IMAGINE

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

   In 1999, more than 1.83 million coronary angiograms were performed in the US. Only 1/3 rd 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 2004;232:18-37

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 2004;232:18-37

 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 2004;232:18-37

 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 64–detector row CT scanners with further reduced gantry rotation times and minimum beam collimation widths of less than 1 mm.

 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 2004;232:18-37

ECG-synchronized CT Scan Acquisition



Prospective Triggering:

A trigger signal is derived from the patient’s 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 2004;232:18-37

 To improve temporal resolution, scan data are only acquired during a partial scanner rotation (approximately two-thirds of a rotation with 240 °–260° 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.

 However, only rather thick section collimation (3 mm with EBCT, 1.5 mm with 16 —detector 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 2004;232:18-37



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 multi –detector 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 2004;232:18-37

 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.

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 2004;232:18-37

   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 2004;232:18-37

 Optimizing Spatial Resolution Spatial 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. 2003;23:S111-S125

 Optimizing Temporal Resolution The 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 100 –150 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. 2003;23:S111-S125

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 2004;232:18-37

 For high spatial resolution (1.00

four –1.25-mm beam collimation), a retrospectively ECG-gated acquisition), and routine scanner settings with –detector 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.

CR Conti. Clin. Cardiol. 28:450-453

CR Conti. Clin. Cardiol. 28:450-453

Contrast Injection

  Scanning times for imaging of the heart with 8 or 16 detector row CT scanners range from 20 to 40 seconds, 80 –120 mL contrast medium injected at a rate of 3–5 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 2004;232:18-37

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 2004;232:18-37

  

(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 aspe

ct

of the left main coronary artery

(LM)

wall.

U. Joseph Schoepf. Radiology 2004;232:18-37

 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 2004;232:18-37

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.



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.

U. Joseph Schoepf. Radiology 2004;232:18-37

 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 2004;232:18-37

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 2004;232:18-37

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 2004;232:18-37

 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 multi –detector row CT.

1  In a somewhat larger patient population investigated with four –detector row CT, overall sensitivity and specificity values for bypass occlusion of 97% and 98%, respectively, were reported.

2

1.U. Joseph Schoepf. Radiology 2004;232:18-37 2. Ropers D. Am J Cardiol 2001; 88:792-795

Thomas Schlosser. JACC 2004; 44:1224-1229

 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; 44:1224-1229

    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; 44:1224-1229

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 (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.

U. Joseph Schoepf. Radiology 2004;232:18-37

 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 2004;232:18-37

(a)

Colored 3D volume rendered view from right posterior oblique perspe

ct

ive reveals luminal narrowing (arrowhead) of artery proximal to the stent.

(b)

Maximum intensity proje

ct

ion 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 proje

ct

ion confirms stent patency and presence of stenosis.

U. Joseph Schoepf. Radiology 2004;232:18-37

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 <50% 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

 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

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; 44:1230-1237 Ricardo C. Cury. AJC. 2005; 96:784-787

  

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; 46:552-557



(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. of the same arteries calcific plaque.

(C, D)

Maximum-intensity projection demonstrates severe soft plaque stenosis of the right coronary artery and superficial same arteries

(E, F)

Invasive coronary angiography of the

Gilbert L. Raff . JACC 2005; 46:552-557

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; 46:552-557

     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

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; 46:92-100

 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 >50 % DS.

Joëlle Kefer. JACC 2005; 46:92-100

    Typical examples of reformatted magnetic resonance (MR)

(left panels)

, and multidetector row computed tomography (MDCT)

(center panels)

images and corresponding quantitative coronary angiography (QCA)

(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; 46:92-100

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.

  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).

The “ sensation 64”

Thanks!