Three-dimensional tracking of cardiac catheters using an inverse geometry
x-ray fluoroscopy system
Michael A. Speidela?
Department of Medical Physics, University of Wisconsin–Madison, Madison, Wisconsin 53705
Michael T. Tomkowiak
Department of Biomedical Engineering, University of Wisconsin–Madison, Madison, Wisconsin 53705
Amish N. Raval
Department of Medicine, University of Wisconsin–Madison, Madison, Wisconsin 53792
Michael S. Van Lysel
Department of Medicine and Department of Medical Physics, University of Wisconsin–Madison,
Madison, Wisconsin 53792
?Received 22 June 2010; revised 21 October 2010; accepted for publication 23 October 2010;
published 23 November 2010?
Purpose: Scanning beam digital x-ray ?SBDX? is an inverse geometry fluoroscopic system with
high dose efficiency and the ability to perform continuous real-time tomosynthesis at multiple
planes. This study describes a tomosynthesis-based method for 3D tracking of high-contrast objects
and present the first experimental investigation of cardiac catheter tracking using a prototype SBDX
Methods: The 3D tracking algorithm utilizes the stack of regularly spaced tomosynthetic planes
that are generated by SBDX after each frame period ?15 frames/s?. Gradient-filtered versions of the
image planes are generated, the filtered images are segmented into object regions, and then a 3D
coordinate is calculated for each object region. Two phantom studies of tracking performance were
conducted. In the first study, an ablation catheter in a chest phantom was imaged as it was pulled
along a 3D trajectory defined by a catheter sheath ?10, 25, and 50 mm/s pullback speeds?. SBDX tip
tracking coordinates were compared to the 3D trajectory of the sheath as determined from a CT
scan of the phantom after the registration of the SBDX and CT coordinate systems. In the second
study, frame-to-frame tracking precision was measured for six different catheter configurations as a
function of image noise level ?662–7625 photons/mm2mean detected x-ray fluence at isocenter?.
Results: During catheter pullbacks, the 3D distance between the tracked catheter tip and the sheath
centerline was 1.0?0.8 mm ?mean ?one standard deviation?. The electrode to centerline distances
were comparable to the diameter of the catheter tip ?2.3 mm?, the confining sheath ?4 mm outside
diameter?, and the estimated SBDX-to-CT registration error ??0.7 mm?. The tip position was
localized for all 332 image frames analyzed and 83% of tracked positions were inside the 3D sheath
volume derived from CT. The pullback speeds derived from the catheter trajectories were within
5% of the programed pullback speeds. The tracking precision of ablation and diagnostic catheter
tips ranged from ?0.2 mm at the highest image fluence to ?0.9 mm at the lowest fluence. Track-
ing precision depended on image fluence, the size of the tracked catheter electrode, and the contrast
of the electrode.
Conclusions: High speed multiplanar tomosynthesis with an inverse geometry x-ray fluoroscopy
system enables 3D tracking of multiple high-contrast objects at the rate of fluoroscopic imaging.
The SBDX system is capable of tracking electrodes in standard cardiac catheters with approxi-
mately 1 mm accuracy and precision. © 2010 American Association of Physicists in Medicine.
Key words: catheter tracking, inverse geometry, fluoroscopy, tomosynthesis, image guidance
Catheter ablation has become a widespread treatment for a
range of cardiac arrhythmias, including forms of atrial fibril-
lation and ventricular tachycardia. In the traditional electro-
physiology approach, a multielectrode catheter is guided in-
side the cardiac chambers under x-ray fluoroscopy. The
endomyocardial surface is probed to create a map of electri-
cal activation and then energy is delivered through the tip to
destroy triggers or pathways of arrhythmia propagation. Re-
cently, ablation strategies have been developed to electrically
isolate cardiac anatomy that is critical to arrhythmia propa-
gation. For example, in atrial fibrillation, there may be mul-
tiple trigger sites within the sleeves of atrial tissue extending
into the pulmonary veins.1Instead of targeting all of these
sites separately, operators ablate circumferentially around the
pulmonary vein ostia2or antrum.3These anatomic ablation
6377 6377 Med. Phys. 37 „12…, December 2010 0094-2405/2010/37„12…/6377/13/$30.00© 2010 Am. Assoc. Phys. Med.
strategies demand accurate real-time knowledge of catheter
tip position relative to moving 3D cardiac structures.
