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Introduction to Cardiac CT

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Victor Micallef

on 4 November 2016

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Transcript of Introduction to Cardiac CT

Reconstruction Algorithms sharpen or smooth the contours of objects enhancing or degrading detection of image detail.
No other anatomic structure poses a greater challenge for cross sectional imaging than the coronary arteries.
curving vessel course
Coronary artery characteristics
CT performance characteristics
small vessel size
contrast resolution and image noise
complex vessel motion
temporal resolution
image reconstruction and reformation
spatial resolution
vessel lumen detection
ability of an imaging system to separate adjacent objects within a volume as being distinct.
Each voxel is the smallest element of image data and can only represent one attenuation value.
For a fov of 500mm diameter and 512*512 pixels image matrix, in plane x and y axis resolution is approximately 1*1 mm
If fov is reduced to 250mm as in coronary CTA, the in plane resolution is improved to 0.5*0.5mm
z axis detector rows are typically 0.5-0.625mm wide
ability of an imaging system to distinguish regions of differing density or attenuation as distinct
Under optimal scanning conditions, MDCT is able to resolve density differences as small as 3 Hu or 0.3% difference in tissue density.
Image Noise
random fluctuation on CT density values
Contrast to noise ratio defines the ratio of true signal to random noise in an object of interest.
Image noise has the greatest impact when imaging small objects with low contrast - such as in coronary CTA, where high levels of noise can exceed the actual density differences among tissues.
Low contrast resolution is dependent on these parameters
Tube current
The more photons used in the x- ray beam the less the statistical variation in attenuation values – lower image noise.
There is linear relationship between changes in mAs and the patient radiation dose.
Noise varies with the square root of change in mAs.
Halfling the mAs will reduce the radiation dose by half and increases the image noise by a factor of 1.4.
X-ray energy
Expressed in kilovolts (kV)
Higher energy results in more photons reaching the detectors thus lower image noise.
A doubling of kV results in a fourfold increase in radiation dose to the patient.
higher kV translates to decrease in image noise.
Pixel size and slice thickness
The smaller the dimensions of an image voxel, the fewer photons contribute to calculating its attenuation value and the greater the image noise.
Pitch
the lower the pitch, the greater the radiation exposure to the patient.
Radiation dose is inversely proportional to the pitch;
less projection data is available to create each image.
noise increases with increasing pitch
Reconstruction algorithm
Independent of the number of photons reaching the detectors, image noise is greatly affected by the specific mathematical tools utilised to reconstruct the CT data.
Choosing among reconstruction algorithms represents a balance between higher spatial resolution with increased noise and higher contrast resolution with decreased noise.
This is the ability to resolve fast moving objects as if they were motionless.
For coronary CTA, temporal resolution faster than 100 msec is desirable.
to freeze motion the scanner rotation speed must be faster than the moving anatomy to obtain an image free of motion artifact.
Digital coronary angiography capture images at 30-50 frames per sec, achieving temporal resolution of 20 msec or less, well suited to freezing coronary motion of high quality imaging
In CT, motion artifact degrades image quality when artery velocity exceeds the temporal resolution of the scanner.
to completely suppress coronary arteries motion artifacts a temporal resolution of between 35 and 75 msec is necessary
Fastest gantry rotation speeds are around 300msec.
Rotation speeds has shown continuous improvement through the evolution of MDCT but it appears that now we are approaching the practical engineering limits for existing system designs.
While future engineering, materials and design breakthroughs are unknown, achieving temporal resolution of 35-75 msec seems well beyond what is physically possible for current MDCT scanners.
Strategies to improve temporal resolution
Half scan or 180 degree reconstruction
effective temporal resolution can be improved to 150-200 msec using half scan image reconstruction technique alone.
Current MDCT scanners have rotation speeds between 300 and 400 msec.
A minimum of 180 degrees of attenuation projection data is mathematically required for image reconstruction in CT;
Cardiac phase and ECG gating
Because the velocity of coronary arteries varies throughout the cardiac cycle, synchronising the acquisition of CT data to the ECG provides a unique opportunity to further improve effective temporal resolution.


