Myocardial viability assessment using nuclear imaging

Utility of cardiac imaging in patients with ventricular tachycardia

Ventricular tachycardia (VT) is a life-threatening arrhythmia that may be idiopathic or result from structural heart disease. Cardiac imaging is critical in the diagnostic workup and risk stratification of patients with VT. Data gained from cardiac imaging provides information on likely mechanisms and sites of origin, as well as risk of intervention. Pre-procedural imaging can be used to plan access route(s) and identify patients where post-procedural intensive care may be required. Integration of cardiac imaging into electroanatomical mapping systems during catheter ablation procedures can facilitate the optimal approach, reduce radiation dose, and may improve clinical outcomes. Intraprocedural imaging helps guide catheter position, target substrate, and identify complications early. This review summarises the contemporary imaging modalities used in patients with VT, and their uses both pre-procedurally and intra-procedurally.

Substrate-based approaches in ventricular tachycardia ablation

Catheter ablation for ventricular tachycardia (VT) in patients with structural heart disease is now part of standard care. Mapping and ablation of the clinical VT is often limited when the VT is noninducible, nonsustained or not haemodynamically tolerated. Substrate-based ablation strategies have been developed in an aim to treat VT in this setting and, subsequently, have been shown to improve outcomes in VT ablation when compared to focused ablation of mapped VTs. Since the initial description of linear ablation lines targeting ventricular scar, many different approaches to substrate-based VT ablation have been developed. Strategies can broadly be divided into three categories: 1) targeting abnormal electrograms, 2) anatomical targeting of conduction channels between areas of myocardial scar, and 3) targeting areas of slow and/or decremental conduction, identified with “functional” substrate mapping techniques. This review summarises contemporary substrate-based ablation strategies, along with their strengths and weaknesses.

Myocardial viability imaging using a novel non-breath-hold cardiac MRI protocol: a comparative assessment with 18F-FDG PET

Background

Delayed contrast enhancement on cardiac magnetic resonance imaging is a well-established MRI technique for the evaluation of myocardial tissue viability. A comprehensive cardiac MRI protocol for myocardial viability comprises of multiple breath hold sequences to provide information regarding chamber volumes, myocardial mass, and function in addition to viability. However, its routine use is limited mainly by lengthy acquisition time and patient’s inability to hold breath in multiple breath hold sequences. The important question to which the referring cardiologists/cardiac surgeons are seeking answer is that “What are the viable vascular territories that will benefit from revascularization?” In this study, we have analyzed the utility of non-breath-hold rapid delayed contrast-enhanced cardiac MR imaging protocol for myocardial viability assessment with 18-flourodeoxyglucose positron emission tomography (18F-FDG PET) as the reference standard.

Results

Forty patients of ischemic heart disease who met the inclusion criteria were included. All patients underwent both 18F-FDG PET and delayed contrast-enhanced cardiac magnetic resonance imaging as per the non-breath-hold protocol. In this free-breathing protocol, the breath hold cine sequences were omitted and, after localizers, post-contrast scans were obtained with a time gap of 15 min post-contrast administration. A total number of 680 myocardial segments and corresponding 120 vascular territories were assessed. MRI and 18F-FDG PET images were analyzed using a 17-segment model as proposed by AHA. Sensitivity, specificity, positive predictive value, and negative predictive value of non-breath-hold rapid delayed contrast-enhanced cardiac MR imaging protocol for assessing myocardial viability (on segment analysis) was 95.5%, 65.59%, 88.0%, and 84.72%, respectively, and of vascular territory analysis were 96.77%, 92.59%, 97.83%, and 89.29%, respectively, in relation to 18F-FDG PET used as the reference standard. Spearman’s rank correlation coefficient is 0.62.

Conclusion

This modified non-breath-hold delayed contrast-enhanced cardiac MR imaging protocol is a reliable tool to answer the clinically relevant question of myocardial viability with a significant reduction in acquisition time and overcomes the limiting need of breath hold.

Clinical significance of quantitative assessment of glucose utilization in patients with ischemic cardiomyopathy

BACKGROUND

The aim of this study was to prospectively quantify the rate of myocardial glucose uptake (MRGlu) in myocardium with different perfusion-metabolism patterns and determine its prognostic value in patients with ischemic cardiomyopathy.

