Angiographic computed tomography (ACT) is a new imaging modality that provides cross-sectional CT images from a rotational angiography run using a C-arm–mounted flat-panel detector. The volume set obtained can be manipulated on a separate workstation in the interventional suite to generate a 3-dimensional (3D) angiographic image combined with CT-quality soft tissue imaging that can be used in real time during the procedure. Designed originally for use in interventional neurovascular procedures (1- 8), its use has extended into other interventional radiology realms, including abdominal (9- 11), spine (12), and hepatic vascular (13- 18). Its application outside interventional radiology is quite limited. Noelker et al. (19) reported the use of ACT for mapping of the left atrium during atrial fibrillation ablation procedures. There have also been early reports of its use in adult thoracic vascular interventions (20). To our knowledge, the use of ACT for patients with congenital heart disease has never previously been described. We describe our initial experience using ACT in a pediatric cardiac catheterization laboratory.

This study was conducted with approval from our Institutional Review Board (IRB # 2009-1-6377). Where appropriate, informed consent was obtained.

A retrospective review was performed of all cases from our cardiac catheterization laboratory, which utilized ACT between July 1, 2008, and June 30, 2009. Identified cases were reviewed for demographic, clinical, and procedural details. ACT images obtained were reanalyzed and compared with standard cineangiographic images to determine cases where ACT provided diagnostic-quality information. The reviewer was blinded to the demographic and clinical details of the case. As much as practically possible, the reviewer was also unaware of the clinical outcome of the case. Diagnostic-quality information is subjectively defined as being equivalent to the information obtained (or expected to be obtained) by traditional 2-dimensional projectional cineangiography. Further, cases were identified in which ACT provided additional clinically relevant information.

ACT images were acquired using a C-arm–mounted flat-panel biplane angiographic system (Artis zee, Siemens Medical Solutions, Forchheim, Germany) using commercially available software (syngo DynaCT, Siemens Medical Solutions). Post-processing was performed on a separate workstation in the interventional suite (Leonardo, Siemens Medical Solutions). ACT data acquisition was performed with the following parameters: 190° rotation, 5-s ungated acquisition time, projection on either 20-cm × 20-cm or 30-cm × 40-cm flat-panel detector size, 48-cm field of view. We displayed the tomographic volume set at a slice thickness of 0.3 mm, although this is adjustable during post-processing. The degree of thresholding during 3D image reconstruction was determined by the operator to optimize image quality. For patients mechanically ventilated during the catheterization, ventilation was held during the period of image acquisition. In awake patients, spontaneous breath-holding was performed when possible. Radiation dose programs were modified from the installation settings in order to minimize radiation exposure while maintaining image quality. Intravenous contrast media (Ioversol 74% [350 mg/ml], Optiray, Covidien Imaging Solutions, Hazelwood, Missouri) was diluted 1:2. Contrast volumes were weight-based and administered with a 0.5- to 5-s X-ray delay based on the clinical situation.

Radiation dose testing was performed using radiologic phantoms (ATOM, Computerized Imaging Reference Systems, Inc., Norfolk, Virginia) utilizing 6 different available ungated cardiac ACT acquisition programs across a range of dose/frame and frame rates. The dose-area product as measured by the Artis zee system (Siemens Medical Solutions) during phantom testing for each program was converted to an estimated total effective dose (mSv) using the Monte Carlo method on commercially available software (PCXMC v2.0, STUK, Helsinki, Finland). For comparative purposes, radiation exposure from standard fluoroscopy and biplane cineangiography was also measured.

Summary data are presented as frequency with percentage of total for count data, median with range for non-normally distributed continuous variables, and mean with standard deviation for normally distributed continuous variables. Difference in the frequency of diagnostic-quality images obtained by ACT across anatomic categories was assessed using Pearson chi-square. Logistic regression was used to attempt to identify demographic, clinical, or procedural variables associated with obtaining diagnostic-quality images. Statistical significance was established at p < 0.05. All statistical analyses were performed using STATA version 10 (StataCorp LP, College Station, Texas).

