He is board certified in Nuclear Medicine and his research is focused on molecular imaging with PET and PET/MRI and targeted radionuclide therapies (theranostics) in cancer.

He has published more than 400 peer-review articles, filed 10 patents, supervised more than 40 PhD students and received numerous prestigious scientific awards over the years.

[11] He served as president of the Scandinavian Society for Clinical Physiology and Nuclear Medicine (SSCPNM) 2006-12 [12] and currently is editor-in-chief of Diagnostics (Basel)[13] as well as on advisory boards and committees.

Kjær's research has focused on translational molecular imaging in with PET and PET/MRI and on development of targeted radionuclide therapies (theranostics) for use in precision medicine in cancer.

[17][18][19][20] His group also demonstrated the prognostic value of FDG-PET in neuroendocrine tumor patients[21][22] and were first to combine 13C-hyperpolarized MRSI and PET on a clinical PET/MRI scanner, a technique they named hyperPET.