1 | INTRODUCTION
Cardiac computed tomography (CT), fluoroscopy and trans-esophageal
echocardiography (TEE) are three major imaging modalities, which support
the increasing complexity of trans-catheter interventions in congenital
and structural heart diseases. 1–3 These modalities
are based on different mathematical coordinate systems, and are often
demonstrated at different visual perspectives. Multi-planar
reconstruction (MPR) of cardiac CT, commonly used as pre-operative
planning tool, is based on the standard Cartesian coordinate system
(Figure 1A). Although manual adjustment is allowed for plane rotation
and movement of the origin of the coordinate system into the region of
interest (ROI), the re-sliced 2D images can still be intuitively
understood because CT datasets include abundant isotropic details in
cardiac anatomy. Instead of multi-slice imaging, fluoroscopy provides
with the X-ray projection from the visual perspective of the image
intensifier, whose trajectory is clinically described by the spherical
coordinate system (Figure 1B) as right anterior oblique (RAO)/left
anterior oblique (LAO) and cranial (CRA)/caudal (CAU). The coordinate
system of TEE is the most complex so the same structure scanned at
different esophageal levels requires mental co-registration in
understanding (Figure 1C, right). The TEE is basically a cylindrical
coordinate system (Figure 1C, left) and its monitor produces 2D
anatomical information on the polar coordinate reference plane, similar
to the axial and the coronal CT MPR planes for trans-esophageal and
trans-gastric windows, respectively. However, the longitudinal axis
(similar to z-axis in Cartesian system) of such cylindrical coordinate
system is not a straight line, not to mention the anteflex and retroflex
probe manipulation; rather, the longitudinal axis actually means the
esophageal tract. Furthermore, the reference plane is allowed to rotate
around its polar axis (the mid-line of the scanning sector) and the
geometry of flex to the right/left maneuver is even more complex. As a
result, both the acquisition and the understanding of TEE images need
learning curve.
Mental co-registration of these modalities is the foundation to
facilitate heart-team communication. 4,5 For quick
switch between imaging modalities with different coordinate systems,
assistive theory and technology have been developed.
Echocardiography-fluoroscopic fusion imaging transforms the coordinate
system of TEE probe to that of the image intensifier so that 2D and 3D
TEE images can be calibrated and projected onto the 2D fluoroscopy.6,7 As a result, images of these two modalities can be
viewed in the same visual perspective 4,5,8 with
acceptable mean target registration error. Piazza and colleagues9,10 introduced the concept of optimal projection
curve that comprises all the projection angles of which the X-ray
tube-to-image intensifier direction is orthogonal to the normal vector
of the interested anatomical targets. Their method is applied to
minimize the parallax during the deployment of a quasicylindrical device
into an anatomical structure of variable geometry. However, the bridge
between cardiac CT and TEE is lacking. Adjusting the MPR crosshairs into
the center of mitral valve plane and crossing the third MPR line through
the LV apex are the traditional methods to obtain LV four-chamber,
two-chamber and long-axis CT re-slices without foreshortening.11 These views precisely describe the LV segments and
mitral apparatus, but are usually at a different reference plane from
that of TEE imaging. Although TEE simulator based on CT images has been
developed as a valuable teaching tool, 12 it
sacrifices the wide visual field of CT scan and loses image quality
during datasets transformation. Due to rapid development of new
trans-catheter devices, more and more distinct imaging requirements are
called for. As a result, it is mandatory to bridge the gap between the
pre-operative planning tool and the intra-operative imaging guidance.
For this purpose, we developed the methods of stepwise cardiac CT MPR
manipulation to mimic trans-esophageal echocardiography. On the other
hand, from the patient-specific information of cardiac CT, we can also
plan the TEE imaging in a preoperative rehearsal to make the desired
standardized TEE views more reproducible.
2 | METHOD (Figure 2)
Since the motion of TEE probe is confined to the esophagus-stomach
tract, it is difficult for TEE sonographers to reproduce the cardiac CT
MPR views used for preoperative plan. By contrast, it is easier to use
CT datasets to simulate TEE imaging. There are three basic MPR planes –
axial, coronal and sagittal – that are perpendicular to each other. One
can adjust the cross-sectional orientation of the other two planes by
moving and rotating their MPR lines on the selected MPR plane (i.e., we
can adjust coronal and sagittal MPR lines on axial plane and vice
versa). To represent the TEE scanning sector, we imagine a circular
sector with the origin in the MPR crosshairs on the axial plane and the
sector has a line of symmetry as the sagittal MPR line. Once we move the
MPR crosshairs into the esophagus with the following sequential steps,
we can translate basic TEE probe movements into corresponding CT MPR
manipulation with the following steps.
Step 1: Manipulation on the sagittal plane: advance, withdrawal,
anteflex and retroflex
The first step to simulate TEE on CT axial MPR plane is to invert the
z-axis (the coronal MPR line on the sagittal MPR plane), because CT
offers a perspective from a caudal direction on the axial MPR plane but
TEE, the cranial perspective on the reference plane. Next, we simulate
the advance/withdrawal by moving the crosshairs along the esophageal
tract into the gastric space. Moreover, we can simulate the anteflex by
clockwise rotation of the axial MPR line on the sagittal MPR plane and
the retroflex by counterclockwise rotation.
Step 2: Manipulation on the axial plane: turn to the left/right
Because the sector width of TEE is limited, sonographers have to turn
the TEE probe rightward to observe the right-sided structures and vice
versa. We can simulate this maneuver by rotating the sagittal MPR line
on the axial MPR plane. Step1 and Step 2 can be repeated randomly until
satisfactory image is obtained on the axial MPR plane before moving on
to the step 3.
Step 3: Manipulation on the coronal plane: rotation, flex to the
left/right
Since the z-axis has been inverted at the very beginning, the anterior
posterior direction on the coronal MPR plane is reversed, too. As a
result, the counterclockwise rotation of TEE scanning plane will be
simulated by a clockwise rotation of the axial MPR line on the coronal
MPR plane. TEE probe is steerable for delicate adjustment of imaging
orientation. The flexible joint is around 5cm above the scanning center.
To simulate the “flex to the right,” two steps have to be taken.
Firstly, we rotate the crosshairs about the flexible joint of TEE probe.
Secondly, a negative rotation with the same degree that the crosshairs
are rotated should be added. Examples of flex to the right at
mid-esophagus level and flex to the left at transgastric level are
illustrated in Figure 3 and Figure 4 respectively.