Between July 1, 2008, and June 30, 2009, 1,083 total cases were performed in our cardiac catheterization laboratory, of which 705 were diagnostic and/or interventional cases utilizing angiography. Of these, ACT was utilized in 41 (5.8%), of which 12 were diagnostic and 29 were interventional. Demographic, clinical, and procedural details of the reviewed cases are summarized in (Table 1). ACT was used across a wide range of patient ages, sizes, and total fluoroscopy times, reflective of the range of case complexity in which ACT was utilized. ACT was performed with breath-holding in a total of 32 cases (78%): 25 were with ventilator-hold in an anesthetized and intubated patient, and 7 were with the patient awake and able to perform spontaneous breath-hold during the 5 s of image acquisition. In 9 cases, ACT was performed with a patient under conscious sedation, but too young to cooperate with spontaneous breath-hold. In all of these 9 cases, we were imaging central structures, and respiratory movements did not significantly interfere with image quality. Total contrast administered was within the acceptable range, with the volume used for ACT constituting, on average, 35% of the total contrast load. Patients were classified into 5 anatomic categories based on the region of interest for visualization by ACT. There were 4 cases in which ACT was used to assess 2 different anatomic regions, so these 4 patients counted for 2 different categories. Anatomic categories consisted of the following:

There were 20 procedures performed in patients at a median age of 3.5 years (range 0.4 to 58.8 years) and weight of 18.7 kg (range 5.1 to 74 kg). These patients had undergone surgical repair of tetralogy of Fallot (n = 8), truncus arteriosus (n = 4), double outlet right ventricle (n = 4), or d-transposition of the great arteries (n = 3); 1 had undergone a Ross-Kono procedure. The median fluoroscopy time was 29.4 min (range 11.3 to 134.3 min) with a mean total case contrast volume of 4.2 ± 1.3 ml/kg and mean contrast volume for ACT of 1.3 ± 0.4 ml/kg. During ACT, contrast was injected either into the right ventricle (n = 11) or right ventricular outflow tract (RVOT) (n = 9) with an X-ray delay of 1 s. A representative image is shown in (Figures 1, 1a) and Online Video 1.

There were 11 procedures in patients at a median age of 7.6 years (range 1.1 to 21.7 years) and weight of 22.5 kg (range 8.4 to 114.4 kg). Eight patients had undergone Fontan procedures, 2 bidirectional Glenns, and 1 a hemi-Fontan operation. The median fluoroscopy time was 44.2 min (range 15.2 to 119.5 min) with a mean total case contrast volume of 4.0 ± 1.8 ml/kg and mean contrast volume for ACT of 1.2 ± 0.4 ml/kg. During ACT, contrast was injected in the superior vena cava for superior cavopulmonary connections (CPC) and in the Fontan baffle or conduit for those with a completed Fontan, with an X-ray delay of 0.5 to 1 s. Representative images are shown in (Figures 2, 2a) and Online Video 2.

There were 5 patients at a median age of 3.5 years (range 0.4 to 14.2 years) and weight of 11.1 kg (range 5 to 50 kg) including 3 with pulmonary vein stenosis, 1 after prior correction of total anomalous pulmonary venous return, and 1 after a Senning procedure. The median fluoroscopy time was 29.8 min (range 16.5 to 84 min) with a mean total case contrast volume of 4.1 ± 1.6 ml/kg and mean contrast volume for ACT of 1.4 ± 0.4 ml/kg. During ACT, contrast was injected in the pulmonary arterial bed with an X-ray delay time up to 5 s depending on the best estimate of the pulmonary transit time.

Four patients underwent ACT to evaluate distal pulmonary arteries (PAs) at a median age of 9.0 years (range 0.4 to 16.3 years) and weight of 26.9 kg (range 5 to 54.9 kg). Two had undergone unifocalization procedures, 1 a modified Blalock-Taussig shunt, and 1 had a stent fragment embolized to a distal PA. The median fluoroscopy time for cases was 71 min (range 42.3 to 102.3 min) with a mean total case contrast volume of 4.5 ± 1.5 ml/kg and mean contrast volume for ACT of 1.2 ± 0.3 ml/kg. During ACT, contrast was injected into the pulmonary arterial bed proximal to the area of interest with an X-ray delay time of 1 s. A representative image is shown in (Figures 3, 3a) and Online Video 3.

Five other procedures were performed at a median age of 4.8 years (range 0.5 to 32 years) and weight of 16.1 kg (range 7.3 to 60.9 kg) in patients with the following diagnoses: patent ductus arteriosus, right ventricle to aorta, left superior vena cava to coronary sinus, pulmonary hypertension, and aortopulmonary collaterals ((Figures 4, 4a),Online Video 4). The median fluoroscopy time was 17.2 min (range 1.7 to 46.4 min) with a mean total case contrast volume of 3.7 ± 2.4 ml/kg and mean contrast volume for ACT of 1.4 ± 0.7 ml/kg.

Overall, diagnostic-quality images were obtained from ACT in 71% of cases (Table 2). In 19 of 41 total cases (46%), the ACT image was obtained instead of a standard cineangiogram, so a direct head-to-head comparison of ACT and cineangiogram quality could not be performed. In these cases, the reviewer subjectively determined whether the ACT was able to provide the diagnostic detail that could reasonably be expected from a standard cineangiogram. There was no statistically significant difference in the frequency of diagnostic-quality imaging across anatomic categories (Pearson chi-square = 2.2, p = NS). Further, univariate logistic regression failed to identify demographic, clinical, or procedural variables associated with obtaining diagnostic-quality images.

