IMAGINE

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IMAGINE

• Maximiliano Arroyo
• UT Division of Cardiovascular Diseases
• January 19th, 2006

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• In 1999, more than 1.83 million coronary
  angiograms were performed in the US. Only 1/3rd
  were performed in conjunction of an
  interventional procedure.
• CT is the premier noninvasive modality for
  vascular imaging of the thorax the heart,
  however, has always been technically challenging
  because of its continuous motion.
• The cross-sectional nature of CT may enable
  assessment of the vessel wall. The potential for
  noninvasive identification, characterization, and
  quantification of atherosclerotic lesions and
  total disease burden within the coronary arteries
  is currently being evaluated.

U. Joseph Schoepf. Radiology 200423218-37
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Modalities

• EBCT Introduced in 1984, was the first system to
  enable ECG-synchronized CT imaging of the cardiac
  anatomy. With presently available scanners, the
  routine protocol comprises a collimation of 3 mm,
  a temporal resolution of 100 msec, and
  prospective ECG triggering for sequential
  acquisition of transverse images consistently at
  the same phase of the cardiac cycle, typically
  during diastole.

U. Joseph Schoepf. Radiology 200423218-37
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• MDCT Introduced in 1998, mechanical spiral CT
  systems with simultaneous acquisition by four
  detector rows and a minimum rotation time of 500
  msec were introduced. This provided a
  substantial performance increase over the spiral
  CT systems that had been available until then.

U. Joseph Schoepf. Radiology 200423218-37
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• A higher temporal resolution is enabled by means
  of faster gantry rotation speed combined with
  dedicated image reconstruction algorithms. The
  strategy that has been pursued to further
  improve fast high-resolution volume coverage is
  to increase the number of sections that are
  simultaneously acquired. So far, this has
  resulted in the introduction of eight, 10, 16,
  32, 40, and 64detector row CT scanners with
  further reduced gantry rotation times and minimum
  beam collimation widths of less than 1 mm.

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• Presence of severe calcification is a limitation
  of contrast-enhanced CT coronary angiography
  because beam-hardening and partial-volume effects
  can completely obscure the cross section of the
  vessel and prevent assessment of the patency of
  the coronary artery lumen. Owing to similar
  effects, metal objects such as stents, surgical
  clips, and sternal wires can also interfere with
  the evaluation of underlying structures. Use of
  the thinnest possible section width reduces
  partial-volume artifacts to some extent and
  improves assessment of calcified coronary
  segments.

U. Joseph Schoepf. Radiology 200423218-37
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ECG-synchronized CT Scan Acquisition

• Prospective Triggering
• A trigger signal is derived from the patients
  ECG on the basis of a prospective estimation of
  the present R-R interval, and the scan is started
  at a defined time point after a detected R wave,
  usually during diastole. With MDCT, several
  sections are obtained simultaneously during one
  scan acquisition with a cycle time that
  ordinarily allows image acquisition at every
  other heartbeat. In general, this strategy
  results in shorter breath-hold times, and
  respiratory artifacts are less likely to occur.

U. Joseph Schoepf. Radiology 200423218-37
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• To improve temporal resolution, scan data are
  only acquired during a partial scanner rotation
  (approximately two-thirds of a rotation with
  240260 projection data), which covers the
  minimum amount of data required for image
  reconstruction. In this way, prospective ECG
  triggering is the most dose-efficient method for
  ECG-synchronized scanning.

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• However, only rather thick section collimation (3
  mm with EBCT, 1.5 mm with 16detector row CT) is
  usually being used for a prospectively
  ECG-triggered acquisition. Thus, the resulting
  data sets are less suitable for 3D reconstruction
  of small cardiac anatomy. Also, the prospectively
  ECG-triggered technique greatly depends on a
  regular heart rate of the patient and is bound to
  result in misregistration in the presence of
  arrhythmia.

U. Joseph Schoepf. Radiology 200423218-37
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• Retrospective Gating
• An alternative approach is retrospective ECG
  gating. This generally enables greater
  flexibility for phase-consistent image
  reconstruction when examining a patient with a
  changing heart rate during acquisition.
  Retrospective ECG gating requires multidetector
  row spiral scanning with a slow table motion and
  simultaneous recording of the ECG trace, which is
  used for retrospective linkage of scan data with
  particular phases of the cardiac cycle.

