CT / CAT Scan - Frequently Asked Questions


Q. What is a CT Scan?

A. CT stands for Computerized Tomography, which refers to the way this exam is performed. A CT scan is an X-ray procedure which is enhanced by a computer. This enhancement results in a three-dimensional view (referred to as a "slice") of a particular part of the body.

During a routine X-ray, dense tissues can block other areas. Aided by the computer, a CT scan is able to put together the different "slices" and create a three-dimensional view, clearly showing both bone and soft tissue.


Q. What will the exam be like?

A. The patient will meet with a CT technologist whose primary concern is the patient’s care and well-being. Our technologists have completed a rigorous course of education and training, and they work under close supervision of our radiologists to assure the most accurate results from the examination.

Prior to the start of the CT exam the technologist will explain the procedure to the patient. This is done for two reasons; to put the patient at ease as to what is taking place and ask for the patient’s cooperation. After the explanation is completed, the patient is positioned and secured on the examination table. It is important for the patient to be secured, because even the slightest movement can blur the picture and result in the need for repeated scans. The technologist will have the patient in full view at all times and be in constant communication via two-way microphones. During the time in the scanner, the patient will hear the humming of the equipment as it produces images. The patient may also feel slight movement of the table as it prepares for the next scan.


Q. What is contrast for CT and MR?

A. Any material administered to patients prior to or during an exam such as CT or MR to improve the identification or visibility of a structure or abnormality. Oral contrast, given by mouth, consists of dilute barium or iodinated compound, which is meant to fill and distend the small bowel primarily. Oral contrast may require a delay of approximately 45 minutes in order for it to take full effect, to allow for the contrast to travel through the length of the bowel. Oral contrast may also include bicarbonate pellets ("fizzies") that release CO2 in the stomach to "open up" the stomach for better analysis. Intravenous contrast, either iodine for CT or a Gadolinium compound for MR, helps to analyze the solid organs, liver, spleen, pancreas and kidneys, as well as help characterize certain abnormalities and "masses" in the musculoskeletal and central nervous systems. The analysis of blood vessels and vascular structures also most often requires contrast. Contrast may also be given through the rectum, if necessary, (in the form of air, CO2 or liquid contrast) to distend and better analyze parts of the colon. The decision to use contrast is based on: the type of examination requested, the presumed diagnosis, (or diagnosis of exclusion) and information from the patient history or discussion with the referring physician. Most often, the final decision to use contrast is made by the radiologist, after determining that contrast use would be both safe and effective for the given patient.


Q. What is a contrast media?

A. This substance is given to highlight various body parts and is eliminated in a day or two. It is usually given by mouth or injection. It is normal for the patient to feel a warm sensation as the dye makes its way through their body.


Q. How long will the exam take?

A. Patients may spend 30 to 60 minutes in the office. This allows for the preparation as well as time for the computer to generate the images. Exam time may vary significantly depending on the nature of the study.


Q. When will the patient know the results?

A. The radiologist will study your films and report the findings to the referring physician within 24 hours. The referring physician will discuss the CT scan results with the patient.


Q. How do you decide whether to use CT or MR for a given problem?

A. Each modality has its advantages and limitations, which differ from one organ system to another.  Part of a radiologist's job, in consultation with the referring physician, is to help choose the safest, most effective means of evaluating a specific patient. This includes radiology-based vs. non-radiological laboratory testing, and choosing among the various imaging examinations, CT, MR, Ultrasound, Scintigraphy and fluoroscopy/X-ray.

Generally speaking, MR is most useful for neurological and musculoskeletal applications, due to its high sensitivity to subject contrast i.e. ability to discriminate between different types of solid tissue, both normal and abnormal. MR uses no X-rays, and may require an intravenous injection of Gadolinium contrast.

CT is a computerized X-ray procedure with advantages related to spatial resolution, (i.e. in the submillimeter range), freezing of physiologic motion, and detection of calcium. CT is well suited to the lungs, coronary arteries and gastrointestinal and urinary tract. CT applications often require the use of intravenous iodine contrast.

