Abstract: X-ray CT provides imaging guidance for cryosurgery that allows 3D visualization of frozen and unfrozen tissue. Temperature in the tissue water-ice interface (0 to -10 

George A Sandison

Jikun Wei

Laigao Chen

Lisa X Xu

Yun Liang

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X-RAY CT HIGH DENSITY ARTIFACT SUPPRESSION IN CRYOSURGERY 
 
Laigao Chen1, Yun Liang2, Lisa X. Xu3, Jikun Wei1, George A. Sandison1 
 
1School of Health Sciences, Purdue University, West Lafayette, IN 47907 
2Department of Radiology, Indiana University Medical School, Indianapolis, IN 46202 
3School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907 
Proceedings of IMECE2002 
ASME International Mechanical Engineering Congress & Exposition 
November 1722, 2002, New Orleans, Louisiana 
IMECE2002-33698 
 
Abstract: 
X-ray CT provides imaging guidance 
for cryosurgery that allows 3D visualization of 
frozen and unfrozen tissue. Temperature in the 
tissue water-ice interface (0 to –10 oC) may be 
calibrated to Hounsfield units. However x-ray 
CT images and their thermal calibration can be 
compromised by the cryoprobes generating high-
density streak artifacts. A new subtraction 
technique for suppression of these artifacts is 
proposed and tested in prostate cryosurgery 
simulations. By subtracting the profile without 
cryoprobes from the profile with cryoprobes and 
iceballs, we can get the combined profile of the 
cryoprobes and a low value background. A 
polynomial interpolation is performed to obtain 
the background profile, which is then added back 
to the original profile without probes. The 
resulting profile is then fed to a conventional 
filtered back-projection routine to reconstruct the 
probe-free image. Finally the cryoprobe pixels in 
the originally constructed image with probes are 
added back to the probe-free image to get the 
final artifact-suppressed image. The major 
advantage of this subtraction technique is that it 
can successfully suppress the high-density 
artifacts in bone abundant body regions. X-ray 
CT images of cryoprobe arrays in a 
homogeneous gelatin phantom and the pelvic 
region of an anthropomorphic Rando phantom 
containing a human skeleton were generated. 
After suppression, cryoprobe metal artifact 
streaks are reduced and visualization of the 
positions and dimensions of the cryoprobes are 
well preserved. In-vivo canine studies are 
planned to test this new suppression technique 
for cryosurgery use. 
 
Introduction: 
Cryosurgery is a therapeutic procedure 
to ablate tissue by freezing injury. Hollow 
metallic tubes, known as cryoprobes, within 
which a gas or liquid cryogen is caused to flow 
are implanted in the tissue and ablating iceballs 
grow about them. Image guidance in cryosurgery  
 
