History of two dimensional intracardiac ultrasound systems
The first application of ICE was done using mechanical ultrasound systems, which were introduced in the 1980s. These systems provided high resolution imaging, but because of the high frequency of the transducers (20–40 MHz), tissue penetration was only limited and anatomic intracardiac overviews could not be obtained. The subsequent development of lower frequency transducers allowed imaging of intracardiac structures, but these systems were still limited by the low steerability and over-the-wire design of the catheters.2
The clinical use was improved by the development of flexible lower frequency transducers (9 MHz), but depth control of these catheters still was not sufficient to allow visualisation of the whole heart from the right side of the heart. In the 1990s, systems modified after transoesophageal echocardiographic probes were introduced. In these systems depth was improved by the use of lower frequencies (5 MHz). However, the large size of these transducers limited the clinical use. In recent years the development of steerable phased array ultrasound catheter systems with low frequency and Doppler qualities has expanded the clinical use of ICE.
What are the technical requirements?
Mechanical ultrasound tipped catheter
The mechanical ultrasound transducer tipped catheter (Clearview, Cardiovascular Imaging Systems Inc, Fremont, California, USA) can be used for both intravascular and intracardiac imaging. For intracardiac use, a 9 MHz single element transducer is incorporated in an 8 French catheter. A piezoelectric crystal is rotated at 1800 rpm in the radial dimension perpendicular to the catheter shaft, thus providing cross sectional images in a 360° radial plane. A sheath surrounding the imaging transducer is necessary to prevent contact of the imaging transducer with the cardiac wall. The ICE catheter needs to be filled with 3–5 ml sterile water before it is connected to the ultrasound machine (Boston Scientific Corp, San Jose, California, USA). Three dimensional reconstruction of the acquired data can be created.
Phased array ultrasound tipped catheter
This system uses a 10 French ultrasound catheter (Acunav Diagnostic Ultrasound Catheter, Acuson Corporation, Mountain View, California, USA), which is positioned in the right atrium (RA) or right ventricle (RV) via a femoral approach, through a 10 French introducer. The ultrasound catheter consists of a miniaturised 64 element, phased array transducer, which is incorporated in a single use catheter. The transducer scans in the longitudinal monoplane, providing a 90° sector image with tissue penetration of approximately 15 cm. Two planes of bidirectional steering (anterior–posterior and left–right, each in a direction of 160°) are possible by using a mechanism on the handle of the catheter. The high resolution multiple frequency transducer (5–10 MHz) allows tissue penetration enhancement, thus allowing depth control. Measurements of haemodynamic and physiologic variables can be made using Doppler imaging. The catheter is connected to an ultrasound system (Acunav/Seqouia, Acuson Corp, Mountain View, California, USA).
- ARVD/C: arrhythmogenic right ventricular dysplasia/cardiomyopathy
- ASD: atrial septal defect
- ICE: intracardiac echocardiography
- LA: left atrium
- LAA: left atrial appendage
- LV: left ventricle
- PFO: patent foramen ovale
- PV: pulmonary vein
- RA: right atrium
- RV: right ventricle
- TOE: transoesophageal echocardiography
- TTE: transthoracic echocardiography
- VT: ventricular tachycardia
Mechanical versus phased array ultrasound tipped catheter
Although the mechanical ultrasound systems can be used at a considerably lower cost compared to the phased array systems, there are several disadvantages as compared to the phased array transducers. Currently, no functional analysis can be performed with mechanical systems, due to the lack of colour and pulsed Doppler features. Since these systems use a single ultrasound frequency of 9 MHz and provide a limited radial depth of view (5 cm), imaging of left sided structures is not feasible with the ultrasound catheter placed in the right heart. This could increase the risk of thromboembolic complications when using the device for imaging of left sided structures. Furthermore, the design of the mechanical rotating catheter shaft, and the lack of an articulation mechanism at the handle, limits steering of the transducer and thus a dynamic view on intracardiac structures. In the remainder of the current report we mainly focus on the use of phased array ICE in guiding percutaneous interventional procedures.
Advantages and limitations of ICE during interventional procedures
In the past, if percutaneous interventions needed guiding, fluoroscopy, transthoracic echocardiography (TTE), or transoesophageal echocardiography (TOE) were used. The use of ICE has several advantages over these other techniques. No radiation is needed. In comparison to TOE, patient discomfort is less and general anaesthesia is not needed, allowing communication with the patient during the procedure. As compared to TTE, the transvenous access has the advantage that it is not necessary to position a transducer in a sterile field.
General advantages of ICE are the availability of direct online information on the position of catheters and devices, and the possibility of direct monitoring of acute procedure related complications (such as thrombus formation, pericardial effusion, etc).
Several limitations of ICE currently exist. First the considerable shaft size (10 French) and the lack of additional catheter features, such as ports for guidewires, therapeutic devices and pressure, form a limitation. Second, the phased array catheters are expensive and single use only. Third, phased array ICE provides only monoplane image sections. Although this can partly be overcome by the steerability of the catheter, operators who are used to multiplane TOE transducers may have difficulty obtaining the same views. Moreover, no standard views for ICE are currently defined, as are available for TTE and TOE.
Which views can be obtained with ICE?
Using phased array ICE, views of all anatomic landmarks can be obtained with the ultrasound catheter positioned in either the RA or the RV. Although images quite similar to TOE can be obtained, the relatively “loose” position of the ultrasound probe in the heart may give the inexperienced operator a feeling of disorientation. To overcome this, structured introduction and steering/manipulation of the ultrasound tipped catheter can ultimately provide orientation, supported by recognition of the anatomical landmarks. The catheter is advanced via the femoral vein, into the middle part of the RA via the inferior caval vein, thus visualising the RA, tricuspid valve, and RV (fig 1A). This view with the catheter positioned in the middle of the RA can be used a “basic point of orientation”, from which other views can be derived. Counterclockwise rotation with the catheter positioned in the superior part of the RA provides imaging of the terminal crest, whereas clockwise rotation of the catheter from the inferior RA provides a view of the Eustachian ridge with the tricuspid–caval isthmus (fig 1B); these are important target structures in atrial flutter ablation.
Figure 1 Different views obtained with the intracardiac echocardiography (ICE) transducer in either the right atrium or the right ventricle. See text for explanation of panels A–H. Ao, aorta; AP, pulmonary artery; (more ...)
By turning the catheter around its axis (clockwise fashion as seen from the operator), imaging of the aortic valve, the right ventricular outflow tract, and the pulmonary artery is feasible (fig 1C). Long axis views of both aorta and pulmonary trunks can be imaged in the same plane, which is not feasible with TOE. By rotating the catheter clockwise from the low right atrium, a short axis view of the coronary sinus can be obtained, while left-to-right movements of the ICE catheter provide a long axis view of this structure (fig 1D, E). When the catheter is further rotated, the next anatomical structure important in interventional cardiology to be recognised is the atrial septum (important for septal puncture, see below). By counterclockwise movement of the catheter from this position, imaging of the left atrium (LA), mitral valve, and left ventricle (LV) is performed (fig 1F). By increasing the depth setting of the catheter with the transducer directed at the left atrial posterior wall, imaging of the left and right pulmonary veins and the left atrial appendage (LAA) is performed from the position of the atrial septum (important for pulmonary vein (PV) ablation, see below). As is it sometimes difficult to distinguish between the LAA and the left superior PV, Doppler capacities can be used to differentiate. Finally, by advancing the catheter into the RV, detailed imaging of the LV can be obtained. Both long and short axis views of the LV can be obtained (fig 1G, H).