METHODS AND RESULTS

79 patients with ischemic cardiomyopathy were prospectively enrolled for dynamic cardiac FDG PET, and then followed for at least 6 months. Perfusion-metabolism patterns were determined based on visual score analysis of 201Tl SPECT and FDG PET. MRGlu was analyzed using the Patlak kinetic model. The primary end-point was cardiovascular mortality. Significantly higher MRGlu was observed in viable compared with non-viable areas. Negative correlations were found between MRGlu in transmural match and a history of hyperlipidemia, statin usage, and triglyceride levels. Diabetic patients receiving dipeptidyl peptidase-4 inhibitors (DPP4i) had a significantly lower MRGlu in transmural match, mismatch, and reverse mismatch. Patients with MRGlu in transmural match ≥ 23.40 or reverse mismatch ≥ 36.90 had a worse outcome.

CONCLUSIONS

Myocardial glucose utilization was influenced by substrates and medications, including statins and DPP4i. MRGlu could discriminate between viable and non-viable myocardium, and MRGlu in transmural match and reverse mismatch may be prognostic predictors of cardiovascular death in patients with ischemic cardiomyopathy.

Cardiac Imaging in Patients With Ventricular Tachycardia

Ventricular tachycardia (VT) is a major cause of sudden cardiac death. The majority of malignant VTs occur in patients with structural heart disease. Multimodality imaging techniques play an integral role in determining the underlying etiology and prognostic significance of VT. In recent years, advances in imaging technology have enabled characterization of the structural arrhythmogenic substrate in patients with VT with increasing precision. In parallel with these advances, the role of cardiac imaging has expanded from a largely diagnostic tool to an adjunctive tool to guide interventional approaches for treatment of VT. Invasive and noninvasive imaging techniques, often used in combination, have made it possible to integrate structural and electrophysiological information during VT ablation procedures. An important area of current development is the use of noninvasive imaging techniques based on body surface electrocardiographic mapping to elucidate the mechanisms of VT. In the future, these techniques may provide a priori information on mechanisms of VT in patients undergoing interventional procedures. This review provides an overview of the role of cardiac imaging in patients with VT.

Interpolated average CT for cardiac PET/CT attenuation correction

BACKGROUND

Previously, we proposed interpolated averaged CT (IACT) for improved attenuation correction (AC) in thoracic PET/CT. This study aims to evaluate its feasibility and effectiveness on cardiac PET/CT.

METHODS

We simulated (18)F-FDG distribution using the XCAT phantom with normal and abnormal cardiac uptake. Average activity and attenuation maps represented static PET and respiration average CT (ACT), respectively, while the attenuation maps of end-inspiration/expiration represented 2 helical CTs (HCT). IACT was obtained by averaging the 2 extreme phases and the interpolated phases generated between them. Later, we recruited 4 patients who were scanned 1 hr post 315-428 MBq (18)F-FDG injection. Simulated and clinical PET sinograms were reconstructed with AC using (1) HCT, (2) IACT, and (3) ACT. Polar plots and the 17-segment plots were analyzed. Two regions-of-interest were drawn on lesion and background area to obtain the intensity ratio (IR).

RESULTS

Polar plots of PETIACT-AC were more similar to PETACT-AC in both simulation and clinical data. Artifacts were observed in various segments in PETHCT-AC. IR differences of HCT as compared to the phantom were up to ~20%.

CONCLUSIONS

IACT-AC reduced respiratory artifacts and improved PET/CT matching similarly to ACT-AC. It is a promising low-dose alternate of ACT for cardiac PET/CT.

Role of PET-CT in the assessment of myocardial viability in patients with left ventricular dysfunction

AIM

Role of PET-CT in assessment of myocardial viability in patients with LV dysfunction.

METHODS

This prospective study included 120 patients with LV dysfunction who underwent 99mTechnetium-Sestamibi myocardial perfusion SPECT-CT and 18FFDG cardiac PET-CT. They also underwent serial echocardiography and coronary angiography along with myocardial perfusion and FDG PET study.