Conversely, data from periods of significant coronary motion can be specifically secluded from image reconstruction.
Projection data used for image reconstruction can be captured during specific desirable segments of the cardiac cycle.
Cardiac phase
Each heartbeat can be divided into two stages
Systole is the period of ventricular contraction and ejection of blood from ventricular cavities
Diastole is the period of ventricular filling and the brief period of ventricular relaxation immediately before filling
Systole
Diastole
The R-R interval of an ECG represents one complete heartbeat, encompassing one sequence of systole and diastole over the seven physiological phases that comprise the cardiac cycle.
4. Isovolumetric relaxation
5. Rapid ventricular filling
6. Reduced ventricular filling-diastasis
7. Atrial systole
In physiologic terms, the cardiac cycle can be further divided in to 7 distinct phases
1. Isovolumetric contraction
2. Rapid ventricular ejection
3. Reduced ventricular ejection
In coronary CTA, the term phase is applied to a regular and arbitrary division of the R-R interval of the ECG rather than the physiologic division of the cardiac cycle.
The R-R interval is typically divided into discrete 10 phases of 10% of contiguous or 20 phases of 5% increments, dividing a heartbeat equal parts
The actual time length of each phase depends on the patient's heart rate, varying over time and from beat to beat.
● HR = 60bpm
● 1 beat per second (1000ms)
● 10% phase = 100 msec
● HR = 120bpm
● 2 beat per second (1 per 500ms)
● 10% phase = 50 msec
By applying ECG gating and limiting imaging reconstruction to specific narrow phases of cardiac cycle where motion is expected to be the least, the best possible images can be obtained.

While the absolute length of each phase in ms will vary with patients’ heart rate, the percent phase will always represent the same relative position in each ECG R-R interval, and to a certain extent, the same physical position of the heart and coronary arteries within a 3d space.
The late ventricular filling stage of diastasis is the most extended period of cardiac stillness. Thus it is usually the best period for best CTA images.

With increasing heart rate there is non-proportional shorting if diastole as compared with systole, narrowing the potential window for optimal imaging.
For heart rate above 83 bpm, the trough in cardiac motion is actually lower in systole than in diastole.
The image quality in MDCT decreases with increasing heart rate variability, even for a low heart rate.
Significant non-proportional shortening and prolongation of physiologic cardiac phases occurs with variable heart rates, confounding the ability of ECG gated imaging to repeatedly capture still coronary arteries and precisely align anatomic structure in a series of beat to beat images.
Accordingly, the reduction in heart rate variability afforded by the use of beta blockers appears to benefit image quality even for patients with lower heart rates.
Two major techniques can be used to exploit ECG sync in coronary CTA,
Retrospective ECG Gating
Traditional coronary CTA is acquired with continuous irradiation of the patient, with imaging synchronized to the patient's ECG tracing and using a small pitch.
In this manner, the coronary anatomy is oversampled at multiple points in the cardiac cycle.
This allows for retrospective image reconstruction to isolate the narrow moments of least coronary artery motion using processing tools after the scan is acquired.
Retrospective ECG gating provides the most flexible and forgiving data acquisition at the cost of substantially increased radiation dose.
Retrospective ECG gating has been the most common method used in coronary CTA imaging as this technique gives the greatest likelihood of diagnostic success.
The optimal phase for image reconstruction frequently differs for the various major coronary arteries;

the left anterior descending artery (LAD) may be seen best at the 70-75% R-R interval