1 
 
edings.asmedigitalcollection.asme.org on 06/30/2019 Terms of Use: his usually performed using ultrasound. However 
x-ray CT imaging is an alternative means of 
guidance having the advantage of displaying 3D 
structure of the iceball, including any tissue 
architecture within. Another advantage of the x-
ray CT modality is that temperature in the 
density change region of the water-ice interface 
can be calibrated to allow almost real-time 
isotherm contour overlays on the iceball images. 
Unfortunately, the x-ray CT images are 
compromised by the presence of artifacts caused 
by the array of metal cryoprobes [1-6]. One 
major source of metal artifacts in CT images is 
the beam hardening correction typically used in 
commercial CT scanners. CT reconstruction 
theory assumes that the incident x-ray beam is 
mono-energetic. In practice the x-ray beam in 
commercial CT scanners is poly-energetic and 
typically contains an energy spectrum from 
20keV to 120keV or 140keV. To remedy this 
problem, CT manufacturers typically apply a 
polynomial correction function to the measured 
attenuation data to approximate the ideal 
attenuation data for a mono-energetic x-ray beam 
[7]. This beam hardening correction function is 
usually derived and calibrated for body tissues 
and is therefore not accurate for metal objects. 
Broad dark streaks emanating from the 
cryoprobes result in the reconstructed CT image. 
Another important source of these metal streak 
artifacts stems from the poor signal-to-noise ratio 
due to photon starvation of the projection rays 
passing through the high attenuation metal 
cryoprobes. Resulting high fluctuations in the 
photon number for these projection rays lead to 
thin streaks (alternately dark and bright) along 
radial directions in the reconstructed image.  
In this paper, we propose a subtraction 
technique to reduce the artifacts caused by 
cryoprobes. This technique significantly 
suppresses the artifacts while still faithfully 
reproducing the positions and dimensions of the 
cryoprobes. Furthermore, the suppression 
technique is computationally efficient and easily 
adapted to commercial CT systems.  Copyright © 2002 by ASME 
ttp://www.asme.org/about-asme/terms-of-use
Downloaded From: https://Methods: 
The region of interest is scanned before 
the cryoprobes are inserted and scanned again 
after the cryoprobes are put in place. (Repeated 
scanning is required to follow the evolution in 
growth of the iceball formed during a typical 
freezing procedure). Hereafter we will use 
P1(m,n) to denote the parallel beam projection 
profile from the first scan, namely the one 
without any cryoprobe; and similarly P2(m,n) for 
profile from the second scan. And m=1,2…M, 
n=1,2…N are the indices of detectors and 
projection views respectively. If the original 
scanning geometry is in fan beam format, we 
assume it has already been rebinned to parallel 
beam format. These two profiles then go through 
the following steps to reduce the artifacts. 
Step 1: Finding the metal probe regions 
in the profile. A conventional filtered back-
projection routine is applied to reconstruct an 
initial image from P2(m,n). Metal probe pixels 
are extracted with a suitable threshold, which is 
normally 2000~3000 HU, from the initial image.  
For each view, the metal pixels are reprojected to 
form clear delineation of their corresponding 
projection channels. Instead of the typical 
projection ray driven reprojection technique [8], 
pixel driven reprojection technique is used. 
Specifically, each metal pixel is reprojected onto 
all view angles. This pixel driven reprojection 
technique is very computational efficient in our 
particular application due to the fact that the total 
number of metal pixels only occupy a very small 
portion of the whole image matrix. Then the 
metal probe regions in the profile can be 
determined as: 
 R(n) = {[s(i,n), e(i,n)]},   i=1,…I(n)       (1) 
where I(n) is the total number of metal probe 
regions within the n-th projection view; s(i,n) 
and e(i,n) are the starting and ending channels of 
the i-th metal probe region within the n-th 
projection view. 
Step 2: Subtraction and interpolation. 
By subtracting P1(m,n) from P2(m,n), which is 
the profile with the presence of cryoprobes and 
iceballs, we get the combined profile of the 
probes and a background. 
P3(m,n) = P2(m,n) - P1(m,n)             (2) 
There are several reasons for the presence of the 
low value background profile: (a) Since the 
density of ice is slightly lower than that of body 
tissues, there will be a negative background 
profile; (b) Body tissues may move a little during 
cryosurgery; (c) Intrinsic randomness of x-ray 
absorption and detection differences between 
two scans. 2 
 
