Introduction
In the physiology of human handling, the hand always follows the eye. Hence, optimal
vision is indispensable to perform surgery. With the advent of minimally invasive
cardiac surgery, the unobscured direct vision was no longer possible, and therefore
endoscopy was introduced to enable it. Advanced endoscopic techniques, as well as
catheter-based interventions, demanded improved preoperative planning based on echocardiographic
or computed tomographic (CT) data. Therefore, cardiac surgeons had not only to understand
and interpret imaging data but also integrate them into their daily practice. Prof.
Mohr was one of the pioneers in the field as very early he realized the importance
of imaging in the curriculum of a cardiac surgeon, and promoted the acquisition of
imaging skills during the residency program in Leipzig. Consequently, several surgeons
and anesthesiologists actively developed research projects in the field of image-guided
therapy.
Early Days
During his research fellowship at Cedars-Sinai Medical Center, Prof. Mohr started
his experimental and clinical research in the field of thermal coronary angiography.[1] This technique was developed for intraoperative noninvasive testing of anastomotic
patency and coronary flow by visualizing small temperature differences between adjacent
structures. A young research colleague of Prof. Mohr at that time was Volkmar Falk,
who wrote his thesis on this specific topic and they continued working in this imaging
field in the early Leipzig days to develop thermal coronary angiography as a reliable
intraoperative visualization tool to assess graft patency.[2]
[3]
[4]
[5] Besides the academic interest in cardiac imaging, Prof. Mohr established an echocardiography
group within the surgical department, to perform routine echocardiographic examinations
and to foster research related to image-guided surgery. In the mid-1990s an echocardiography
laboratory run by cardiac surgeons was quite rare in Germany but served as a testimony
to the vast interest of Prof Mohr with regards to imaging as an integral part of a
surgical career.
Expanding of Imaging to the Next Level
Expanding of Imaging to the Next Level
In 2005, the “innovation center for computer assisted surgery (ICCAS)” was founded
in collaboration with the Leipzig Heart Center. One of the early foci of ICCAS was
to develop the field of image-guided robotic surgery including augmented reality applications.[6]
[7]
[8] This comprised planning, simulation, and performance of augmented reality enhanced
robotically assisted endoscopic bypass grafting. Another augmented reality application
was developed for determining the optimal annuloplasty ring size during mitral valve
repair using a template overlay on top of three-dimensional transesophageal echocardiography
(3D-TEE).[9]
TAVI Imaging in Daily Cardiac Surgical Practice
TAVI Imaging in Daily Cardiac Surgical Practice
With the early introduction of transcatheter aortic valve implantation (TAVI) at the
Leipzig Heart Center in 2005,[10] a new demand for advanced imaging became obvious. Imaging was no longer only important
for supporting intraoperative strategies but became indispensable to guide the procedure
and to help to avoid life-threatening complications. The closed-heart situation of
TAVI was a new setup for cardiac surgeons who were used to have direct vision of the
operating field and the aortic valve. This led to the need for precise preoperative
and intraoperative planning and sizing of the aortic root anatomy including the aortic
annulus. Initially, TEE was the main imaging technique for preoperative annulus sizing.[11]
[12] With growing experience in TAVI and the finding of a high percentage of paravalvular
leakage the complex 3D geometry of the aortic annulus came into focus.[13] First experiences with aortic annulus sizing in 3D-TEE showed precise measurements,[14] but soon the potential advantage of CT for routine TAVI screening became evident.
CT allows for a 3D view of all anatomic structures of interest, including calcification
patterns. The Leipzig group adopted this technique quickly and presented the concept
of the definition of an effective aortic annulus diameter based on the luminal circumference
of the aortic annulus and the resulting calculated area.[15] Ever since the effective diameter has become the standard now routinely used for
CT-based annulus diameter measurements.
Intraoperative guidance and template-based planning are other important aspects of
TAVI procedures. In close cooperation with ICCAS and Siemens Healthcare new concepts
of intraoperative TAVI imaging were evaluated at the Leipzig Heart Center, in Zürich,
Switzerland and in Bad Nauheim, Germany. Tracking of landmarks during live 2D fluoroscopy
was developed as a tool to enable precise implantation of TAVI prostheses. In an ICCAS
project, imaging-registration flowcharts were developed and integrated into fluoroscopy.[16] The so-called “DynaCT,” an intraoperative 200 degrees rotational angiography to
generate 3D CT-like images ([Fig. 1]) using a C-arm[17]
[18] was for the first time applied to visualize the aortic root on top of the life fluoroscopy
during TAVI procedures. This technique, which was also patented by the Leipzig team
in conjunction with Siemens, was further developed clinically at the Leipzig Heart
Center and the Kerckhoff Klinik in Bad Nauheim. DynaCT is now routinely used to optimize
C-arm angulation. The latest generation of DynaCT software allows for annulus diameter
and coronary ostia distance measurements.[19] Early ideas of template-based overlay of prostheses models into the aortic root
to predict implantation height and valve size[20] were also integrated into this software.
Fig. 1 Three-dimensional model of the aortic root generated by DynaCT: The C-arm angulation
is presented in the right upper corner, the red line under the aortic annulus indicates
a perpendicular angulation of the C-arm to the annulus, the green, and blue dots represent
the right and left coronary ostia, the three red dots represent the three nadirs of
the aortic valve, the yellow centerline is interpolated in the aorta, and the purple
dots and lines indicate the commissures and their course.
Advanced Mitral Valve Imaging
Advanced Mitral Valve Imaging
The Leipzig Heart Center is well known for its minimally invasive mitral valve repair
program. Advanced mitral valve imaging was always one of Prof. Mohr's major fields
of interest. There was an early adoption of 3D-TEE for mitral valve surgery planning
and judgment.[8]
[21] Detailed anatomical and pathological, as well as a physiological understanding of
the mitral valve apparatus, are indispensable for successful mitral valve surgery.
Real-time 3D-TEE allows for examining all relevant structures preoperatively and intraoperatively
and serves as a quality control postoperatively. Intraoperative TEE is also able to
detect most of the severe complications that may occur during mitral valve surgery,
such as iatrogenic injury of the circumflex coronary artery which was systematically
explored in a large case series.[22] The use of 3D-TEE soon became clinical routine in mitral valve surgery centers worldwide.
Lately, augmented-reality applications, model base planning, and 4D modeling were
clinical research areas at the Leipzig Heart Center.[8]
[23]
[24] Different types of mitral valve models have been evaluated and established with
different levels of complexity in calculating and understanding. Dynamic mitral valve
models that can be reconstructed using 4D CT or real-time 3D-TEE have been found especially
useful as they allow the evaluation of the mitral valve throughout the entire cardiac
cycle. In the near future, structure–fluid interaction models will be explored. First
experiences with 4D modeling of the mitral valve in real-time 3D-TEE ([Fig. 2]) have been published[23] and the clinical use in complex mitral valve repair planning has been demonstrated
in a live case at 2016 EACTS annual meeting, held at Barcelona, Spain.
Fig. 2 MV assessment by four-dimensional quantification software. Three-dimensional transesophageal
echocardiography image in mid-systole shows severe P2-prolapse (A). Assessment of MV morphology in mid-systole by MV model before surgical MV repair
(B). Assessment of MV in mid-systole after surgical MV repair. MV, mitral valve.