MR is also useful for evaluating solid organs, to clarify indeterminate thoraco-abdominal CT or ultrasound findings. MR can also be substituted for CT in certain situations, such as patients with allergy to iodine contrast, young, or chronically ill patients in whom repeated exposure to X-Rays ant CT could be of concern.


Q. How much radiation related cancer risk is inherent to Computer Tomography, CT ?

A. A recent article in the Archives of Internal Medicine regarding "projected cancer risks from Computer Tomography" has caused a great deal of anxiety among patients and their loved ones as a result of the ensuing publicity.  We feel that it is important to clarify the situation, in order to allay everyone's fears.

There is serious question in the scientific community as to the validity of studies which equate the effects of radiation exposure  in atom bomb survivors in Japan to present day patients undergoing CT scans.

According to the United States Nuclear Regulatory Commission, the annual background radiation exposure to an individual in the United States is approximately 300 millirem (3 msv), while those living at high altitude experience much greater background exposure To quote the USNRC, "These people living in areas having high levels of background radiation above 1000 millirem (10 msv) per year, such as Denver, Colorado, have shown no adverse biological effects". As an example, a typical chest CT in our office is approximately 100-150 millirem, 1-1.5 msv. Abdominal and pelvic CT scans are typically about 1000 millirem (10 msv). About the yearly background radiation in Denver, another example, an average airline flight crew receives an additional 300-600 millirem (3-6 msv) per year.


Q. What  measures can patients take to guard against being  'overexposed'?

A. To this end, we have added a very significant upgrade to our volume 64 (64 detector) CT scanner- Adaptive Statistical Iterative Reconstruction (ASIR). This software upgrade allows us to lower radiation dose by approximately 40% while improving resolution. We are proud to be the first radiology practice in the Metropolitan area to have added this upgrade.

In addition, we have independantly begun to "flag" patients whose past  medical radiation exposures exceeds a certain amount, based on having undergone multiple CT scans in our office or elsewhere. These patients are scanned  very dili-"gently" and advised, if necessary,  to discuss other choices for further imaging  with their treating physicians .

CT scanning is an extremely powerful and safe diagnostic tool. This trust placed in NYMI Associates for diagnostic imaging by doctors and their patients is paramount. We will continue to do everything in our power to safeguard it by limiting patients’ exposure.


Q. How can radiologists affect even the small risk that patients take when undergoing X-ray based procedures?

A. CT scans performed in an office such as ours are always monitored by our physicians and performed by very experienced staff, each of whom has more than 25 years of experience. We have a physicist on staff and our machines are constantly calibrated. All of our equipment and staff are accredited by the very stringent American College of Radiology. We participate in the ACR's "Image Gently" program. We adhere to the principle of ALARA- As Low As Reasonably Achievable in determining techniques for each examination. We pride ourselves in using the lowest radiation doses possible to achieve studies that are of superior diagnostic quality.


Q. What radiation doses do patients receive during CT examinations at NYMI Associates?

Typical Adult VCT / ASIR Effective Doses:

  • Low Dose CT CHEST: ~ 1.5 mSv
  • CT CALCIUM SCORE: ~ 1.5 mSv
  • CT COLONOGRAPHY: ~ 3 mSv
  • CT STONE STUDY: ~ 5 mSv
  • CT ENTEROGRAPHY:  ~ 9 mSv

Annual Natural Background Radiation:

  • Airline Pilots (allowed): ~ 50 mSv
  • Radiologists  (allowed):  ~ 50 mSv
  • NY Resident: ~ 3 mSv
  • Denver Resident: ~ 6 mSv

Effective Dose Of A Study Varies:

  • from patient to patient
  • from scan to scan

As A Result Of:

  • Patient size and age
  • Number of images taken
  • Type of resolution required (spatial vs. contrast)
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