proceedings.asmedigitalcollection.asme.org on 06/30/2019 Terms of Use: hA baseline profile B(m,n) can be 
obtained for all regions in R(n) by performing a 
polynomial interpolation to the projection data 
on both sides of the metal probe region 
[s(i,n),e(i,n)].  We denote this interpolation as an 
operator O : ˆ
B(m,n) = O  Pˆ 3(m,n)                     (3) 
Then this background profile is added back to 
the profile without probes P1(m,n) to get the 
probe-free profile. 
P(m,n) = P1(m,n) + B(m,n)            (4) 
Step 3: Filtered back-projection and 
adding back of the probe pixels. P(m,n) in Eq.(4) 
is the processed final profile that is fed to a 
conventional filtered back-projection routine to 
reconstruct the artifact-suppressed image. Those 
metal probe pixels obtained from step 1 by 
thresholding are now added back to the 
reconstructed image so as to reproduce the 
probes in the same positions and with the same 
dimensions.  
Overall, the above processing procedure 
in the projection profile domain can be expressed 
as:  
P(m,n) = P1(m,n)  + [PÔ 2(m,n) - P1(m,n)]  (5) 
and it can be automated and easily implemented 
on current commercial CT scanners.   
To illustrate the above procedure, the 
new method has been applied to a homogeneous 
gelatin phantom. As proposed, an axial CT scan 
was performed without any probe or iceball. 
Then five cryoprobes were inserted into the 
phantom to form a pentagon shape. An argon 
cryogen was used with the probes to cause the 
gelatin to freeze and form an iceball around each 
probe. After all five iceballs coalesced to be a 
single one, a second scan was performed. The 
whole phantom volume scanned comprised 32 
images of slice thickness 2.5mm. 
 To also test this subtraction method in 
the presence of bone structures, the pelvic region 
of an anthropomorphic Rando phantom 
(Phantom Laboratory Inc., Salem, NY) was used. 
The anthropomorphic Rando phantom represents 
an average man made from tissue-equivalent 
materials that is transected into transverse slices 
of 2.5 cm (1 in.) thickness. When assembled the 
36 slices, numbered 0-35, provide a head and 
torso with skeleton, lungs, and air passages. 
Scans of the phantom and five cryoprobes were 
obtained for slices 33 and 34. 
All CT scans were performed on a 
Philips MX8000 quad-slice spiral CT scanner 
(Philips Medical Systems). The scanning Copyright © 2002 by ASME 
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Downloaded From: https://parameters set were 120 kVp, 250 mA and slice 
thickness 2.5 mm. After scanning, the raw 
projection data were downloaded from the 
scanner to a SUN workstation. All the following 
processing procedures including rebinning, 
filtered back-projection and metal artifacts 
suppression were implemented on the SUN 
workstation using Interactive Data Language 
(Research Systems, Inc.). For the 3D rendering 
of the homogeneous gelatin phantom, the 
reconstructed image sets before and after 
correction are sent back to a dedicated view 
workstation of the MX8000 CT scanner (Philips 
Medical Systems) for, multi-planar reformation 
(MPR) and maximum intensity projection (MIP).  
 
Results and Discussions: 
As shown in Fig. 1(a), for the 
homogenous gelatin phantom, severe streak 
artifacts in the original x-ray CT image were 
caused by the cryoprobes. After correction, the 
major high density streak artifacts are 
successfully reduced, as shown in Fig. 1(b).  The 
correction is not perfect and a few new artifacts 
are introduced due to the intrinsic inconsistency 
of the polynomial interpolation among different 
projection profiles. Fig. 1(c) and Fig. 1(d) are 3D 
renderings of the cryoprobes and iceballs. Before 
correction, the iceballs are rendered in irregular 
shapes because of the streak artifacts while after 
correction the five iceballs are clearly rendered 
around each cryoprobe. Fig. 2(a,b,c) illustrate 
one of the projection profiles and its correction 
procedure. In Fig. 2(a) the solid line is the 
original profile where the peaks are due to the 
pentagon shaped array of metal cryoprobes. The 
dotted line is the profile before the probes were 
inserted to the phantom. Because the density of 
ice is lower than that of gelatin, the dotted line is 
a little higher than the solid line. After 
subtraction, we get the solid curve in Fig. 2(b) 
and perform the interpolation which is plotted as 
dotted line in this figure. Then the dotted line 
profile in Fig. 2(b) is added back to the dotted 
line profile in Fig. 2(a) and the result is plotted in 
Fig. 2(c), where the dotted line profile in this 
figure is the final processed profile that is fed to 
a back-projection routine to reconstruct the 
probe-free image.   
For the pelvic region of the Rando 
phantom which has large bone structures, Fig. 
3(a) shows the original reconstructed image with 
streak artifacts emanating from the probes. Fig. 
3(b) is the result after applying this new 
subtraction technique. Since it is not possible to 
form iceballs in the Rando phantom because it is 3
 
proceedings.asmedigitalcollection.asme.org on 06/30/2019 Terms of Use: hresin-based, the background profile mainly 
comes from the random noise of the two scans. 
Therefore a zero interpolation baseline was used. 
It is clearly shown that metal probe artifacts are 
successfully removed.  
 This artifact suppression technique does 
not require body tissues be fixed between the 
two x-ray scans because an interpolation is used 
to simulate the tissue profile. However, the bone 
structures do need to be well fixed between scans 
since the bone structure profile is difficult to 
simulate by interpolation. Movement of bony 
structure may be minimized by, for example, a 
stereotactic body frame [9]. Such frames may 
have an added bonus that is precision guidance 
of the cryoprobes. 
 