U. Joseph Schoepf. Radiology 200423218-37
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• Retrospectively ECG-gated CT of the heart
  requires a highly overlapping spiral scan with a
  spiral table speed adapted to the heart rate to
  ensure complete phase-consistent coverage of the
  heart with overlapping image sections.

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At heart rates less than a predefined threshold,
one segment of consecutive multisection spiral
data is used for image reconstruction. At higher
heart rates, two or more subsegments from
adjacent heart cycles contribute to the partial
scan data segment. In each cardiac cycle, a stack
of images is reconstructed at different z-axis
positions covering a small subvolume of the heart
.
U. Joseph Schoepf. Radiology 200423218-37
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• The continuous spiral acquisition enables
  reconstruction of overlapping image sections,
  with a longitudinal spatial resolution of up to
  0.6 mm.
• Retrospectively ECG-gated acquisition is the
  preferred method for contrast-enhanced
  high-spatial-resolution imaging of small cardiac
  structures, especially the coronary arteries.
• Diastole is usually chosen for image
  reconstruction because it is the phase of the
  cardiac cycle with the least motion however,
  owing to the highly overlapping acquisition,
  image data can be reconstructed for the entire
  course of the cardiac cycle.

U. Joseph Schoepf. Radiology 200423218-37
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• Optimizing Spatial ResolutionSpatial resolution
  is largely dependent on the type of scanner
  available. The smallest detector widths range
  from 0.5 to 1.25 mm.
• The spatial resolution of four detector row CT is
  0.6 x 0.6 x 1.0 mm, that of electron-beam CT is
  0.7 x 0.7 x 3 mm, and that of magnetic resonance
  (MR) coronary angiography is 1.25 x 1.25 x 1.5
  mm. Spiral CT allows volume acquisition and
  reconstruction of overlapping sections, which
  improve z-axis resolution. The resolution of 16
  detector row CT is up to 0.5 x 0.5 x 0.6 mm. This
  resolution is approaching, but remains inferior
  to, that of conventional angiography, which is
  0.2 x 0.2 mm.

Harpreet P.Radiographics. 200323S111-S125
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• Optimizing Temporal ResolutionThe temporal
  resolution is the amount of time it takes to
  acquire the necessary scan data to reconstruct an
  image. The temporal resolution of electron-beam
  CT is 100 msec, and that of MR imaging is 100150
  msec. For multisection CT, it is primarily
  dependent on the time taken by the scanner to
  complete one gantry rotation but can be modified
  by using partial scan reconstruction techniques.

Harpreet P.Radiographics. 200323S111-S125
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Radiation Dose

• Relatively high radiation exposure is involved
  with retrospectively ECG-gated imaging because of
  continuous x-ray exposure and overlapping data
  acquisition at a slow spiral table feed, a
  substantial portion of the acquired data and
  radiation exposure are redundant and do not
  contribute to image generation.
• There is considerable disagreement in the
  literature as to the actual radiation dose,
  because the lack of standardization of the
  protocols.

U. Joseph Schoepf. Radiology 200423218-37
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• For high spatial resolution (1.001.25-mm beam
  collimation), a retrospectively ECG-gated
  acquisition), and routine scanner settings with
  fourdetector row CT, an exposure limit of
  approximately 10 mSv is applied, which is two to
  three times the average annual background
  radiation in the United States. Comparable to the
  exposure received during a typical routine
  diagnostic coronary angiogram. As progressively
  thinner beam collimations are used for scanner
  types with added detector rows, radiation dose
  generally increases.

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CR Conti. Clin. Cardiol. 28450-453
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CR Conti. Clin. Cardiol. 28450-453
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Contrast Injection

• Scanning times for imaging of the heart with 8 or
  16 detector row CT scanners range from 20 to 40
  seconds, 80120 mL contrast medium injected at a
  rate of 35 mL/sec is needed to maintain
  homogeneous vascular contrast throughout the
  scan.
• Saline chasing (eg, bolus of 50 mL of saline
  injected immediately after the iodinated contrast
  medium bolus) has proved to be helpful for better
  contrast medium bolus utilization, for high and
  consistent vascular enhancement, and for
  prevention of streak artifacts, which frequently
  arise from dense contrast material in the
  superior vena cava and right atrium and sometimes
  interfere with the evaluation especially of the
  right coronary artery.

