARA principle is followed at the center (As low as Reasonably achievable)
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Ask the imaging center to see their ACR (American College of Radiology) accreditation.
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Ask your doctor if your clinical situations allows for imaging at longer intervals.
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Ask if MRI can be used instead of CT, particularly if you are young and if your past exposure has been high. New ACR accredited MRI can do a good job of staging lymphomas. Both CT and MRI have strengths and weaknesses depending on the area of the body being looked at.
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Clinical trials: Trials can sometimes require patients to get imaging done more often and a closer intervals (as short as 2 months) When making your case to the investigator about using MRI instead of CT, provide the background facts in summary form, and include your estimated
cumulative effective dose. Inform the investigator that the FDA probably considers ACR accredited MRI images equivalent to CT.
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Expert
commentary by Orhan H Suleiman MS, Ph D, FAAPM |
It's not easy to quantify the risk: Comparisons of CT exposures with exposures to background radiation received in air travel may be useful. According to the FDA: One pelvic CT is roughly equivalent to exposures to cosmic radiation from 333 flights from Los Angeles to New York and back (.03 mSv)
(Radiation
Safety).
A twenty-year career as a member of a flight crew might add up to roughly: 300 days x 20 years, or 6,000 days of exposure to cosmic radiation. Dividing this number by 333 gives us a very rough equivalent of 18 pelvic CT scans (10 mSv).
Note that despite the increased exposure, the study found "No consistent association between employment period or duration and cancer mortality was observed." Mortality
from cancer and other causes among male airline cockpit crew in
Europe.
..." key factor in the body's
response to ionizing radiation is the relative sensitivity to
radiation of the various cell types that comprise body tissues.
Bergonié and Tribondeau's Law (1906) implies that rapidly dividing
cells (e.g., cells of the blood forming tissues and certain groups of
immature sperm cells) are among the most sensitive to acute effects of
radiation. The more highly differentiated cells (e.g., muscle and
nerve cells) are less vulnerable to acute injury by radiation.
Other
factors that influence the expression of the deterministic effects of
radiation include the region of the body irradiated and variation
between individuals in their physiologic response to radiation."
http://books.nap.edu
Related articles
| What are the Radiation Risks from CT? FDA
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| Breast Shield Cuts CT Scan Radiation www.realage.com
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| Radiation Biology By: Scott Williams, MD auntminnie.com
Comprehensive review of risks ... "it is reasonable to
assume that certain factors can modify the ultimate effects of
radiation [3]. Factors such as age at time of exposure and the
manner in which the radiation was received can affect the risk
relationship."
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| Mortality from cancer and other causes among male airline
cockpit crew in Europe.
Int J Cancer. 2003 Oct 10;106(6):946-52. PMID:
12918075
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Maintaining a Proper Perspective of Risk Associated with
Radiation Exposure PDF
Michael A. Thompson, Division of Medical Imaging and Therapy,
University of Alabama at Birmingham, Birmingham, Alabama
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Radiation Risk From CT Scans: A Call for Patient-Focused Imaging
~ Richard C. Semelka, MD
Medscape Radiology 6(1), 2005. © 2005 Medscape
Richard Semelka, MD, is one of the preeminent practitioners of abdominal MRI in the United States and is a frequent contributor to Medscape Radiology.
The hot-button issues raised in this opinion piece call into question some of the current practices surrounding the use of CT scanning. It's the opinion of this editor that a topic of importance, such as the safety and health of patients referred for diagnostic imaging evaluation, merits a broad dialogue in an open, spontaneous, and timely forum.
full text Medscape
(free login req.)
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Doctors' knowledge of radiation exposure: questionnaire study,
2003 BMJ
Conclusion: "Overall, 97% of the answers were underestimates
of the actual dose; six (5%) doctors did not realize that
ultrasound does not use ionising radiation; and 11 (8%) did not realize
that magnetic resonance imaging does not use ionizing radiation."
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| How many CTs are too many? Lymphomation.org
"In order to make truly informed
decisions, it's important to put the risks of procedures into context
and not to magnify them."
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Radiation Dose Comparisons FDA
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| Radiation doses in computed tomography - The increasing doses of radiation need to be controlled
BMJ 2000;320:593-594 (4 March)
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| Radiation dose in [CT] scans could be halved
BMJ 2001;323:185
(28 July)
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Second malignancy after treatment of childhood non-Hodgkin
lymphoma.
Cancer. 2001 Oct 1;92(7):1959-66. PMID: 11745271 PubMed
Provides clues about long-term risks.
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Resources and
Research News
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