In 12 cases, the anatomic information obtained from ACT was felt to augment in a clinically meaningful way the information obtained from standard cineangiography. In 10 of these, ACT provided an improvement in relevant anatomic detail, including 7 cases of imaging the RVOT/central PAs ((Figure 1), Online Video 1) and 3 cases of imaging the CPC ((Figure 2), Online Video 2). In the majority of these cases, the relevant anatomic information could only be obtained by rotating the 3D reconstructed image to an axial projection in order to fully delineate the anteroposterior contour of the vessel of interest. This is an imaging angle that cannot be achieved by standard cineangiography. In the remaining 2 cases, ACT imaging was able to aid in a complex catheter manipulation by overlaying the 3D reconstructed image onto the fluoroscopy screen. In 1 instance, this overlay facilitated cannulation of a unifocalized collateral vessel from the PA ((Figure 3), Online Video 3). In the other, it aided catheterization of an aortopulmonary collateral vessel from the posterior aspect of the descending aorta ((Figure 4), Online Video 4). There was no statistically significant difference in the frequency of superior-quality images across diagnostic categories (Pearson chi-square = 2.6, p = NS). Further, univariate logistic regression failed to identify demographic, clinical, or procedural variables associated with obtaining superior-quality images.

All radiation dose testing was performed after first increasing the kilovolt limit to 102 kVp to allow for copper filtration to be used during image acquisition at a higher kV range. The radiation exposure from various ACT programs as measured by the dose-area product (μGy·m²) and the total effective dose (mSv, as estimated by dose calculation software) are summarized in (6). Variability in radiation dose from a single 5-s image acquisition sequence is based on patient size and the dose program selected, with estimated dose exposures ranging from <0.1 to 3.5 mSv. Our currently preferred ACT image acquisition program uses a dose of 0.17 μGy/frame and a frame rate of 60 frames/s. This results in similar radiation exposure as a comparable 5-s biplane cineangiogram across all phantom sizes (approximately 0.2 to 1.4 mSv) while maintaining image quality.

During our first year with ACT technology available in our pediatric cardiac catheterization laboratory, we performed ACT image acquisition in 41 procedures across a wide range of patient ages, sizes, and anatomic diagnoses. ACT technology was originally designed for application in interventional neuroradiology procedures (1- 6,8). The technical details supporting this new technology have been well described (21- 24). Its use has extended to other interventional radiology realms (9- 14,16- 18,25) and more recently into the adult cardiac electrophysiology suite (19). To our knowledge, this is the first report of the use of ACT technology in a cardiac catheterization laboratory for patients with congenital heart disease.

ACT provided diagnostic-quality imaging in over 70% of cases. The image quality from ACT has also proven to compare quite favorably with the current imaging standard in a variety of other arenas (7,16,26- 31), although not all (15,32- 34). In addition, in 12 cases, the information obtained from ACT augmented in a clinically meaningful way the information obtained from traditional angiography. In 10 patients, this included relevant anatomic information that was not attainable by standard angiography, typically complex geometric relations that are best viewed in an axial projection. In 2 other cases, ACT aided in complex catheter manipulations where the 3D ACT image was used as an overlay on the fluoroscopy screen.

The issue of radiation exposure is not entirely straightforward. The factory-installed default ACT programs result in substantial radiation exposure. However, we were able to reduce this to a more acceptable range through a number of efforts. First, we changed the kilovolt limit for all ACT programs to 102 kVp in order to allow for copper filtration to be used during image acquisition. Second, we removed the antiscatter grid for all subjects <15 kg. Third, we collimated top to bottom prior to image acquisition. Finally, we performed extensive testing of various dosing programs on phantoms to determine the optimal compromise between radiation exposure and image quality (6). Currently, we find the best balance of image quality and radiation dose is achieved with the 0.17 μGy/frame dosing program at 60 frames/s. This results in a comparable radiation exposure to a 5-s biplane cineangiogram. The radiation exposure from clinical applications of ACT has not previously been reported. However, the exposure measured during our phantom testing is similar to that previously reported from 3D rotational angiography (35) and from phantom testing of a similar C-arm CT system for neurovascular interventions (36). It is also well within the total estimated exposure during a routine pediatric cardiac catheterization (37- 41). Finally, our measured exposure from ACT is significantly lower than the median exposure of 12 mSv for adult cardiac CT angiography in clinical use across 50 study sites recently reported by Hausleiter et al. (42).

