A. Definitions: Based on Ci.
This unit of absolute activity is abbreviated Ci and is expressed in terms of disintegrations per second (dps). A Curie is represented by a sample with a decay rate of 3.7 X 10¹⁰ dps or 2.22 X 10¹² dpm.

B. Definitions: Based on Bq

C. Useful Equivalents

37 MBq = 1 mCi

1 MBq = 0.027 mCi = 27 μCi

D. Tracer level work

Most tracer level work performed in Nuclear Medicine is on the μCi to mCi range. Less than 1 μCi is required for a measurement in a well counter. Imaging procedures require many μCi and often mCi

E. Potential source of confusion

One disintegration, representing the disintegration of a nucleus, is an absolute event that took place. Unfortunately, our detectors are never 100% efficient and in fact their efficiency generally falls in the range of 40 – 60%. In addition, counting geometry is never ideal. Depending upon detector efficiency and counting geometry, we may or may not be able to detect an individual event. 1 count may therefore not represent 1 disintegration.

F. Detector Efficiency

Detector efficiency indicates what fraction of total disintegrations is recognized by the detector. If we count a standard whose activity is precisely known, then mathematically,

G. Mathematical Problem Solving

Let’s assume that the detector efficiency of a particular detector is 47%. In problem solving, one may use either

H. Specific Activity Specific activity is defined as the activity per unit mass (e.g., mCi/g).

It is unrelated to concentration in solution, which is activity per unit volume (e.g., mCi/ml). Specific Activity must contain a term related to mCi or disintegrations and a term related to mass such as a gram or mole term.

For example, which one of the following is NOT an example of specific activity?

mCi/mg

kCi/μmole

cpm/mmole

dps/g

ANSWER:cpm/mmole.
Since specific activity is defined as activity/unit mass, the numerator must be directly relatable to disintegrations. Counts can not be related to disintegrations unless the detector efficiency is known.