Can you be too thin for hi def lipo? I have a BMI of 21.5. I am 5 ft tall and weigh 110 pounds

No, you can still get high definition VASER liposuction. Considering your BMI you likely want to focus more on contouring and definition than comprehensive fat removal. High definition VASER liposuction is the most efficient technique for body etching. A Skype or FaceTime consultation with a VASER specialist will help you get an accurate recommendation and pricing.
Respectfully, 
Arian Mowlavi MD, FACS
Realself 100 Hall of Fame Inductee

some people can get great liposuction results when it comes to getting definition. You should not expect to see significant contour changes in regards to overall body proportions since you don’t have enough fat to make changes that way. For example your clothes will fit the same but your muscle wall of your abdomen could look better. Personally I would approach high definition to work with a great deal of caution. This is generally based on purposely giving people an abnormal fat distribution with the Intention of trying to make fat look like muscle. To me the results look like what they are. An artificial attempt to make fat look like muscle. It’s definitely a much better procedure for thin people then people With even modest amounts of body fat. This procedure is extremely unforgiving for technical errors and any results obtained can generally never be reversed. If those lines are etched incorrectly you will live with them for the rest of your life. Also, if you gain weight in the future then the results can look ridiculous. Personally I prefer a more natural thin even fat distribution that is natural and mimics extreme weight loss.For sure this should only be done by those who Have a lot of experience. Preferably many thousands of cases under their belt. This is not a good procedure to have done by someone who bought a VASER in the last 12 months and just took the body contouring course by Alfredo Hoyos.Best, Mats Hagstrom MD