Comments on 'Cerenkov radiation allows in vivo optical imaging of positron emitting radiotracers'.

In a recent paper (Spinelli et al 2010 Phys. Med. Biol. 55 483-95) the authors report on their measurements and observations regarding the use of optical imaging of Cerenkov radiation to observe the distribution of radiotracer in a mouse. The paper, while broadly correct, develops a detailed model of the Cerenkov radiation spectrum that does not appropriately consider the particle energy and the distance travelled while velocity exceeds the Cerenkov threshold. Also, we note the authors' two different methods for determining the depth of the source appear in fact to be the same method if the first method properly accounts for the spectrum of the emitted radiation.