RESULTS

Thirty-three patients had single vessel disease, 48 had triple vessel disease, and rest had double vessel disease. Among 786 segments, matched defects were seen in 432 (55%) and mismatched defects in 354 (45%) segments. 78 patients were surgically managed, and 42 were medically managed. The change in LVEF after surgical management was statistically significant compared to medical management.

CONCLUSION

Viability assessment should be performed in patients who present after 12h of acute myocardial infarction or with LV dysfunction due to ischemic heart disease to decide upon appropriate surgical management.

Derivation of a respiration trigger signal in small animal list-mode PET based on respiration-induced variations of the ECG signal

BACKGROUND

Raw PET list-mode data contains motion artifacts causing image blurring and decreased spatial resolution. Unless corrected, this leads to underestimation of the tracer uptake and overestimation of the lesion size, as well as inaccuracies with regard to left ventricular volume and ejection fraction (LVEF), especially in small animal imaging.

METHODS AND RESULTS

A respiratory trigger signal from respiration-induced variations in the electro-cardiogram (ECG) was detected. Original and revised list-mode PET data were used for calculation of left ventricular function parameters using both respiratory gating techniques. For adequately triggered datasets we saw no difference in mean respiratory cycle period between the reference standard (RRS) and the ECG-based (ERS) methods (1120 ± 159 ms vs 1120 ± 159 ms; P = n.s.). While the ECG-based method showed somewhat higher signal noise (66 ± 22 ms vs 51 ± 29 ms; P < .001), both respiratory triggering techniques yielded similar estimates for EDV, ESV, LVEF (RRS: 387 ± 56 µL, 162 ± 34 µL, 59 ± 5%; ERS: 389 ± 59 µL, 163 ± 35 µL, 59 ± 4%; P = n.s.).

CONCLUSIONS

This study showed that respiratory gating signals can be accurately derived from cardiac trigger information alone, without the additional requirement for dedicated measurement of the respiratory motion in rats.

Evaluation of Flap Tissue Viability by F-18 FDG PET/CT

A 60-year-old man underwent vertical rectus abdominis myocutaneous flap to reconstruct a left lateral chest wall defect. For assessment of viability of muscle flap, F-18 fluoro-2-deoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) was performed 15 days after surgery. The FDG PET/CT showed a small metabolic defect in the left lateral chest wall. During follow-up, necrotic change of the graft was observed, and the site was in accordance with the area where the metabolic defect was observed in the FDG PET/CT. As a result, debridement and wound closure was performed. This case suggested that the FDG PET/CT should be a useful method for the monitoring of muscle viability after flap surgery.

Intracardiac acoustic radiation force impulse (ARFI) and shear wave imaging in pigs with focal infarctions

Four pigs, three with focal infarctions in the apical intraventricular septum (IVS) and/or left ventricular free wall (LVFW), were imaged with an intracardiac echocardiography (ICE) transducer. Custom beam sequences were used to excite the myocardium with focused acoustic radiation force (ARF) impulses and image the subsequent tissue response. Tissue displacement in response to the ARF excitation was calculated with a phase-based estimator, and transverse wave magnitude and velocity were each estimated at every depth. The excitation sequence was repeated rapidly, either in the same location to generate 40 Hz M-modes at a single steering angle, or with a modulated steering angle to synthesize 2-D displacement magnitude and shear wave velocity images at 17 points in the cardiac cycle. Both types of images were acquired from various views in the right and left ventricles, in and out of infarcted regions. In all animals, acoustic radiation force impulse (ARFI) and shear wave elasticity imaging (SWEI) estimates indicated diastolic relaxation and systolic contraction in noninfarcted tissues. The M-mode sequences showed high beat-to-beat spatio-temporal repeatability of the measurements for each imaging plane. In views of noninfarcted tissue in the diseased animals, no significant elastic remodeling was indicated when compared with the control. Where available, views of infarcted tissue were compared with similar views from the control animal. In views of the LVFW, the infarcted tissue presented as stiff and non-contractile compared with the control. In a view of the IVS, no significant difference was seen between infarcted and healthy tissue, whereas in another view, a heterogeneous infarction was seen to be presenting itself as non-contractile in systole.