the right coronary artery (RCA) is seen best at the 40% R-R interval in a given patient during the same coronary CTA scan.
By acquiring data with retrospective ECG gating, there is great latitude in optimizing image quality on a vessel-by-vessel or even segment-by-segment basis.
As the precise phase that will be required for optimal image reconstruction cannot be determined in advance, retrospective ECG gating requires the use of a low pitch, usually around 0.2.
At a pitch of 0.2, each z-axis position is oversampled by a factor of 5, conferring a fivefold increase in radiation dose over the single z-axis image sampling, compared with prospective ECG gated step-and-shoot imaging.
Retrospective ECG gating is often selected for patients with higher and more variable heart rates to capitalize on the potential for post scan image optimization.
Prospective ECG Gating
In prospective ECG gating, the phase of image acquisition is preselected based on analysis of the ECG tracing just before scanning.
The scanner samples 3-7 heartbeats immediately before the diagnostic image acquisition.
an optimum phase of the cardiac cycle is selected to trigger scanning.
With the patient motionless on the CT table data acquisition is triggered at a specific phase of the cardiac cycle.
The table is moved to the next range of contiguous anatomy and data acquisition is again triggered at the same ECG phase.
The process is repeated until the entire heart is imaged.
This form of scanning is referred to as step-and-shoot imaging, as it describes the alternating sequence of scan-move-scan-move-scan taking place.
With wide arrays of detectors of 40 mm and greater, step-and-shoot coronary CTA scanning has become possible with the heart imaged in 3-4 prospectively triggered contiguous scans over 7-9 heartbeats during a single breath hold.
Each level of axial anatomy is scanned only once and irradiation occurs only during the triggered window of the ECG cycle, completing the mandatory 180 degree scanner rotation.
This limited acquisition includes only one complete sampling of each level of axial anatomy
Post-acquisition phase selection to optimize image reconstruction is not possible as in retrospective scanning.
In addition, as only one narrow portion of the cardiac cycle is imaged; ejection fraction, myocardial function, and valve motion cannot be evaluated in prospective ECG-gated MDCT.
Relatively low radiation doses can be achieved for coronary CTA when prospective ECG gated step-and-shoot technique is used.
Average radiation dose for the prospective ECG-gated studies is 2.84 mSv, compared with 18.4 mSv in the retrospective ECG-gated; an 83.2% dose reduction for prospective ECG-gated studies.
Multisegment Reconstruction
In single-segment CT reconstruction, the projections used to form an image are all derived from one heartbeat of data.
As phase length is dependent on heart rate, the proportion of the R-R interval that is used for image reconstruction is adjusted to meet the minimum required data acquisition.
For a heart rate of 60 bpm, a 10% window of the cardiac phase occurs over only 100 msec --- less time than the 150 msec needed to complete a 180 degree axial image reconstruction on a scanner with a 300 msec rotation speed.

