Computerized tomography (CT)

CT scans involve larger radiation exposure than conventional x-ray imaging.

Computed tomography, more commonly known as a CT or CAT scan, is a diagnostic medical imaging test. 


Cross-sectional images generated during a CT scan can be reformatted in multiple planes, and can generate three-dimensional images. 

Such images can be viewed on a computer monitor, printed or transferred to a CD or DVD.

CT images of internal organs, bones, soft tissue and blood vessels provide greater detail than traditional x-rays, particularly of soft tissues and blood vessels.

More than 19,500 CT scans performed daily in the United States.

Estimated 81 million CT sans performed in US in 2014.

Annual increase in use is about 8% in the last decade.

In a study (Stopsack KH) of 54,447 adults, 48.4% underwent at least one CT scan in a ten-year period: 10 year radiation doses from CT scan were 0.12 9.9 mSV in 15.8% of the population, 10-24.9 mSv in 16.9%, 25-99.9 mSv in 13.8% and, and 100 mSv or greater in 1.9%.

Fetal exposure from a CT scan of the abdomen and pelvis may increase the risk of leukemia by a factor of 1.5 over the background rate of approximately one in 3000.

A typical abdominal/pelvic CT scan deliveries 10-25 mGy of radiation with the fetal exposure of radiation as low as 4.8 mGy.

Based on measurement of the amount of energy absorbed as a beam of radiation passes through tissues from a source to a detector.

The radiation source and detector are mounted opposite to one another along a circular track that can rotate rapidly and synchronously around the table on which the patient lies.

Measurements are made of many projections through the tissues and stored in a computer.

The computer controls the radiation source, its rotation and that of the detector, and the movement of the table, and generates slices of the measured projections.

In modern CT, the source/detector makes at least a complete 180-degree rotation about the subject obtaining a complete set of data from which images may be reconstructed. 

The slices are at right angles to the long axis of the body.

Volume CT dose index (CTDIvolume) in milligray is commonly used metric to describe radiation output from a CT scanner

Volume CT dose index Does not represent a patient’s absorbed dose.

Site-specific dose estimate (SSDEs) calculates the mean absorbed dose at the center of the scan range for organs.

The study captures the density of tissues which allows discrimination between different densities of air, water, fat, bone and other bodily constituents.

Images are presented on the computer and can also be optimally displayed to depict the varying tissues by modification of densities by the process referred to as windowing.

Dense material appears bright on images and tissues that are less dense, like fat or water, appear dark.

Estimates of patient dose must take into account a patient’s habitus into account.

Hounsfield units are the standard measure of density.

Hounsfield units range from -1000 (air) to over 1000 (cortical bone).

The Hounsfield scale applies to medical grade CT scans



Soft Tissue−300 to -100


Water 0

CSF 15

Blood+30 to +45


Bone+700(cancellous bone)to +3000 (dense bone)

Each year in the US 10% of the population undergoes a CT scan.

Use grows by 10% a year.

Radiation doses from CT scans are 100-500 times those from coventional radiography, depending on the part of the body being imaged.

Risk of cancer from a single CT scan could be as high as 1 in 80 (Smith-Bindman R et al).

Evidence suggests that the radiation dose of CTs can be reduced by 50% without reducing diagnostic accuracy (Catalano C et al).

Physiological risks of CT scans include possible contrast-induced nephropathy, and allergic reactions to contrast.

Approximately half of the studies done in adults are diagnostic scans of the body, and one third of the scans are of the head.

Recent increase in pediatric studies and in adult screening tests.

Pediatric studies make up 6-11% of CT scans.

Utilization in children increasing because of the short amount of time needed to perform the examination and the elimination of the need for anesthesia to keep the child still.

Increased use in pediatrics for the diagnosis of appendicitis.

Organ dose to the stomach is at least 50 times greater than the .25 mGy dose from a conventional anterior posterior abdominal x-ray.

Brain dose from a head CT is approximately 50-60 mGy.

Colon dose from abdominal/pelvis CT scan is approximately 15-20 mGy.

Subjects each patient to the equivalent of 30 to 442 chest radiographs per scan.

Almost 20% of patients receiving radiation exposure between 3 and 20 mSv and as many as 2% receive high or very high doses from 20 mSv to more than 50 mSv.

