Final answer:
MRI is the investigation of choice for lateral sinus thrombosis, and ultrasound is the safest imaging method during pregnancy because it does not use ionizing radiation, while CT and PET scans pose a higher risk due to exposure to radiation.
Step-by-step explanation:
The investigation of choice for lateral sinus thrombosis is Magnetic Resonance Imaging (MRI). An MRI provides detailed images of the brain and the blood vessels, making it highly effective for detecting conditions such as sinus thrombosis. When it comes to imaging during pregnancy, it is important to use methods that do not pose a risk to the fetus. Among the imaging techniques mentioned, ultrasound is considered the safest option for use during pregnancy, as it does not use ionizing radiation. While CT scans are sometimes used during pregnancy when necessary, they are generally avoided due to the risks associated with radiation exposure. In contrast, PET scans employ radiopharmaceuticals, which also carry a risk of radiation and are typically not recommended during pregnancy.