U. Joseph Schoepf. Radiology 200423218-37
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Data Display

• Maximum intensity projection Not only displays
  coronary artery CT data in a more intuitive
  format but also condense diagnostic information
  into a few relevant sections or views. For
  routine visualization of large-volume CT coronary
  angiography data sets, many centers perform three
  dedicated maximum intensity projection
  reconstructions to create views of the left and
  right coronary arteries and of the entire
  coronary arterial tree from a cranio-oblique
  perspective.

U. Joseph Schoepf. Radiology 200423218-37
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• (a) RAO view along the interventricular groove
  shows LAD, with mixed atherosclerotic lesion
  (arrowhead) with calcified components in the
  proximal course of the vessel.
• (b) LAO view in plane RCA with calcified nodules
  (arrowheads) along the course of the vessel.
• (c) LAO "spider" view shows (LAD and its diagonal
  branches, with soft-tissue-attenuation plaque
  (arrowhead) in the anterior aspect of the left
  main coronary artery (LM) wall.

U. Joseph Schoepf. Radiology 200423218-37
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• Multiplanar reformations image data can be
  rearranged in arbitrary imaging planes, with
  image quality comparable to that of the original
  transverse sections.

left anterior descending coronary artery in a
patient with CAD.
U. Joseph Schoepf. Radiology 200423218-37
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• Three-dimensional display 3D post processing is
  a means of displaying information in an intuitive
  fashion. The most commonly used technology for 3D
  display of the coronary arterial tree is volume
  rendering.

Left Anteroposterior cranial projection shows
LAD and Cx. Right Volume rendering in
anteroposterior cranial projection shows left
main coronary artery with its branches, LAD and
Cx.
U. Joseph Schoepf. Radiology 200423218-37
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• Contrast-enhanced 16-detector row CT coronary
  angiography. Colored volume rendering of right
  coronary artery (RCA) displayed in slightly
  cranial right anterior oblique.

U. Joseph Schoepf. Radiology 200423218-37
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Contrast-enhanced CT of Coronary Artery
Anomalies, Bypass Grafts, and Stents

• MR imaging is limited with regard to
  determination of the distal coronary arterial
  course. Therefore, CT is the preferred modality
  for evaluation of small collateral vessels,
  fistulas, and vessels originating outside the
  normal sinuses.

U. Joseph Schoepf. Radiology 200423218-37
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Patient with superdominant anomalous right
coronary artery (AnRCA) supplying the majority of
the myocardium. (a) Selective conventional
angiographic image and (b) volume-rendered 3D
reconstruction (cranial right anterior oblique
perspective) from contrast-enhanced 16-detector
row CT coronary angiography.
U. Joseph Schoepf. Radiology 200423218-37
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• Bypass graft imaging more clinically relevant,
  is complex functional assessment of bypass flow,
  accurate detection of graft lesions, and reliable
  visualization of (distal) anastomoses. Data on
  the accuracy of CT for the detection and grading
  of hemodynamically significant graft stenosis are
  still rather sparse and are ordinarily based on
  small patient populations studied with
  electron-beam or multidetector row CT.1
• In a somewhat larger patient population
  investigated with fourdetector row CT, overall
  sensitivity and specificity values for bypass
  occlusion of 97 and 98, respectively, were
  reported.2

1.U. Joseph Schoepf. Radiology 200423218-37
2. Ropers D. Am J Cardiol 2001 88792-795
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Thomas Schlosser. JACC 2004 441224-1229
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• All IMA grafts could be visualized with
  diagnostic image quality, whereas only 28 of 37
  (76) of the distal anastomoses to the LAD and 3
  of 5 (60) of the distal anastomoses to the
  diagonal branches could be evaluated.
• A total of 11 of 42 (26) of the distal IMA
  anastomoses were classified as unevaluable due to
  poor opacification and artifacts caused by metal
  clips.