Conventional x-ray fluoroscopy offers high resolution
real-time imaging of metallic catheter electrodes. However,
the lack of depth information and poor soft tissue contrast in
a 2D x-ray projection limits the utility of fluoroscopy alone
for anatomic-based ablation. Biplane x-ray fluoroscopy im-
proves the ability to appreciate catheter position in 3D but
the ionizing radiation dose to the patient is a source of con-
cern. Radiofrequency catheter ablation ?RFCA? is associated
with long fluoroscopic imaging times and radiation-induced
skin injuries have been reported.4A study of 28 RFCA pro-
cedures reported the fluoroscopic time from both planes av-
eraged 120.8 min ??62.6 min?, with peak skin dose increas-
ing with both fluoroscopic time and patient weight.5The
mean effective radiation dose from biplane fluoroscopy dur-
ing ablation of atrial fibrillation has been reported as 15.2–
39.0 mSv, increasing with body mass index.6
Nonfluoroscopic 3D electromagnetic ?EM? catheter track-
ing systems have emerged as an important tool for catheter
ablation. These systems perform 3D tip tracking using a local
magnetic field emitter and a field-sensing catheter tip.7The
tracked positions can be used to generate 3D electroanatomic
maps of activation time and can be merged with preacquired
volume images of cardiac anatomy ?e.g., cardiac CT?.8EM
tracking facilitates catheter ablation but has several limita-
tions. Specialized catheters and external equipment are re-
quired. Trackable catheters are limited to those offered by the
tracking system vendor. The specialized hardware compo-
nents built into the catheter constrain device profile and me-
chanical performance. Since these systems do not provide
live imaging, they are typically used in combination with
conventional x-ray fluoroscopy. Low dose imaging methods
that could offer 3D localization and tracking of any catheter
device would be an appealing alternative.
Inverse geometry x-ray fluoroscopy, based on scanning
beam digital x-ray ?SBDX? technology, has the potential to
substantially reduce patient x-ray doses and also provide
three-dimensional tracking of unmodified catheters by using
real-time tomosynthesis.9–12SBDX performs fluoroscopy
and angiography at 15–30 frames/s using a rapidly scanned
narrow x-ray beam directed at a small-area photon-counting
detector array ?Fig. 1?. As detailed in Ref. 10, the use of a
narrow x-ray beam, distant detector, and thick CdTe x-ray
detector results in low levels of detected scatter and high
x-ray stopping efficiency over a range of kVps, which in turn
allow a given image signal-to-noise ratio ?SNR? to be
achieved with lower x-ray output and lower patient dose.
Inverse geometry spreads the source x-rays over a larger area
at the patient entrance, further reducing skin dose. Signal-to-
noise ratio and entrance exposure measurements on a proto-
type SBDX system have demonstrated the potential for 84%
entrance exposure reduction without loss of SNR compared
to a conventional cardiac angiographic system at equal
The inverted system geometry also gives SBDX a unique
real-time tomosynthesis capability. SBDX simultaneously re-
constructs multiple tomosynthetic images spaced throughout
the patient, each of which portrays in-plane objects in focus
and out-of-plane objects as blurred. A 2D multiplane com-
posite is generated for live display.10Recently, algorithms
were described that use the tomosynthetic images generated
by SBDX to perform 3D tracking of high-contrast objects
such as catheter electrodes.12Computer simulations of
simple geometric phantoms indicated 3D tracking with sub-
millimeter accuracy and precision was feasible, depending
on image SNR, object velocity, and system tomographic
In this paper, we present the first investigation of the
SBDX catheter tracking algorithm using images acquired
with an SBDX prototype. The catheter tracking algorithm is
detailed and two phantom studies are reported. In the first
study, the accuracy and precision of catheter tracking is
evaluated in the presence of anatomic background for differ-
ent catheter velocities. In the second study, the precision of
3D localization is determined for six different cardiac cath-
eter geometries as a function of image SNR.
The 3D catheter tracking algorithm is an extension of the
tomosynthetic image reconstruction and plane scoring tech-
nique that is used to generate the live multiplane composite
display for SBDX. We begin with a brief review of the
SBDX scanning and image reconstruction method and then
describe the catheter tracking algorithm.
II.A. Scanning and image reconstruction
The SBDX x-ray source consists of a magnetically de-
flected focal spot, a planar transmission target, and a multi-
hole collimator. The collimator holes define a set of narrow
overlapping x-ray beams directed at the x-ray detector. There
are 100?100 focal spot positions on a 2.3 mm pitch. The
detector is located 1500 mm above the target plane in the
x-ray source. The source and detector are mounted to a gan-
FIG. 1. The SBDX system uses a raster scanned focal spot, transmission
target, multihole collimator, and hardware based reconstructor. Boxes with
solid lines indicate the steps of image formation and image presentation in
the current SBDX prototype. The proposed catheter tracking steps are
shown with dashed lines.
6378 Speidel et al.: 3D catheter tracking using inverse geometry x-ray fluoroscopy6378
Medical Physics, Vol. 37, No. 12, December 2010
The tracking algorithm is based on the concept that object
blurring versus plane position can be treated as a distribution
function whose center-of-mass represents the true object po-
sition. It is designed to make use of the tomosynthetic recon-
structions and plane scoring techniques that were developed
for real-time fluoroscopic image display. Tomosynthesis-
based 3D localization has been reported in other applica-
tions, for example, brachytherapy seed localization.19Track-
ing in cardiac applications is challenging, however, due to
the high object velocities involved. To our knowledge,
SBDX is the only tomosynthesis system with the scanning
speed, x-ray output, and continuous imaging capability re-
quired for both live x-ray fluoroscopy and frame-by-frame
3D cardiac catheter tracking.