For a heart rate of 60 bpm, for this scanner, at least 15% of the R-R interval must be included to reconstruct an image.
At higher heart rates, an even larger proportion of the R-R interval must be included; for a heart rate of 90 bpm, phase data from 23% of the R-R interval is required, and for a heart rate of 120 bpm, 30% of the R-R interval is required.
The widening of required phase with increasing heart rate may lead to incorporation of undesirable portions of the R-R interval into image reconstruction and introduction of artifacts from periods of significant coronary artery motion.
While the mathematical requirement for 180 degree data acquisition is absolute, in retrospective ECG gating, it is possible to interpolate the minimum 180 degrees of data from more than one heartbeat to create an image--a process known as multisegmental reconstruction.
The effective temporal resolution is equal to the acquisition time of the longest segment included in the multisegmental reconstruction.
In three-segment reconstruction, three contiguous 6o degree segments of projection data are interpolated from three adjacent heartbeats to comprise the minimum arc of data for image reconstruction.
The effective temporal resolution is reduced to one-sixth of the nominal scanner rotation time, or about 55 msec
In four-segment reconstruction, four contiguous 45 degree segments of projection data are interpolated from four adjacent heartbeats to comprise the minimum arc of data for image reconstruction; the effective temporal resolution is reduced to about 42 msec.
Dual Source CT
dual-source CT (DSCT) places two x-ray sources at a 90 degree angle on the spinning gantry ring of the scanner in an effort to achieve cardiac imaging independent of heart rate.
For two x-ray tubes rotating at 330 msec, the effective temporal resolution for a full rotation is cut by half to 165 msec.
Applying 180 degree half-scan reconstruction brings the effective temporal resolution to 83 msec using a single segment of helical data for image reconstruction;
More recently, a DSCT device with a gantry rotation speed of 280 msec and a single-segment temporal resolution of 75 msec has been introduced.
Temporal resolution of 100 msec or better is necessary to reduce or eliminate coronary artery motion artifacts.
Single-source MDCT uses multisegment reconstruction to optimize temporal resolution in patients with high heart rates.
Because of its greater temporal resolution, DSCT is uncoupled from multisegment reconstruction and its associated artifacts.
With DSCT, motion-free coronary CTA appears possible for heart rates up to 92 bpm , but this technique still performs best for heart rates less than 8o bpm.
Despite the effective temporal resolution of 83 msec for DSCT, image quality in these systems is still affected by heart rate and, perhaps more importantly, by heart rate variability.
In DSCT, both heart rate variability and coronary artery calcification are still factors that continue to degrade image quality and diagnostic accuracy of coronary CTA.
A perfect image is a contrast-filled left heart, a saline-filled right heart, and a motion-free right coronary artery.
Patient Instructions
Patients must be provided with a set of pretest instructions and their active cooperation and compliance are essential.
Patients should be instructed to abstain from stimulants such as nicotine or caffeine for 12 hours preceding the exam and asked to refrain from phosphodiesterase type 5 inhibitors (e.g. Viagra, Cialis, and Levitra) 24 hours prior to the study due to potential for hypotension with concomitant sublingual nitroglycerin.
Due to the use of intravenous contrast and potential for nausea, no oral intake 4 hr prior to the study is requested
It is important to note that breath-holding during image acquisition significantly lowers the mean heart rate by approximately 4 bpm, which should be considered if contemplating beta-blocker administration at a borderline optimal heart rate.
Heart Rate Control
The beta-blocker, metoprolol tartrate, is used due to its fast onset; and short half-life, and it can be administered either via an intravenous or oral route.
Although the multisegment recontruction results in faster temporal resolution at higher heart rates, image quality suffers from spatial inconsistencies due to cardiac motion variability over these two or more sampled heart beats.

