Dose of Intravenous Contrast Agents for Enhanced Spiral CT Scanning

Dose of intravenous contrast agents for enhanced spiral CT scan, body weight considerations.

The contrast agent used during the time of development of these protocols, was Ultravist 300 from Bracco Diagnostics Inc.. It is a non-ionic agent, and contains 300mg of Iodine per ml. Other contrast agents could be substituted (such as Omnipaque 300, Isovue 300, etc). The table's Dose column shows the volume of Ultravist 300 to be injected, in milli-litres.

Fixed dose versus weight-based contrast doses.

Historically, enhanced CT scanning has been performed using a fixed dose of contrast agent for all patients. Typically 100ml (or 150ml) is used for all patients regardless of body weight. Using a weight-based scheme for contrast dose is less common, but there is certainly radiology literature support for it^1,2. An arithmetic formula is sometimes used to calculate the contrast dose, (eg 1.5ml/kilogram) and sometimes a simpler division of patients into 2 or 3 categories is used. (eg. small/medium/large.)

One of the barriers against using a weight-based dose, is that the CT technologists fear it will take more time. Indeed, it does take extra time to ask the patient "how much do you weight?", and if they don't know, it takes extra time to ask the patient to stand on a scale. Our weight-based method uses a look-up table, so there is no formula to calculate. The numbers in the lookup table can be memorized by the CT technologists, so there is virtually no extra time needed to use the look-up table. It takes extra time to type the parameters for injection rate and scan delay into the power injector machine. Our own CT technologists were initially reluctant to switch to using the weight-based contrast doses, but with experience, they estimate that the extra time is less than one minute per patient.

There are several advantages to using a weight-based contrast dose.

All patents get a uniformly good quality scan. In contrast, with fixed doses, the lighter patients get too much contrast, and the heavier patients get too little (if 100ml) or barely enough (if 150ml).
Radiologist's dose variability can be eliminated. Radiology groups big and small, are familiar with the difficulties in getting a group of radiologists to agree on a technical parameter. Some radiologists will want to save money and use 100ml per patient, which is known to be "satisfactory". Other radiologists will want to use 150ml, since higher doses give better quality scans. If a CT practice allows its radiologists to individually determine their own dose prescriptions, the result will be an unpredictable mixture of doses. Isn't it silly, for a patient to get 150ml on one CT scan, and 100ml on a followup CT scan? Our own CT practice once had this problem. For example, a patient with cancer had 4 followup scans and received 100ml, 150ml, 120ml, and 80ml on successive scans. The liver metastases changed apparent size on each scan, simply due to the technical variations. It was precisely this type of problem that prompted us to standardize our doses. Implementing a weight-based scheme was a good way to get everyone to agree.
Overall quality can be improved without raising costs. Compared to a fixed dose of 100ml, weight based doses cost only 3% more, in our patient population. Compared to a fixed dose of 150ml, the weight-based doses cost 31% less! With the weight-based approach, the median dose is 100ml. That is, most commonly, the average patient receives a 100ml dose. The average dose is 103.0ml within the population. Use this number for cost calculations.

Compare the Weight-based contrast doses to graphs of the former standards.

The adult standard fixed doses of 100ml or 150ml, and the pediatrics standard of 2.0 ml/kg.

A problem? There is no logical transition between pediatrics to adult fixed doses.

The Weight-based Doses, shown by the thicker line, allows a much better transition between pediatrics and adult doses.

How about using a specific formula to calculate dose, instead of a lookup table? What formula would be good?
For arguments sake, set all formulas to give a 100ml dose to the average patient. The possibilities include a "per kilogram" formula, or a "per BSA" formula (calculated from height and weight). There are at least 2 formulas to calculate Body Surface Area (BSA), including the DuBois method and the Mosteller method.

The thicker line, shows the actual doses, and the other lines show the theoretical doses that would have been given, if a formula calculation had been used, for example, using per kilogram dose (1.416ml/kg) or per BSA doses using the the DuBois formula ( 55.02ml/m²) or the Mosteller formula (54.3ml/m²).

As the graphs show, the actual doses given, from the weight-based look table, are a good compromise between "per kilogram" and "per BSA" formulas.

References

Uchida M, Edamitsu O, Chang Y, Nishimura H, Hayabuchi N, Dynamic study of the liver with helical scanning: determination of hepatic contrast enhancement in routine studies. Nippon Igaku Hoshasen Gakkai Zasshi. 1996; 56:502-6
Heiken JP, Brink JA, McClennan BL, Sagel Ss, Crowe TM, Gaines MV, Dynamic incremental CT: effect of volume and concentration of contrast material and patient weight on hepatic enhancement. Radiology 1995; 195:353-7

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