The SBDX study of a moving ablation catheter in a chest
phantom demonstrated 3D tip tracking with 1.0 mm mean
error after registering and comparing tracked positions to CT.
Imaging was conducted at an intensity similar to that which
may be encountered clinically. The study of stationary cath-
eters versus image intensity demonstrated z-coordinate stan-
dard deviations ranging from 0.2 to 0.9 mm for ablation and
diagnostic catheter tips depending on background image in-
tensity. Smaller deviations were observed in the x- and
y-coordinates. Our experiments show that SBDX may offer
similar catheter tip tracking accuracy to commercially avail-
able nonfluoroscopic EM tracking systems which are in use
for radiofrequency catheter ablation procedures. EM tracking
systems have been shown to provide ?1 mm relative dis-
tance error and location standard deviation.20
Tracking precision depended on image intensity, catheter
electrode size, and electrode contrast. The improvement in
tracking precision with background intensity was consistent
with expectations. This may be understood as the result of
improved object signal-to-noise ratio in each tomosynthetic
image, which yields more reliable gradient estimates at any
object point, and also a reduction in background gradient
values, which gives more freedom in the selection of detec-
tion threshold. A previous work has shown that z-axis preci-
sion is also dependent on the tomographic angle of the
SBDX system.12In a detector redesign currently underway,
the detector width has been doubled. The increase in both
tomographic angle and image SNR anticipated for this new
detector is expected to improve tracking precision compared
to the SBDX prototype used in this study.
Tracking performance depends on the proper selection of
filtering kernels and score thresholds. In this study, several
guidelines for the selection of these parameters are pre-
sented. Although the algorithm could be optimized to yield
the best precision for individual electrodes and electrode ori-
entations, there is value in finding a set of parameters that
work for a variety of objects simultaneously. In this study, it
was possible to simultaneously track the electrodes on five
different catheters across a range of image intensities. Mul-
tiple catheter tracking may be useful in a scenario where a
navigated catheter tip is tracked relative to a second catheter
positioned at an anatomic landmark ?e.g., in the coronary
Several limitations of the tracking algorithm should be
noted. The tracking algorithm used here assumes that elec-
trodes are nonoverlapping in the image field-of-view. An al-
ternative algorithm that enables tracking of overlapping ob-
jects separated by several cm has been described; however, it
is more computationally expensive.12We note that the non-
overlapping condition is often satisfied during clinical fluo-
roscopy since the interventionist generally avoids end-on
views. Although a variety of velocities and 3D trajectories
were tested in phantoms, true 3D catheter motion in the clini-
cal setting may be more complicated and will include respi-
ratory motion. Studies of tracking performance in animal
models are underway.
Ultimately, a real-time implementation of the tracking al-
gorithm is desired. Presently, only multiplane tomosynthesis
and plane scoring may be performed in real-time SBDX
hardware. Tracking was performed offline in software using
detector data recorded and downloaded from the reconstruc-
tor. We note that the design of a next generation SBDX sys-
tem is underway and the real-time reconstruction hardware
for that system has been designed to enable both imaging
and independent analysis of tomosynthetic plane stacks ?e.g.,
catheter tracking?.21For a clinical implementation of SBDX
catheter tracking, a method for registering tracked 3D coor-
dinates to preacquired volume images ?e.g., CT or MR? is
also desired. The external fiducial technique used in this
study to register SBDX-to-CT is one possibility, as is the use
of internal landmarks or internally placed catheters. This is
left as a topic for further investigation.
High speed multiplanar tomosynthesis with an inverse ge-
ometry x-ray fluoroscopy system enables accurate and pre-
cise 3D tracking of high-contrast electrodes in standard car-
diac catheters. Phantom studies with the SBDX prototype
system demonstrate catheter tip tracking accuracy and preci-
sion of 1 mm or better is feasible. The combination of low
dose fluoroscopic imaging and 3D catheter tracking offered
by SBDX technology is well-suited to anatomically targeted
interventional procedures such as radiofrequency catheter ab-
lation of cardiac arrhythmias.
FIG. 13. Standard deviation in proximal electrode z-coordinate versus mean
6388 Speidel et al.: 3D catheter tracking using inverse geometry x-ray fluoroscopy 6388
Medical Physics, Vol. 37, No. 12, December 2010
Financial support for this work was provided by NIH
Grant No. R01 HL084022. The authors wish to thank Dr.
Douglass Kopp and Dr. Andrew Klein for their helpful dis-
cussions and assistance. The authors also thank Triple Ring
Technologies, Inc. and NovaRay Medical, Inc. for technical
support for the SBDX prototype system.
a?Author to whom correspondence should be addressed. Electronic mail:
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