Technical principles of computed tomography in patients with congenital heart disease

Careful preparation is a prerequisite for CT in both paediatric and adult patients. We do not perform sedation in neonates. If sedation is necessary in children, we prefer a short-term sedation through the administration of midazolam hydrochloride (0.1 mg/kg body weight) via the intranasal route.

Non-ECG-synchronised CT provides a fast acquisition of the cardiac and extracardiac structures in patients with CHD but lacks the visualisation of small cardiac and coronary structures because of heart motion artefacts (Fig. 1). A pitch of around 1 to 1.5 usually represents a good balance of image quality, radiation dose and acquisition time provided. Thicker detector collimation is preferable for non-ECG-synchronised CT to reduce radiation dose and improve image quality impression at the expense of reduced detail visualisation of small structures. In addition, the required CT acquisition time is lower when using thicker collimation than for thin collimation imaging, and consequently respiratory motion artefacts might be reduced. However, if evaluation of small intracardiac structures is required, thin collimation should be used. In general, non-ECG-synchronised CT in CHD patients may be acquired during breath-hold. If the patient is not able to hold his or her breath, CT could be performed with a free-breathing technique but the patient should be advised to perform calm, shallow breathing and to avoid excessive respiratory excursions. However, respiratory artefacts are less remarkable with modern CT systems, and it has been reported that in 82% of patients with CHD the origins and proximal segments of the coronary arteries are evaluative in non-ECG-synchronised CT acquisitions [8]. The radiation dose of this imaging technique is commonly less than 1 mSv in young children [9].

In retrospective ECG-gated CT the data acquisition is performed with continuous imaging table motion and spiral acquisition with overlap of the radiation beam on each spiral synchronised to the ECG (Fig. 2). Hence, low pitch values are necessary for gapless data acquisition, and radiation dose is substantially higher than when using non-ECG-synchronised CT. However, this results in oversampling of information across different phases of the cardiac cycle and across several consecutive heart beats. Nevertheless, the synchronisation to the ECG signal provides artefact-free visualisation of cardiac and coronary structures even at elevated heart rates when using modern CT systems with high temporal resolution. A multisegment reconstruction algorithm using more than one heartbeat for image reconstruction might be applied to further improve temporal resolution. However, we usually prefer a monosegment algorithm because the multisegment algorithm prolongs the acquisition time and might introduce additional motion artefacts in the case of heart rate variability [10]. Using retrospective ECG-gated CT, the effective radiation dose is around 2–6 mSv for patients with CHD [11]. Although ECG-synchronised CT allows for the assessment of left and right ventricular function, performing a non-ECG-synchronised CT for morphology and an additional echocardiography for functional information is usually the right way to make a diagnosis.

Conversely, in prospective ECG-triggered CT irradiation is exposed at user-selectable predefined heart phases only and the table is moved in a sequential manner with the table remaining stationary while data are acquired (Fig. 3). This technique has the potential of a smaller effective patient dose, which has been reported to be 1–3 mSv in adults [12] and 0.2–0.7 mSv in newborns and infants [11]. However, this technique is limited by the fact that information on ventricular function can not be assessed and image quality can be impaired in the case of arrhythmia. To date, prospective ECG-triggered CT has almost completely replaced retrospective ECG-gated acquisition in patients with CHD. For patients with heart rates <75 bpm, we predefine the data acquisition in the diastolic phase at 70% relative to the R-R interval and for patients with a heart rate of more than 75 bpm in the systolic phase at 30%. We still perform a retrospective ECG-gated CT—if ECG-synchronisation is required—in patients with arrhythmia and if multi-phase functional evaluation is required. Most recently an adaptation of the prospective ECG-triggered CT technique to a dual-step mode has been introduced to provide ventricular functional analysis at a low radiation dose [13].

Electrocardiography-synchronised CT is not commonly used in children because of the associated higher radiation dose. As the greater cardiac structures are affected by motion to a minor degree, ECG synchronisation is unlikely to contribute to improved image quality. On the other hand, ECG-synchronised CT is associated with a longer imaging time, and, therefore, motion artefacts due to respiration or body motion might be more crucial than the beneficial effects of cardiac motion suppression.

Recently, the dual-source technique has been refined to allow CT acquisitions at a high-pitch value of up to 3.4 [14]. In single-source CT, the applicable pitch is limited to 1.5 to ensure gapless volume coverage. In the high-pitch acquisition mode available at dual-source CT, the second detector system can be used to fill these gaps [14]. The high-pitch acquisition mode is fast enough to reduce the acquisition time for the entire chest to below 1 s [15] and therefore might allow artefact-free visualisation of the coronary arteries [15] and the aortic root [16] in routine non-ECG-synchronised chest CT studies or to compensate for respiratory motion. In ECG-gated cardiac CT acquisitions, the high-pitch mode has been demonstrated to provide artefact-free visualisation of the coronary arteries at a radiation exposure below 1 mSv [14, 17]. However, to the best of our knowledge this novel technique has not yet been investigated in CHD patients.