Rationale for Heart Rate Control
Improved Image Quality
Prolong Diastole
Assessibility of the mid-RCA is especially improved by low heart rates.
Single-segment Reconstruction
Low heart rates improve image quality with heart rates of 65 bpm or lower required for optimal image quality on 64-MDCT.
Slow heart rates prolong the two relatively motion-free phases of the cardiac cycle, end systole, and mid-diastole so that reconstruction windows can be safely placed within these phases without needing data from portions of the cardiac cycle that include motion.
Slow heart rates also improve image quality by enabling the use of single-segment reconstruction rather than multisegment reconstruction.
Single-segment reconstruction utilizes data from a single heart beat to reconstruct an image, whereas multisegment reconstruction uses data from two or more consecutive heart beats to reconstruct data to form a single image.
Reduction in Radiation dose
Dose modulation can be set as tight or wide as one deems reasonable for a particular patient, but in reality, software adaptations have become so advanced that ECG adaptive triggering will automatically adjust or terminate tube current modulation during image acquisition to meet the needs of a particular heart rate.
Prospective Triggering
Heart rate control can result in considerable radiation dose reduction in patients by two methods
First, slow-and-steady heart rates allow for prospective cardiac gating whereby image data are obtained at a single predetermined point in the cardiac cycle with an incrementally moving table to cover the heart with minimal overlap.
Dependent upon the detector width, coverage of the heart requires two to eight steps of table movements.
Because the number of cardiac phases acquired is limited, prospective triggering is reserved for those patients with heart rates less than 65 bpm with little to no heart rate variability.
The second method whereby heart rate control decreases radiation dose is in retrospectively gated studies by allowing the use of ECG tube current modulation.
In retrospective gating, the x-ray generator is on and acquires data throughout the entire cardiac cycle.
Heart rate irregularity or variability can be dealt with by using large volume datasets reconstructed at various phases of the cardiac cycle.
Radiation dose can be substantially decreased in retrospectively gated studies by employing ECG tube current modulation, but is limited to patients with slow heart rates where image reconstruction will predictably occur in diastole.
With ECG tube current modulation, tube output reaches the prescribed level for a short interval in mid to late diastole where data for coronary artery analysis likely to be reconstructed.
During the remainder of the cardiac cycle, tube output is decreased by 8o-96% of the prescribed dose, resulting in dose savings of 20-50% depending on the heart rate.
Retrospective Triggering
Image quality suffers but remains sufficient for cardiac functional analysis.
At the scanner
Calcium Score CT
Most institutions perform this non-contrast-enhanced study as part of a standard cardiac CT examination.
Coverage is based on the scout topogram with the superior level beginning at the carina extending inferiorly to the cardiac apex.
This prospectively triggered scan, obtained at 3 mm slice thickness, offers valuable information regarding calcified plaque burden and risk stratification, which are value additive data to the CTA.
This low radiation dose scan also allows for tight CTA coverage, resulting in minimization of the total slice numbers for the thin-section CTA examination.
Finally, the calcium score affords a heart rate prediction during a breath hold to determine if ECG tube current modulation or prospective gating can be incorporated, thereby reducing the total radiation dose.
Nitroglycerin
Nitroglycerin improve detection of obstructive lesions by increasing flow in the epicardial coronary arteries via vasodilatation and have more effect on the diameter of nonstenotic coronary artery segments than stenotic segments.
In addition, sublingual nitroglycerin also allows for better, visualization of branch vessels without adverse side effects or decreased image quality.
The preferred method is to administer a single dose of sublingual nitroglycerin spray (0.4 mg/l).
Contrast Timing
(3) timing bolus.
The three methods to determine IV contrast timing are
(1) preset time,
(2) computed assisted triggering or bolus tracking
Preset time is the method whereby contrast infusion is delivered according to a preset time (e.g., 23 sec) without bolus tracking or timing bolus assistance.
Although this does save a step in the image acquisition process, preset timing is not the recommended delivery timing method because it does not allow for consistently optimal arterial contrast enhancement.
The inflow of the contrast medium is monitored, and when the contrast medium reaches a predetermined level (e.g., 100 HU), CT image acquisition starts automatically.
Computer-assisted bolus tracking allows image acquisition through automated timing.
This decreases the possibility of respiratory motion from misunderstanding breath hold instructions or patient motion due to the unexpected "flushing" sensation of contrast injection.

Despite adding an additional step, a timing bolus is the most commonly used and preferred method to synchronize image acquisition with contrast delivery.
A timing bolus of 20 mL of iodinated contrast is injected at the prescribed injection rate, the same rate that will also be used for the CTA portion of the examination.
Following a 10-sec delay, scanning is repeat every 2 sec at the same z-axis position until contrast peaks in the ascending aorta.
The z-axis position is determined from the calcium score CT and is at the level of the aortic root, one slice or 3 mm above the left main coronary artery origin.
A region of interest is chosen in the ascending aorta, and a curve is produced that indicates peak contrast opacification and allows accurate timing for peak contrast enhancement in the coronary arteries.
The typical timing delay is 23-25 sec.
The timing bolus is also used to ensure a properly functioning IV
A timing bolus is also advantageous because it allows the patient to have a "practice scan" whereby additional breath hold instructions can be rehearsed and contrast infusion can be experienced.
CTA
Coverage is determined from the calcium score CT data

Coverage
The superior extent of the CTA begins 12 mm above the left coronary artery origin, and the inferior coverage is 9 mm below the cardiac apex.
Contrast Delivery
Although not identified in one-quarter of cardiac CT examinations, a patent foramen ovale can be identified on cardiac CT and is readily apparent with a saline-filled (dark) right heart and a contrast-filled (bright) left heart.

