Approaches to Dose Reduction in Molecular Imaging
Biological parameters are the pharmacokinetic properties of the radiopharmaceutical, which influence the biological half-life and distribution pattern, and change dynamically after the application. Effective half-life combines the effects of physical decay (physical half-life) and excretion (biological half-life):
1/T1/2eff = 1/T1/2phys + 1/T1/2biol
The calculation of effective dose in an individual varies due to differences in excretion time, as well as anatomical differences. Dose calculation reflects the absorbed radiation in the body both from the radiopharmaceutical residing in that region and the radiation dose from distant organs/regions. Thus, some simplifications – such as standardized anatomy – are made for the assessment of dose for diagnostic purposes, as in Organ Level INternal Dose Assessment
Package inserts describe the effective doses per administered MBq as well as the organ doses per MBq for, at minimum, critical organs for a standard patient with respect to size, weight, distribution pattern and excretion.
The impact of the molecular imaging system on the dose is only influenced by the system’s minimum activity requirements for a given image quality and acquisition duration.
There is a wide range of typical effective doses for different nuclear medicine procedures. Some are far below 1 mSv (e.g. the Schilling test), while others may exceed 10 mSv (e.g. gallium scintigraphy). The effective dose for a typical 18F-FDG injection used for a PET•CT is 5–7 mSv. Most procedures, however, result in doses between 1 and 10 mSv.
In hybrid devices, the burden of radiation stems not only from the use of radiopharmaceuticals but also from the CT component as well. Thus, advances in scanner technology allow the reduction of injected activity, and hence, the effective dose.
In addition to activity reduction, other considerations include:
1. The usage of ionizing radiation always requires a justified indication. Can the same clinical result reasonably be achieved using any other examination?
2. Can the overall diagnostic dose be reduced by judiciously combining examination types? Good clinical planning may reduce the number of CT examinations if diagnostic CT and CT for hybrid imaging are combined in a single scan.
3.Isotopes with a shorter half-life and favorable radiation type can reduce radiation exposure dramatically. For example, the use of 123I-MIBG instead of 131I-MIBG in the diagnosis of neuroendocrine tumors improves image quality and reduces radiation exposure, due to a much shorter half-life (13 hours instead of eight days) and radiation type (gamma emitter with a main peak of 151 keV versus combined beta and gamma emitter). Another example is the use of 18F-FDG-PET instead of 67Ga scintigraphy in lymphoma and inflammation cases. In addition to a much lower radiation exposure, image quality and precision are much better and the overall time from injection to end of scan is substantially reduced (1.5–2 hours versus 3–4 days).
4. The choice of the radiopharmaceutical can also reduce the radiation burden. 99mTc-MAG3 shows a much higher renal uptake and clearance than 99mTc-DTPA. This allows injected activity to be reduced by a factor of 2 while maintaining the
same image quality and precision with respect to urodynamics.
5. Not every clinical question requires striving to achieve the best possible image quality through the combination of high injected activity and advanced acquisition and reconstruction tools. In some cases, a somewhat reduced image quality, which permits lower injected activity, still fulfills the clinical need.
6. Several countries have issued guidelines for maximum standard activities according to examination type. Pediatric activities can be calculated adherent to special rules. In the past, effective doses in children were rather high due to insufficient adjustments to the guidelines, which have since changed.