Studies have revealed a 13 fold variation between the highest and lowest radiation doses for each type of CT scan studied.

Variations in dosage vary from institution to institution, and within the same institution.

Median effective dose from a typical CT coronary angiogram is 22 mSv and 31 mSv for a multi-phase abdominal-pelvic CT scan.

Estimate for excess cancer mortality from radiation exposure from CT scans is one death

per 2000 scans, assuming an effective dose of 10mSv per scan and the risk of 5% per Sievert.

Estimated 1-2% of all cancers in the United States may be associated with exposure to ionizing radiation from CT scans (Brenner DJ et al, Fazel R et al).

Exposure to medical radiation from CT scans is associated with elevated risk of thyroid cancer and leukemia.

The elevated risk in thyroid cancer and leukemia in association with medical CT was stronger in females than males.

No significant association between the risk of cancer and CT scans was observed in overall patients with NHL, however, increased risks were found in patients ≤45 years of age.

Dose-response relationship is observed in patients ≤45 years of age for thyroid cancer, leukemia and lymphoma.

CT scans may be associated with an increased risk of thyroid cancer and leukemia and those diagnosed with NHL at a younger age.

The elevated risk in thyroid cancer and leukemia in association with medical CT is stronger in females than males.

Estimated that one in 1000 individuals exposed to 10 mSv of radiation will develop a solid cancer or leukemia during their lifetime (National Academies’ Biological Effects of Ionizing Radiation 7th Report).

A scan of the chest delivers more than 100 times the radiation dose of a routine frontal and lateral chest x-ray.

Radiation exposure from CT scans has increased due to the increased speed of the image acquisition, allowing for vascular, cardiac and multiphase examinations.

Radiation doses to patients depend on the number of scans, tube current, scanning times, patient size, degree of overlap between adjacent CT slices, tube voltage and design of the scanner being used.

Sensitivity and specificity for staging of the mediastinum in NSCLC is 52% and 69% respectively.

Compared to MRI CT scans provide better imaging of bony structures.

Has the highest sensitivity (95%) and specificity (98%) for urinary stone detection than any imaging technique.

Abdominal CT preferred hepato biliary imaging technique, with the exception of the gallbladder which is better visualized with ultrasound.

In the evaluation of acute abdominal pain, early abdominal CT scanning provides better diagnostic accuracy then does standard supine abdominal and chest imaging, and less frequent misses unexpected and serious conditions.

CT of the abdomen in evaluating acute abdominal pain has 89% sensitivity and a 77% specificity in diagnosing serious conditions (Lameris W et al).

Increases in CT use for the evaluation of abnormal pain has not resulted in an increase in the rate of diagnosis of significant intra-abdominal conditions (Pines JM et l). are in a

Compared to ultrasound abdominal CT scan allows better visualization of exorability structures, the pancreas, vascular structures and lymph nodes.

Estimated that 1.5-2% of all cases of cancer may be attributed to CT radiation.

CT scans of the head associated with 2mSv, CT cervical spine 6 mSv, CT of chest 15mSv, CT abdomen 8 mSb and CT pelvis 6 mSv.

Average CT scan of the brain .06 Gy.

Estimated that 0.2% of the incident cancers in the UK could be attributable to CT scans (Berrington de Gonzalez A).

Increased risk of cancer among long-term survivors of Hiroshima and Nagasaki, who received exposures of 10 to 100 millisieverts (mSv), and this exposure is similar to radiation exposure from a single CT scan, and particularly in those who receive multiple CT scans over time.

Associated with increased risk of later cancer, although the risk is greatest for individuals imaged as children.

The future risk of cancer needs to be weighed against the information that he can be gained from a CT scan to identify potentially treatable pathology.

Patients with breast cancer who undergo routine staging chest CTs and 15.6% were found to have either lung or liver abnormalities-but only 1.55 of patients had true metastases, all but one metastasis being identified in stage III disease (Kim H).

In breast cancer staging CT studies have detection rates of 4-13% for pulmonary nodules, with few being significant.

Low-dose abdominal CT allows for a 78% reduction in radiation exposure compared to traditional abdominopelvic CT and may be preferable for diagnosing children and young adults in whom exposure to CT radiation is of particular concern. 


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