PET•CT Myocardial Perfusion Combined with CT Angiography in Post-Revascularization Ischemia

Identification of lesion by combining myocardial perfusion, CT angiography and syngo Circulation

Seng Chuan Ong , MD, FRCR, Hee Kit Lai, MD, MRCP and Robert Kwok, MD, FRCR

Case study data provided by Mount Elizabeth Hospital Singapore (Parkway Group), Singapore


 |  2013-06-19

History

A 74-year-old man with history of coronary artery disease treated with bypass grafting (CABG) 4 years ago, presented to the hospital with worsening exertional dyspnea and occasional chest pain. His resting ECG showed sinus rhythm with no acute stress changes. He was referred for a stress-rest Rubidium-82 (Rb-82) PET•CT myocardial perfusion study.
Dynamic Rb-82 stress-rest myocardial perfusion study was performed on a Siemens Biograph™ mCT 40. Dosage of Rb infusion was based on patient’s body weight at 15 MBq/kg. A rest study was first obtained. Imaging acquisition was over 8 minutes starting from time of Rb-82 infusion. This was followed by dipyridamole (persantin) infusion at 0.14 mg/kg/min over 4 minutes. Stress imaging was then acquired at peak stress (8 minutes after start of persantin infusion).
A CT coronary angiography (CTA) was performed at the end of the PET perfusion study. Stress and rest PET perfusion studies were evaluated with 4DMSPECT, and the CT angiography was evaluated using syngo® Circulation. Biograph mCT with LSO crystals and ultraFast electronics provides high count rate capability for excellent image quality in dynamic Rb-82 myocardial perfusion imaging, especially in obese patients. The unique detector design of Biograph mCT avoids detector saturation at higher doses of Rubidium and thus is able to perform dynamic PET studies for myocardial blood flow estimations at the higher Rubidium doses required for obese patients. syngo Circulation is a Siemens software that evaluates CT angiography from all vendors and is able to fuse PET data and CT angiography with volume rendering in order to improve diagnostic confidence. 

 

Examination Protocol
Scanner: Biograph mCT 40
Dose: Rb-82 40 mCi [15 MBq/kg] injection
Acquisition: Dynamic List mode
CT : Low dose for CTAC

 

CT Coronary angiography
Dose: 80 ml contrast injection
CT: 100 kV, 300 eff mAs, 1 mm slice

Diagnosis
Comparison of static short-axis, horizontal-axis and vertical-long-axis slices of stress and rest Rb-82 myocardial perfusion PET•CT studies show a large perfusion defect in the lateral wall in the stress images, which corresponds to territory perfused by the left circumflex artery, with complete reversibility as demonstrated by normal uptake at rest. There is mild to moderate left ventricular dilatation during stress, which reflects the severity of lateral wall ischemia. The rest of the left ventricle (LV) myocardium shows normal perfusion.

Comments
In view of the stress-induced perfusion defect, the CT angiography was performed to evaluate arterial and venous graft patency in this patient with history of CABG.
CT angiography showed patent left internal mammary artery (LIMA) graft to mid left anterior descending artery (LAD), as well as patent venous graft to distal right coronary artery (RCA). The venous graft to first obtuse marginal (OM1) was patent at origin, as well as the entire length of the graft. However, the graft insertion at the level of proximal OM1 appeared irregular with absence of contrast runoff into OM1 distal to the graft insertion. This suggested the possibility of an obstruction to contrast flow at the insertion level, which may explain the stress-induced ischemia in the lateral wall exactly cor¬responding to the OM1 territory.
Native vessels including LAD, left circumflex were heavily calcified and narrow throughout their entire extent, which reflects shunting of blood to the patent grafts.
Fusion of stress and rest PET perfusion and coronary tree derived from CTA using syngo Circulation show the extent of stress-induced ischemia with complete normalization of perfusion at rest (thin arrow). The ischemic zone corresponds to the myocardium supplied by OM1 with irregularities at the insertion of the venous graft to OM1 (thick arrow). The LIMA graft to LAD and venous graft to RCA are distinctly patent with no corresponding ischemic zones.
In view of the patency of all other grafts and absence of ischemia in all of LV myocardium except the lateral wall (OM1) distribution, as well as the irregularities visualized on the CTA at the insertion of venous graft to OM1 and lack of opacification of the mid and distal OM1 on CTA, there appeared to be a strong possibility of a graft block at the OM1 insertion which, if confirmed by catheter angiography, is amenable to an angioplasty. Combined imaging Biograph mCT 40 of Rb-82, stress-rest myocardial perfusion and CT coronary angiography along with hybrid display of coronary tree with volume-rendered, quantitatively accurate PET perfusion data using syngo Circulation was key to identification of the culprit lesion at the graft insertion.

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