Saturday, July 18, 2009

How A CT Abdomen/Pelvis Is Performed

CT Scan of the Abdomen (Source: Wikipedia)

The patient was having nonspecific abdominal pain that had not resolved after several days in the hospital. On the suggestion of her kidney doctors, a computed tomography (CT) scan of the abdomen and pelvis with contrast was ordered. The patient desired the exam in order to find out the etiology (origin) of her pain.

When ordering any imaging study, CTs in particular, it is important to consider several things beforehand:
  1. Is this study necessary?
  2. Is this study appropriate?
  3. What are the risks of this study?
  4. What are the benefits of the study?
  5. Will the study reveal new information that will affect the management of the patient?
  6. For hospitalized patients, does this study need to be performed in-house, or can it done on an outpatient basis?
In the case of this patient, the answers to these questions were as follows:
  1. Yes, to determine the etiology of her pain, which could have potentially been serious.
  2. One could argue about whether an ultrasound should have been performed first, but a CT provides a great deal more information, so the CT was preferred.
  3. The major risks include radiation and contrast-induced nephropathy. The risk of radiation is development of a future cancer, but this is tied to cumulative radiation exposure, so one CT by itself is not a huge harm. Contrast-induced nephropathy is potentially quite serious, but in this patient, her kidney function was so depressed that there was not much kidney function to preserve to begin with.
  4. The benefits would be to rule out any potentially urgent, addressable causes of her pain.
  5. Yes, potentially (see #4)
  6. In practical terms, it made sense to do this study as an inpatient, versus having the patient come back for a separate study.
After the order was sent by the primary team, the radiologist has to look over the order to ensure that it is appropriate and then protocol it. The protocol is the specific routine that the scanner will run while acquiring the images. After approval, the patient was taken down to the CT scanner room. Contrast was infused into her via an intravenous (IV) line. Since the area of interest was any soft tissue mass causing the pain, the scanner was programmed to wait until the contrast had entered the venous phase after perfusing through the arteries, roughly 80 seconds after the infusion began. At this point, the scanner spun into action, using its radiation beam and detectors to create the typical cross-sectional images of a CT scan. The scanner only scanned the patient's abdomen and pelvis in order to ensure a limited dose of radiation. After the scan, the patient was returned to her room. The images were sent to the radiologist's Picture Archiving and Communication System (PACS) machine, where she was able to interpret the images minutes after the scan had been taken.

Fortunately for the patient, the scan was negative and she was discharged home. The contrast would be dialyzed off at a later date.

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