We have identified a number of situations where ACT technology may be particularly suited. First, we have found it useful in providing additional anatomic detail of structures that are best visualized in axial projections. Not only has the reconstructed image been helpful, but we have also found a great deal of useful information in the tomographic volume sets that are generated (Figure 5). Second, the use of the reconstructed 3D image from ACT as an overlay on the live fluoroscopy monitor may aid in complex catheter manipulations. Third, ACT may be useful to define the optimal camera angles prior to a planned intervention under standard biplane (or even single plane) fluoroscopic guidance. Finally, ACT may be useful when CT-quality soft-tissue imaging is desired in addition to the usual hemodynamic and anatomic information obtained at cardiac catheterization.

Regarding the technical aspects of performing an ACT, as a rule, we recommend diluting the contrast 1:2 with saline and administering a contrast volume of approximately 1.5 ml/kg during the duration of the injection. If the catheter is able to be positioned in close proximity to the area of interest, a 1-s X-ray delay setting should suffice. If the pulmonary veins are to be visualized following an injection in the pulmonary arteries, a longer X-ray delay will likely be needed. We currently favor the 5-s ACT program using a radiation dose of 0.17 μGy/frame and a frame rate of 60 frames/s. For extracardiac vascular structures, the ungated program with a breath-hold is sufficient.

We have identified a number of limitations to this imaging modality. First, the images obtained are time-averaged over the duration of the 5-s C-arm rotation. Thus, the temporal information obtained by standard angiography is not available with ACT. Second, we have had situations where there is “drop-out” of the signal in areas of very tight stenosis or adjacent to stented regions. Finally, we view the 3D reconstructed images with the same degree of skepticism that we have for the 3D images generated from standard CT or cardiac magnetic resonance (CMR). That is, a surface-rendered image such as these can be altered somewhat by the degree of windowing that occurs by the operator during post-processing. Thus, we are cautious about using the 3D images for precise measurements, instead using them for a more global appreciation of spatial relations.

There are also limitations to the present study. First, this is a purely retrospective description of our early experience with this new modality in a relatively small number of cases. Second, the procedures during which ACT was used were selected by the operator with the hope that ACT would be a useful imaging technique for that particular anatomy. Thus, this likely represents a biased sampling of cases, the results of which may not necessarily generalize to other anatomic areas or to other operators. Third, determining the “diagnostic utility” of the images obtained involves a degree of subjectivity. Finally, this study focuses largely on the utility of the 3D images generated, with the understanding that there is also information to be obtained from the tomographic volume sets (Figure 5).

Multislice spiral computed tomography (MSCT) is an alternative imaging modality by which detailed 3D cardiovascular anatomy can be obtained and has been used successfully in a variety of forms of congenital heart disease (43- 45). However, in patients where invasive catheter study is indicated for diagnostic or therapeutic purposes, ACT may have advantages over conventional MSCT: 1) the images are easily obtainable without significantly prolonging the catheterization procedure; 2) the contrast dose for an ACT acquisition is less than or equal to that used for MSCT; and 3) the generated images can be immediately and accurately registered with real-time fluoroscopy to serve as a 3D overlay roadmap in the cardiac catheterization laboratory. A more intriguing alternative to ACT for use in the catheterization laboratory is CMR fused with X-ray. Although CMR requires a substantially longer image acquisition time than ACT, there is no ionizing radiation and the functional information obtained by CMR is available. Several investigators have described techniques for the registration of CMR images with biplane fluoroscopy to allow the use of this 3D dataset for roadmapping (46- 49). Much future work will be required to determine the optimal application of these new strategies in transcatheter evaluation and treatment of congenital heart disease.

ACT is a new imaging modality available in the pediatric cardiac catheterization laboratory that provides a complete volume set of contrast-enhanced CT images from a single C-arm rotation. The images, including a 3D reconstruction, can be quickly generated and used in real time during the procedure. We have found that this technique is able to provide diagnostic-quality imaging in over 70% of cases. Further experience will likely only increase this percentage. ACT may provide additional clinically relevant data in situations where an axial projection is needed to best view the area of interest, such as the reconstructed RVOT and the CPC. The 3D images obtained by ACT can also be used to set the ideal camera angles for further angiography or intervention. Further, the 3D images can be used as an image overlay on the live fluoroscopy screen to aid with complex catheter manipulations. With the appropriate technical modifications, the contrast and radiation exposure from ACT can be comparable to a standard biplane cineangiogram. Further work is needed to continue to optimize the technical aspects and define the clinical utility of this new modality.

The authors thank Marily Sarmiento, Siemens Medical Solutions, for technical assistance.

For a supplemental figure, table, and videos, please see the online version of this article.

The Use of Angiographic Computed Tomography Imaging in the Cardiac Catheterization Laboratory for Congenital Heart Disease