This post explains how to do an ultrasound-guided piriformis muscle injection according to Chang et al. 2017:
Entrapment of the sciatic nerve within (a swollen) piriformis muscle can cause pain and numbness at the buttock which may radiate to the ipsilateral thigh and leg. For injection, the patient is positioned prone and the transducer is placed along the long axis of the piriformis muscle. The sciatic nerve should be scrutinized to prevent incidental injury because the nerve may sometimes pass through/above the piriformis muscle. The power Doppler should be turned on to prevent injury of the ascending branches of the inferior gluteal artery, which are located underneath the gluteus maximus muscle. The needle is inserted from the medial to lateral aspect through an in-plane approach, targeting the painful point of the piriformis muscle. The painful point is likely to be found at the muscle tendinous junction, which is easily impinged between the gluteus maximus muscle above and the body of the ilium below.