The term "imprisonment" is synonymous with "incarceration" - or, at least the potential for such incarceration. In many states one can be convicted of a felony and never even go to jail OR prison, but, if the offense has the potential of incarceration for a year or more, then it qualifies. In CA now lesser felonies that had previously gone to state prison are now being served locally .... apparently the state thinks that the counties have more money so they have shifted responsibility for felons back to the counties after they took money away from them and gave a little back to pay for this for a year ... fun being in CA! Do more with less ... and for less pay and fewer benefits!
It would seem to me to be odd that MA has no means by which these convictions can be overlooked for purposes of becoming a peace officer - at least in MA, but, if no such process exists, then it adds a new wrinkle to the idea that your actions as a child can haunt you for the rest of your life.