Do not get get confused, normally an PET scan is associated with tumors, but an MRI is also equally as useful when involving the head.
Let me explain as to the most logical answer. The MRI or magnetic resonance imaging, is going to be the one you want to use for this. An MRI is useful for giving a multi side analysis of organs including of the head. The MRI uses electronic frequencies inside of a magnetic field in order to develop a 3D (not usual 3D, but multiple angles). This type of imaging is useful for finding anything that up with the brain including a tumor.
An PET scan or Positron-emission tomography, which is normally tasked with finding Malignancies is just that. The thing is, that it is absurd to use a PET scan to find a location of said tumor, because the truth is, it only really able to detect malignancies. If the tumor is benign, meaning it is not cancerous, then the PET is useless. The PET scan uses around the same tech as a CT, however it moniters the usage of radioactive glucose that is injected into the body. The reason why it can only detect malignancies is that a malignant growth will take up more sugars than the surrounding tissues.
An Ultrasound could potentially be used, but it just be uncomfortable for the patient, plus it is not useful if the tumor is too deep in the brain. It is more useful for detecting hemtomas or checking blood circulation in the head.