Dobutamine stress tetrofosmin SPECT; evaluation of short rest-stress protocol and head to head comparison with MIBI in detection of coronary artery disease

OBJECTIVE

The purpose of the present study was to evaluate the feasibility and diagnostic accuracy of same day short rest-dobutamine stress Tetrofosmin (TF) SPECT imaging protocol and to compare TF SPECT results with MIBI SPECT in the same subjects who were unable to perform treadmill exercise or were unsuitable for pharmacological vasodilator stress.

METHODS

The study group consisted of 19 patients (2 female and 17 male, with a mean age of 53.8 +/- 7.9 yrs) in whom coronary artery disease (CAD) had been proven or excluded at coronary angiography (CA). MIBI SPECT imaging was performed first. TF SPECT images were obtained one week after MIBI imaging. Immediately after the rest SPECT imaging in both of the MIBI and TF studies, patients underwent dobutamine stress tests. Rest-stress radiotracer doses and dobutamine doses were the same for both TF and MIBI studies. While 60 min waiting periods were applied for MIBI study, only 30 min waiting periods were applied for TF study after the rest and stress injections. Images were evaluated by visual and quantitative analysis.

RESULTS

Dobutamine stress parameters were similar for both studies. Although in TF study, the time between radiopharmaceutical injection and imaging was shorter than in MIBI study, there was no significant difference between heart-to-liver (H/Li) and heart-to-lung (H/Lu) ratios. According to CA results, diagnostic accuracy was similar for TF and MIBI. While sensitivity, specificity and accuracy for TF study were calculated as 82%, 84% and 82%, respectively, the corresponding values for MIBI were 82%, 88% and 84%, respectively. This clinical study has shown comparable diagnostic performance for the detection of CAD between MIBI and TF. Good correlation was found between segmental analysis for both studies.

CONCLUSION

MIBI and TF showed similar perfusion defects and good segmental correlation during dobutamine stress with the same quality images. Both radiopharmaceuticals may be acceptable with this imaging protocol. Besides this, TF study showed better reversibility degree (55%) in a shorter time when compared to MIBI study (25%) in perfusion defects (especially in segments with severely decreased perfusion or no uptake).

Towards a biomechanics-based technique for assessing myocardial contractility: an inverse problem approach

This work presents the initial development and implementation of a novel 3D biomechanics-based approach to measure the mechanical activity of myocardial tissue, as a potential non-invasive tool to assess myocardial function. This technique quantifies the myocardial contraction forces developed within the ventricular myofibers in response to electro-physiological stimuli. We provide a 3D finite element formulation of a contraction force reconstruction algorithm, along with its implementation using magnetic resonance (MR) data. Our algorithm is based on an inverse problem solution governed by the fundamental continuum mechanics principle of conservation of linear momentum, under a first-order approximation of elastic and isotropic material conditions. We implemented our technique using a subject-specific ventricle model obtained by extracting the left ventricular anatomical features from a set of high-resolution cardiac MR images acquired throughout the cardiac cycle using prospective electrocardiographic gating. Cardiac motion information was extracted by non-rigid registration of the mid-diastole reference image to the remaining images of a 4D dataset. Using our technique, we reconstructed dynamic maps that show the contraction force distribution superimposed onto the deformed ventricle model at each acquired frame in the cardiac cycle. Our next objective will consist of validating this technique by showing the correlation between the presence of low contraction force patterns and poor myocardial functionality.

Towards a biomechanical-based method for assessing myocardial tissue viability

This work presents the first steps towards the development and implementation of a novel 3D biomechanical-based method for assessing the viability of myocardial tissue, with particular interest for its application in myocardial infarction (MI) diagnosis. This assessment technique quantifies the myocardial contraction forces developed within the ventricular myofibrils in response to the electrophysiological stimulus. In this manuscript we provide a 3D finite element (FE) formulation of a contraction force reconstruction algorithm based on an inverse problem solution of linear elasticity, along with its implementation using clinical data. This algorithm has been applied to patient-specific models obtained by extracting anatomical features from high-resolution, high-contrast magnetic resonance (MR) cardiac images. The input consists of motion information extracted by nonrigid registration of the mid-diastole reference image to the remaining images of the 4D data set, acquired using ECG-gating throughout the cardiac cycle. The result consists of a display-map of the contraction force distribution superimposed on the anatomical ventricle model, which allows the clinician to identify regions of low contractility in the myocardium.