There are three methods to deliver CM
1. Single-phase injection: Contrast only.
2. Dual-phase: Contrast followed by a saline flush
3. Triphasic injection: Contrast followed by a saline contrast flush mixture, followed by a saline flush.
Finally, the saline flush clears contrast in the upper extremity veins and SVC and thus delivers the entire contrast infusion thereby reducing total contrast volume required.
The dual and triphasic injection protocols have three advantages.
First, this flush clears the right heart of contrast, thereby eliminating streak or beam-hardening artifact from an otherwise densely contrast-filled right ventricle.
Second, the 3D software has a much better probability at probing and delineating the course of either the RCA or LAD with decreased right ventricular attenuation than a densely contrast-opacified right ventricle.
The dual-phase saline flush protocol fills the right atrium and ventricle with low-attenuation saline, making assessment of the right heart structures impossible.
Alternatively, the triphasic saline-contrast combination flush protocol allows interpretation of the right heart chambers, right heart valves, and right ventricle function without the streak artifact caused by contrast infusion alone.
Due to this added benefit, many advocate for the triphasic saline- contrast flush combination with a 30%:70% contrast media- saline mixture rather than the saline flush protocol alone.
The triphasic injection is indeed recommended if the clinical question necessitates evaluation of the pulmonary arteries or right heart structures.
On the other hand, right heart pathology is extremely unusual.
What is not rare, however, is a patent foramen ovale, a finding recognized in 25% of autopsy patients.
Contrast Concentration
The target arterial enhancement of the coronary arteries on 64 MDCT has been determined to be between 300 and 350 HU. Similar to imaging other organs, insufficient arterial enhancement will result in poor diagnostic accuracy.
Iodine concentrations between 320 and 370 mg/mL are recommended.
MDCT routinely allows for contrast volumes below 100 mL. Contrast volume can be further reduced below 8o mL when employing a weight-adjusted approach of 1.0 mL/kg.
IMAGE RECONSTRUCTION AND INTERPRETATION
Image reconstruction parameters are chosen to optimize coronary artery evaluation during the relatively motion-free portion of the cardiac cycle. With prospective triggering options are restricted to the limited dataset/s acquired, but the choices are numerous for a retrospectively gated study.
Typically five datasets, centered in mid-diastole, are reconstructed at the slice thickness equivalent to that of image acquisition. Using the relative time or percentage approach, this would generate five datasets from the 55-75% R-R or 65- 85% R-R interval (scanner dependent) at 5% increments.
Functional analysis is achieved with 10 data sets reconstructed from 0-90% phase at 10% intervals at 3 mm slice thickness.
Spatial resolution for CTA is maximized by image reconstruction at the smallest possible field of view that still includes the coronary artery anatomy.
Post· processing software enables automatic segmentation and generation of 3D images including volume-rendering and curve multiplanar reformations.
This approach is recommended in patients with low, regular heart rates where there is a high likelihood of diagnostic image quality in mid-diastole.
Alternatively, a preview function can be employed to determine the optimal phase for reconstruction. The preview function usually displays 20 images at 5% intervals (0-95% R-R) at the same z-axis position.
The image with the least motion can be chosen to establish the ideal phase for reconstruction.
Image reconstruction in end-systole may be required in patients with elevated heart rates, especially for interrogation of the RCA.
Full field of view reconstructed images are also created at 5 mm slice thickness in lung and mediastinal algorithms to evaluate for mediastinal, lung and upper abdominal organ pathology.
Introduction to Cardiac Ct
At these rotation speeds, forces more than 33 times the force of gravity is exerted upon the spinning internal components.
Diastole is normally longer in duration than systole.
Low points or troughs in coronary artery motion occur in both diastole and systole, varying with heart rate.
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