The work describes a method to quantitatively evaluate spatial resolution in B-mode quality control images as radial modulation transfer function (MTF). The method is easy to use in in-field quality assurance programs in clinical institutions, as it is fast and can be performed with the help of phantoms that are already used for routine quality control. It uses a round insert with different echogenic characteristics compared with the phantom background. The method was automated with a MATLAB® routine, which can be run in full automatic mode or with a grade of interaction by the user, as is sometimes necessary with low-quality images. Radial MTF was evaluated for several ultrasound (US) images produced with different clinical US scanners from various vendors and equipped with different types of transducers (linear, convex, sector, vector) working at various frequencies (from 2 to 14 MHz) and with different modalities. It was also evaluated on various computed tomography (CT) images including round inserts. This was done to compare these results with the MTF calculated from the same images using the well-known method from the point spread function (PSF) of a high-contrast bead. Our radial MTF method, adapted to the specific characteristics of US images, was found to be reasonably robust and in line with other methods commonly used for USs (e.g., the pin method, which has repeatability issues) and in other fields, such as X-ray CT.