For contrast agent application, a right-arm injection is preferable to avoid high-contrast artefacts on the left brachiocephalic vein. For neonates and young children, non-ionic iodinated contrast agent is injected at a dose of 2 mL/kg body weight to a maximum amount of 100 mL. The injection rate is set at 1 mL/s but can be increased to 2 mL/s in patients with a large intracardiac communication. In adults, 1.5 mL/kg body weight is injected at a rate of 3–4 mL/s. To reduce artefacts from undiluted contrast material and to reduce the total amount of contrast material, a saline bolus chasing technique should be applied. We prefer the bolus tracking technique to determine the imaging delay in ECG-synchronised CT. In the paediatric patient, a region of interest is placed in the left ventricle, and a threshold attenuation of 200 HU is set. In adults, the region of interest is placed in the ascending aorta, and the attenuation threshold is set at 140 HU. However, when in doubt about the intracardiac connections and the timing of the contrast attenuation of the different cardiac chambers, monitoring the arrival of contrast material in the cardiac chambers and manually starting the CT acquisition is often the safer way for sufficient contrast attenuation of the cardiac cavities and adjacent arteries and veins. In non-ECG-synchronised CT in paediatric patients a fixed imaging delay is usually sufficient with the start delay set at 12 s for peripheral injection and 8 s for central venous injection.

Several image reconstruction techniques are available for the evaluation of the CT acquisition including multi-planar reformations (MPR), maximum intensity projections (MIP) and the volume rendering technique (VRT). In general, a combination of the 2D and 3D reconstruction techniques is used for the evaluation of CT in CHD patients. The 2D MPR is the most relevant reconstruction algorithm as it is capable of displaying the CT dataset in any plane and even in curved planes if desired. In general, the axial source images and coronal and sagittal MPR are reviewed to evaluate the cardiac and extracardiac structures. Oblique MPR are interactively arranged to review potential intracardiac connections or to evaluate the atrioventricular and ventriculo-arterial junctions. Three-dimensional MIP and VRT are used to image a portion or the complete dataset in one image, which is particularly helpful to get an overview of the cardiac and extracardiac structures and for preoperative planning and postoperative evaluation of the surgical procedures that have been performed.

The z-axis coverage of the CT acquisition needs to be limited to the relevant structures. One has to bear in mind that the CT radiation dose is directly proportional to the imaging range when all other acquisition parameters are similar, and that ECG-synchronised cardiac CT has the highest radiation dose per centimetre imaging range of all CT protocols. A recent study observed that, by minimisation of the imaging range in cardiac CT, 0.7 mSv for each centimetre of the imaging range lost can be saved in adults when using a retrospective ECG-gated technique [18].

One of the easiest and most effective ways to limit radiation dose in CT for CHD imaging is to implement body-size-adaptive CT protocols. Various parameters that reflect the body size might be used to adapt the imaging parameters. In our institution, we find it most practicable to adapt the imaging parameters to the body weight in paediatric patients, while in adults adaptation to the body mass index (BMI) is favourable. For that reason, CT in adults with a normal BMI of ≤25 kg/m² is performed with a tube voltage of 100 kVp and a tube current time-product of 220 mAs/rotation. In overweight patients with a BMI of >25 kg/m² CT is performed at 120 kVp and 330 mAs/rotation. Using these adaptations in adults, a reduction in radiation dose of around 50% has been found for the low kilovoltage compared with the 120-kVp standard protocol [19].

In order to reduce the radiation dose associated with retrospective ECG-gated CT, the tube current should be modulated according to the ECG (aka ‘ECG pulsing’). With this technique, the tube current output outside a predefined phase in the cardiac cycle is reduced to 4–20% of the nominal tube current inside the pulsing window [19]. The image quality is impaired by noise outside the full tube current window, thereby rendering those phases of the cardiac cycle inadequate for detailed morphological evaluation but providing sufficient image quality for ventricular functional analysis. Optimal ECG pulsing windows for cardiac CT depend on the patient’s heart rate and have been recommended as follows [20]: 60–70% of the R-R interval at heart rates of ≤60 bpm, 60–80% at heart rates of 60–70 bpm, 55–80% at heart rates of 70–80 bpm, and 30–80% at heart rates of >80 bpm. The ECG pulsing technique has been reported to reduce the radiation dose by up to 64% [21].

Radiation dose reduction is of highest importance in children because of greater radiosensitivity, a longer life expectancy and the possible requirement of several follow-up studies during their lifetime.

Computed tomography imaging for CHD in neonates and children should always be performed at 80 kVp. Cardiac CT with a tube voltage of 80 kVp has been successfully performed in adults with a body weight below 60 kg [22]. In children, CT with 80 kVp provides sufficient image quality when the tube current output is adapted to the patient’s body weight (Table 2). Besides the advantage of saving radiation dose, the 80-kVp setting allows the total amount of contrast agent to be reduced because iodine has a higher attenuation at lower energy. In addition, non-ECG-synchronised CT is the preferred acquisition technique in neonates and children because of higher heart rates in those patients and shorter acquisition times of non-ECG-synchronised CT, which results in fewer respiratory artefacts. If an ECG-synchronisation technique is required, the CT protocol should be performed with a narrow ECG pulsing window width to limit the radiation exposure.