Thomas Schlosser. JACC 2004 441224-1229
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• MSCT permitted visualization of all proximal and
  distal anastomoses of venous grafts to the LAD.
• Invasive coronary angiography revealed 8 venous
  grafts to the LCX to be occluded, all correctly
  diagnosed by MSCT. All proximal and 25 of 33
  (76) distal anastomoses in the LCX region were
  adequately seen on MSCT. The remaining 8 distal
  anastomoses (24) were classified as unevaluable
  due to poor opacification and/or artifacts caused
  by cardiac motion.
• All proximal and 22 distal anastomoses (63) to
  the RCA, could be visualized. A total of 13 of 35
  (37) of the distal anastomoses were classified
  as unevaluable. Overall, 83 of 112 (74) distal
  anastomoses could be evaluated.
• The unevaluable distal anastomoses were estimated
  as stenotic. This results in a lower specificity
  (68) and positive predictive value (PPV) (37)
  compared with the separate analysis of the
  evaluable segments (specificity 95, PPV 81).

Thomas Schlosser. JACC 2004 441224-1229
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(LIMA) bypass graft. Anastomosis has been created
between left internal mammary artery and left
anterior descending coronary artery (LAD)
territory. Note extensive atherosclerotic changes
in the native vessels.
Colored volume-rendered view from anterior
perspective, derived from 16-detector row CT
angiography, 3 venous bypass grafts VCABG-LAD,
VCABG-Cx, and VCABG-RCA. Additional left internal
mammary artery bypass graft (LIMA-BG), also to
the LAD
U. Joseph Schoepf. Radiology 200423218-37
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• Coronary stents have been notoriously difficult
  to assess with CT. Contrast-enhanced CT can be
  used to assess stent patency on the basis of
  contrast enhancement in the course of the artery
  with the stent, because an unenhanced distal
  coronary artery lumen usually reflects critical
  in-stent restenosis. However, assessment of the
  stent lumen for nonocclusive in-stent restenosis
  due to neointimal hyperplasia remains
  challenging.

U. Joseph Schoepf. Radiology 200423218-37
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(a) Colored 3D volume-rendered view from right
posterior oblique perspective reveals luminal
narrowing (arrowhead) of artery proximal to the
stent. (b) Maximum intensity projection and (c)
multiplanar reformation in oblique coronal planes
show patent stent lumen and mixed atherosclerotic
lesion (arrow) with calcified and noncalcified
components as the cause of stenosis proximal to
the stent. (d) Conventional angiographic image in
left anterior oblique projection confirms stent
patency and presence of stenosis.
U. Joseph Schoepf. Radiology 200423218-37
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Box and whisker plot (median value and quartiles)
of angiographic in-segment coronary stenosis
(measured by quantitative coronary angiography
QCA) for each of the four grades of MDCT
narrowing. Grade 1, none or minimal narrowing
grade 2, moderate but obstructing lt50 of the
lumen grade 3, significant (50) but not severe
narrowing grade 4, severe narrowing to total
occlusion of stented segment.
Tamar Gaspar, JACC 2005 46 1573-1579
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• Five stents that were not assessable by MDCT were
  excluded.
• MDCT excluded restenosis in two-thirds of
  patients. this would result in only 1 in 10
  stents with restenosis being missed (or 13.5 of
  patients).

Tamar Gaspar, JACC 2005 46 1573-1579
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Contrast-enhanced CT Angiography for CAD

• In 763 coronary segments, CCA detected a total of
  75 lesions 50.
• The MSCT correctly assessed 54 of these.
  Twenty-one lesions were missed or incorrectly
  underestimated. Sensitivity was 72, specificity
  97.

Axel Kuettner. JACC 2004 441230-1237 Ricardo
C. Cury. AJC. 2005 96784-787
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• 64 slice MSCT compared to QCA for quantification
  of lesion severity
• 935 of 1,065 segments (88) could be analyzed
  either quantitatively or qualitatively. Of these,
  773 of 935 (83) segments could be quantitatively
  measured by both MSCT and QCA. Of these, 130 of
  773 (17) had stenoses.
• Comparing the maximal percent diameter luminal
  stenosis by MSCT versus QCA. Bland-Altman
  analysis demonstrated a mean difference in
  percent stenosis of 1.3 14.2 .

