Situs Inversus with Dextrocardia MPI Protocol using SPECT/CT

Purpose: Dextrocardia with situs inversus is a condition where a patient’s heart along with their chest and abdominal organs are oriented in a mirror image from the anatomical norm. Therefore, the patient’s heart lies to the right of the midline with the apex of the heart pointing toward the right side of the body. SPECT/CT systems work well when performing myocardial perfusion imaging (MPI) procedures on patients with standard anatomy, but oftentimes the camera protocols fail to easily acquire dextrocardia with situs inversus patients due to their condition. The purpose of this research is to establish a MPI protocol for imaging patients who have dextrocardia with sinus inversus to be utilized by our department.

Methods: A SPECT phantom with a cardiac insert (Data Spectrum Corporation) containing a mid-anterior defect was acquired using a Siemen’s Symbia T-series SPECT/CT system equipped with low energy high resolution (LEHR) collimators. The cardiac insert and the phantom were filled with 0.35 mCi and 4.52 mCi of Tc-sestimibi, respectively. The camera manufacturer’s acquisition protocol for situs inversus used the same parameters as a standard MPI study with the exceptions that the starting angle was at 135 and the camera heads were positioned so that Head 1 was at the right lateral and Head 2 at the anterior. Acquired images were then processed using the camera manufacturer’s situs inversus processing protocol, which flipped the images to appear in standard anatomic position for easier reading by the physicians. Results: The camera manufacturer’s protocols successfully acquired and processed the cardiac phantom placed in a dextrocardia with situs inversus position on the imaging table. During the reconstruction process, masking the background was difficult because the short axis images did not appear on the computer screen. The protocol states that “images might not appear,” however, since this step is required to center the images properly, the user must blindly manipulate the images until they appear in standardized position for reading. The defect in the mid anterior wall of the cardiac phantom was visualized, but the lack of labels made locating the defect extremely difficult. Since this protocol was acquired with a phantom, other issues, such as gating, patient motion, electrocardiogram (EKG) lead placement, etc., may further complicate the protocol when acquiring a patient.

Conclusions: Images acquired using a cardiac phantom placed in the dextrocardia with situs inversus position were successfully acquired and reconstructed, albeit with many difficulties. The camera manufacturer’s protocol, which has not been validated, offered a starting point for our department to successfully develop a protocol for this type of patient.