References: 
 
[1] Glover GH, Pelc NJ. “An algorithm for the 
reduction of metal clip artifacts in CT 
reconstructions.” Med. Phys. 8: 799-807, 1981. 
[2] Kalender WA, Hebel R, Ebersberger J. 
“Reduction of CT Artifacts Caused by Metallic 
Implants.” Radiology 164: 576-577, 1987. 
[3] Zhao S, Robertson DD, Wang G, Whiting B, 
Bae KT. “X-ray CT metal artifact reduction 
using wavelets: an application for imaging total 
hip prostheses.” IEEE Trans. Med. Imaging 19: 
1238-1247, 2000. 
[4] Wang G, Snyder DL, OSullivan JA, Vannier 
MW. “Iterative deblurring for CT metal artifact 
reduction.” IEEE Transactions on Medical 
Imaging, 15: 657-664, 1996. 
[5] Tuy H K. “A post-processing algorithm to 
reduce metallic clip artifacts in CT images.” 
European Radiology 3: 129-134, 1993. 
[6] De Man B, Nuyts J, Dupont P, Marchal G, 
Suetens P. “An iterative maximum-likelihood 
polychromatic algorithm for CT.”  IEEE Trans. 
Med. Imaging 20: 999-1008, 2001. 
[7] Herman GT. “Correction for beam hardening 
in computed tomography.” Phys. Med. Biol. 24: 
81-106, 1979. 
[8] Crawford CR. “Reprojection using a parallel 
back-projector.” Med.Phys. 13: 480-483, 1986. 
[9] Lax I, Blomgren H, Naslund I and Svanstrom 
R. “Stereotactic radiotherapy of malignancies in 
the abdomen.” Acta Oncologica, 33: 677-683, 
1994.  Copyright © 2002 by ASME 
ttp://www.asme.org/about-asme/terms-of-use
Downloaded From: https:/ (a)  
 
(b) 
 
 
(c) 
 
 
(d) 
 
 
Figure 1. Comparison of CT images of cryoprobes and iceballs in a homogeneous gelatin phantom before 
and after correction. (a) image before correction; (b) image after correction. (c) 3D rendering before 
correction; (d) 3D rendering after correction.   
 
 
 
 
 
 
 
 
 
4 Copyright © 2002 by ASME 
 
/proceedings.asmedigitalcollection.asme.org on 06/30/2019 Terms of Use: http://www.asme.org/about-asme/terms-of-use
Downloaded From: https://p(a) 
 
 
(b) 
 
 
(c) 
 
 
Figure 2. Diagrams of the subtraction technique procedure. (a) The two original projection profiles, solid 
line is the one with probes and iceballs, dotted line is without any probe or iceball. (b) Combined profile of 
the cryoprobes and the background, obtained by subtracting dotted line from solid line in (a). Then 
interpolation is performed and represented by the dotted line. (c) Resultant profile after adding the 
interpolated background back. 5 Copyright © 2002 by ASME 
 
roceedings.asmedigitalcollection.asme.org on 06/30/2019 Terms of Use: http://www.asme.org/about-asme/terms-of-use
Downloaded From: https://p 
(a) 
 
(b) 
 
 
Figure 3. CT images of the pelvic region of an anthropomorphic Rando phantom. (a) image before 
correction, there are streak artifacts emanating from the cryoprobes; (b) image after subtraction method, 
metal probe artifacts are removed without introducing bone artifacts. The dark areas surrounding the bright 
circular cryoprobes are due to air inside the holes made for holding the cryoprobes.
 
6 Copyright © 2002 by ASME 
 
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X-RAY CT HIGH DENSITY ARTIFACT SUPPRESSION IN CRYOSURGERY

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X-RAY CT HIGH DENSITY ARTIFACT SUPPRESSION IN CRYOSURGERY

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