Gilbert L. Raff . JACC 2005 46552-557
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• (A) Volume rendering technique demonstrates
  stenosis of right coronary artery below the acute
  marginal branch as well as nodular coronary
  calcifications largely extrinsic to the right
  coronary lumen and (B) normal left coronary
  artery. (C, D) Maximum-intensity projection of
  the same arteries demonstrates severe soft plaque
  stenosis of the right coronary artery and
  superficial calcific plaque. (E, F) Invasive
  coronary angiography of the same arteries

Gilbert L. Raff . JACC 2005 46552-557
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Overall, 935 of 1,065 (88) segments could be
interpreted, 773 of 935 (83) quantitatively and
162 of 935 (17) qualitatively only.
Gilbert L. Raff . JACC 2005 46552-557
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• IVUS in 38 vessels in 20 patients.
• A total of 365 sections were available for the
  comparison with IVUS in 161 of these (26
  vessels), atherosclerotic plaques were present.
• 64-slice CT enabled a correct detection of plaque
  in 54 of 65 (83) sections containing
  noncalcified plaques, 50 of 53 (94) sections
  containing mixed plaques, and 41 of 43 (95)
  sections containing calcified plaques, resulting
  in an accuracy of 90 to detect any plaque (145
  of 161).
• In 192 of 204 (94) sections, atherosclerotic
  lesions were correctly excluded. In addition to
  the ability to classify calcified, mixed, and
  noncalcified lesions, 64-slice CT enabled the
  visualization of lipid pools in 7 of 10 (70)
  sections and enabled us to identify a spotty
  calcification pattern in 27 of 30 (90) sections.
  
• In three sections without evidence for
  echolucency on IVUS, hypodense areas (lipid
  cores) were identified by 64-slice CT. In 314 of
  365 sections (86), consensus between IVUS and
  64-slice CT was achieved regarding the
  morphologic classification. The plaque type was
  misclassified by 64-slice CT in 23 of 145
  atherosclerotic sections.

Alexander W. Leber . JACC 2006 IN PRESS
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Why not MRI?

• 16-MDCT offers better visualization of the
  coronary arteries than MR.
• Using visual assessments of DS severity, both
  MDCT and MR have similar accuracy for detecting
  significant coronary artery disease.
• Quantitative assessment of DS severity
  significantly improves the diagnostic accuracy of
  MDCT, but not that of MR, as compared to visual
  analysis alone.
• Using quantitative assessment of DS severity,
  MDCT has significantly higher diagnostic accuracy
  than MR.

Joëlle Kefer. JACC 2005 4692-100
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• By visual analysis, MR and MDCT had similar
  sensitivity (75 vs. 82, p NS), specificity
  (77 vs. 79, p NS), and diagnostic accuracy
  (77, vs. 80, p NS) for detection of gt50 DS.

Joëlle Kefer. JACC 2005 4692-100
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• Typical examples of reformatted magnetic
  resonance (MR) (left panels), and multidetector
  row computed tomography (MDCT) (center panels)
  and corresponding quantitative coronary
  angiography (QCA) images (right panels)
• (A) Normal right and left coronary arteries by
  MR, MDCT, and QCA.
• (B) Isolated mid-RCA stenosis.
• (C) Two-vessel disease involving the mid-LAD, and
  left circumflex coronary artery

Joëlle Kefer. JACC 2005 4692-100
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Conclusions

• MDCT is now more comparable to QCA, with
  excellent sensitivity and specificity in
  experienced centers.
• Further evolution of MDCT (more and faster
  detectors, software improvement) will likely
  provide a better spatial and temporal resolution.
• Currently, MDCT is not the test of choice in
  patients with prior CABG, stents, severely
  calcified lesions perhaps also patients with
  elevated HR, and obese.

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• MDCT does not have the capability of assessing
  the distribution of various morphologic patterns
  of calcium and their relation to other soft
  plaque components further plaque
  characterization (e.g., lipid pools and fibrous
  tissue), a prerequisite for the identification of
  most vulnerable lesions, is not yet a workable
  reality, even with the 64-slice machines in their
  current configuration.
• The noninvasive identification of plaque
  components subtending vulnerable lesions will
  require additional improvement in the primary
  instrumentation, software, perhaps ?? the use of
  hybrid constructs (e.g., with positron emission
  tomography).

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The sensation 64
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Thanks!