Dual cardiac-respiratory gated PET: implementation and results from a feasibility study

PURPOSE

Spatial resolution in myocardial imaging is impaired by both cardiac and respiratory motion owing to motional blurring. We investigated the feasibility of a dual cardiac-respiratory gated positron emission tomography (PET) acquisition using a clinical PET/computer tomography (CT) scanner. We describe its implementation and present results on the respiratory motion observed.

METHODS

The correlation between diaphragmatic excursion measured by real-time magnetic resonance imaging (MRI) and the expansion of the chest measured with an elastic belt was studied in six subjects. PET list mode acquisitions were then performed in 12 patients, six of them injected with 13N-ammonia and six with 18F-FDG. In parallel, the ECG and respiratory signals of the patients were recorded and the list mode file correspondingly sorted using a dual gated approach. Respiratory motion of the heart was quantified by measuring the displacement between the inspiratory and expiratory images in the diastolic phase by means of intensity-based non-rigid image registration.

RESULTS

The correlation between diaphragmatic excursion and expansion of the chest was excellent (R2=0.91), validating the ability of the elastic belt to provide an adequate respiratory trigger. Respiratory signals corresponding to the chest expansion showed a large inter-patient variability, requiring adapted algorithms in order to define suitable respiratory gates. Dual gated PET series were successfully acquired for both groups of patients, showing better resolved myocardial walls. The average respiratory motion of the heart measured by PET was 4.8 mm, with its largest component in the craniocaudal direction. Moreover, a deformation of the heart with respiration was observed, with the inferior wall moving significantly more than the anterior.

CONCLUSION

Dual gated cardiac PET studies were performed successfully and showed better resolved myocardial walls as compared with ungated acquisitions. The respiratory motion of the heart presented a significant elastic component and was of the same magnitude as the spatial resolution of current PET cameras.

Usefulness of 18-fluorodeoxyglucose positron emission tomography in the evaluation of tumor cardiac thrombus from renal cell carcinoma

PURPOSE

Retrospectively analysed our experience in Renal Cell carcinoma (RCC) patients studied with 18-Fluorodeoxyglucose Positron Emission Tomography (FDG PET) to rule out the incidence and clinical impact of heart metastases.

MATERIAL AND METHODS

In two years, 37 patients with RCC were studied with FDG PET. 10 were studied for initial staging and 27 patients were evaluated for suspected recurrence or re-staging.

RESULTS

In two patients (5%), PET scan showed pathological focal uptake in myocardium. On the bases of this finding, MRI was performed visualizing a myocardial mass in both lesions and confirmed by histology in one of them. The hypothesis of prolepses of the tumour by thrombus in RCC patients justified the surgery.

CONCLUSION

Whole-body FDG PET in RCC patients could help to diagnose cardiac metastasis, and allows the possibility of therapeutic surgery, due to the thrombus significance of heart involvement.

[Cardiovascular monitoring of patients with systemic lupus erythematosus]

Accelerated atherosclerotic cardiovascular disease is increasingly recognized as a major cause of morbidity and mortality in patients with systemic lupus erythematosus (SLE). Cardiac manifestations of SLE are frequent and can involve almost all components of the heart. Pulmonary hypertension often develops during the course of SLE. The high incidence of cardiovascular complications may justify a screening of SLE patients in order to ensure early diagnosis and therapy. Results of diagnostic procedures that detect coronary insufficiency, surrogates of atherosclerotic burden and echocardiographic findings are often abnormal in SLE. However, evidence to support a routine screening for cardiovascular disease is currently not available. Therefore, based on the recommendations that have been proposed for other conditions associated with cardiovascular disease, we suggest assessment of risk factors and the performance of echocardiography at least annually in asymptomatic SLE patients. If two or more risk factors are present, an exercise ECG is recommended. The benefit, however, of screening SLE patients for cardiovascular disease has to be confirmed in prospective studies.

