Intertransverse process block (ITPB) is a thoracic nerve block technique where the LA is injected into the thoracic "intertransverse tissue complex" posterior to the superior costotransverse ligament (SCTL). It may be an attractive alternative to a TPVB for unilateral surgery on the thorax, when considering the risk for pleural puncture, since it does not require direct needle access to the thoracic paravertebral space. However, although ITPB is ultrasound guided, the LA is frequently injected using surrogate bony landmarks in the sagittal sonogram, and in most cases without even identifying the SCTL.

The knowledge regarding the spread of local anesthetic is a major prerequisite for a safe and effective use of all regional anesthetic methods in clinical practice. The primary outcome is the spread of injectate which will be evaluated on axial three-dimensional MRI with contrast medium. The secondary outcome is the sensory dermatomal blockade of this novel ITPB block.

Inclusion Criteria:

• healthy Thai men with no diseases

Exclusion Criteria:

• inability to cooperate or communicate in Thai
• BMI ≥ 30 kg/m2 or body weight ≤ 45 kgs
• allergy to local anesthetics or contrast agents
• infection in the block area
• previous surgery or anatomical abnormalities of thoracic spines closed to block area
• coagulation disorder/thrombocytopenia/anticoagulation therapy
• contraindications to magnetic resonance imaging (MRI) (metal implants - pacemaker and others, claustrophobia)