5 2009 Fermi Symposium, Washington, D.C., Nov. 2-5 Figure 1: VLBI images at 8.4 GHz and radio continuum spectra of sources for which the synchrotron self- absorption turnover frequency was successfully measured. The shaded area on the images marks the region where the synchrotron turnover was detected. The color of the shaded area represents the upper limit of the magnetic field strength ( B$_{⊥}$ ). The values of B$_{⊥}$ and p presented in Table I correspond to this area. On the spectrum plots, the blue curves represent the homogeneous synchrotron source model, while the green lines represent a simple power law model. Best fit parameters of both models are shown on each plot. The † sym- bol marks sources for which the analysis results are preliminary. (a) AO 0235+16 image (b) AO 0235+16 spectrum (c) 0528+134 image (d) 0528+134 spectrum (e) OJ 248 image (f) OJ 248 spectrum (g) W Com image † (h) W Com spectrum † (i) 3C 273 image (j) 3C 273 spectrum (k) 3C 279 image (l) 3C 279 spectrum (m) 4C 38.41 image † (n) 4C 38.41 spectrum † (o) Mrk 501 image † (p) Mrk 501 spectrum † eConf C091122