Final answer:
Ultrasound is the best imaging modality for differentiating between solid and cystic breast masses due to its ability to depict fluid-filled and solid structures without the use of radiation, making it safe during pregnancy as well.
Step-by-step explanation:
The imaging modality that helps to differentiate between solid and cystic breast masses is ultrasound. Ultrasound technology is the least invasive and uses no electromagnetic radiation, making it safe for use during pregnancy as well. It is excellent for distinguishing between solid masses, which could be malignant or benign tumors, and cystic masses, which are typically fluid-filled and often benign. Meanwhile, mammography can indicate the presence of a lump or region of increased density but is not as effective at differentiating between solid and cystic masses as ultrasound. MRI (Magnetic resonance imaging), which uses magnetic fields and radio waves to generate images and is known for not exposing patients to radiation, has been used as a supplement to conventional x-rays to improve detection and reduce false positives in breast imaging. On the other hand, PET scans use radiopharmaceuticals and are not typically used for distinguishing between solid and cystic breast masses.