Heart Failure Following Anterior Myocardial Infarction: An Indication for Ventricular Restoration, a Surgical Method to Reverse Post-Infarction Remodeling

Anterior myocardial infarction produces abrupt left ventricular (LV) dysynergy and global systolic dysfunction. Rapid intense neurohumoral activation, infarct expansion, and early ventricular chamber dilatation all contribute to restoring a normal stroke volume despite a persistently depressed ejection fraction. Continued neurohumoral activation provokes late remodeling of the remote non-infarcted myocardium, characterized by an abnormal progressively increasing LV volume/mass ratio that leads to further LV remodeling. Heart failure is a progressive disorder of LV remodeling. Heart failure from post-infarction remodeling is unique because of the persistent non-functioning scar that self- perpetuates abnormal loading conditions and neurohumoral activation. Medical therapy attenuates remodeling and improves survival but does not change the size of the scar. Surgical ventricular restoration to exclude the non-functioning infarct from the ventricular cavity decreases ventricular volumes, increases global ejection fraction, attenuates neurohumoral activation and yields an excellent 5-year survival. Combined medical and surgical therapy is recommended in this patient population.

Assessment of myocardial viability by intracellular 23Na magnetic resonance imaging

BACKGROUND

Because of rapid changes in myocardial intracellular Na+ (Na+(i)) during ischemia and reperfusion (R), 23Na magnetic resonance imaging (MRI) appears to be an ideal diagnostic modality for early detection of myocardial ischemia and viability. So far, cardiac 23Na MRI data are limited and mostly concerned with imaging of total Na+. For proper interpretation, imaging of both Na+(i) and extracellular Na+ is essential. In this study, we tested whether Na+(i) imaging can be used to assess viability after low-flow (LF) ischemia.

METHODS AND RESULTS

Isolated rat hearts were subjected to LF (1%, 2%, or 3% of control coronary flow) and R. A shift reagent was used to separate Na+(i) and extracellular Na+ resonances. Acquisition-weighted 23Na chemical shift imaging (CSI) was alternated with 23Na MR spectroscopy. Already during control perfusion, Na+(i) could be clearly seen on the images. Na+(i) image intensity increased with increasing severity of ischemia. During R, Na+(i) image intensity remained highest in 1% LF hearts. Not only did we find very good correlations between Na+(i) image intensity at end-R and end-diastolic pressure (R=0.85, P<0.001) and recovery of the rate-pressure product (R=-0.88, P<0.001) at end-R, but most interestingly, also Na+(i) image intensity at end-LF was well correlated with end-diastolic pressure (R=0.78, P<0.01) and with recovery of the rate-pressure product (R=-0.81, P<0.01) at end-R. Furthermore, Na+(i) image intensity at end-LF was well correlated with creatine kinase release during R (R=0.79, P<0.05) as well as with infarct size (R=0.77, P<0.05).

CONCLUSIONS

These data indicate that 23Na CSI is a promising tool for the assessment of myocardial viability.

Prognostic value of ECG-gated thallium-201 single-photon emission tomography in patients with coronary artery disease

BACKGROUND

The phenomenon of reversible impairment in LV function has been well described and is known as myocardial stunning.

OBJECTIVE

Thallium-201 myocardial perfusion gated SPECT was used to evaluate myocardial stunning and its incremental prognostic value in patients with coronary artery disease.

PATIENTS AND METHODS

Fifty-six patients (aged 63+/-11 years) with coronary artery disease were included in this study. All subjects underwent exercise thallium scintigraphy. ECG-gated SPECT was obtained both at post-stress (10 minutes after the injection of 111 MBq of thallium at the time of peak exercise) and at rest (180 minutes). The left ventricular ejection fraction (LVEF) and end-systolic and end-diastolic volume (ESV, EDV) were determined by a quantitative gated SPECT (QGS) program.

RESULTS

Follow-up was complete in all patients (mean 569 days). The magnitude of the depression of post-stress LVEF relative to the rest LVEF was correlated with the severity of ischemia (p < 0.05). The group with a median LVEF of more than 45% had a significantly higher event-free rate (p < 0.01).

CONCLUSION

Assessment of post-stress left ventricular function by gated-SPECT provides incremental prognostic information and is useful in predicting cardiac events in patients with suspected or definite coronary artery disease.