Contrast Material Injection Techniquesfor CT Angiography of the Coronary Arteries 23

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CHAPTER 23 / CONTRAST MATERIAL INJECTION TECHNIQUES 237

237

Contrast Material Injection Techniques

for CT Angiography of the Coronary Arteries

F ILIPPO C ADEMARTIRI , MD AND K OEN N IEMAN , MD

From: Contemporary Cardiology: CT of the Heart:

Principles and Applications

Edited by: U. Joseph Schoepf © Humana Press, Inc., Totowa, NJ

INTRODUCTION

Conventional catheter angiography is based on capturing X-ray images of the iodinated contrast material (CM) while it flows into the vessels. Computed tomography angiography (CTA) is based on the same concept, scanning the patient and his/her heart while a high concentration of iodinated CM flows through the coronary arteries (1). The CM inside the vessels increases the density of the vessel lumen compared to the sur- rounding tissues, and allows us to distinguish between lumen and soft tissues.

Because the CM is administered intravenously in CTA, the optimal phase to scan coronary arteries will be the arterial one.

In fact, during the delayed phase, several tissues enhance due to the CM perfusion, and the CM itself dilutes into the extracel- lular fluid, reducing the intravascular attenuation. Then vessels are no longer easily visualized as in the arterial phase (1).

Spiral computed tomography technology was limited by low speed and spatial resolution. Multislice computed tomography (MSCT) technology provides better image quality because of thinner slice thickness, and faster acquisition time because of reduced scan rotation time and multiple detector rows. The advantages of MSCT technology related to CM use are: (1) use of less CM (30–50%); (2) the injection rate can be increased with a concomitant better enhancement of the vessels; and (3) most of the data can be acquired during a defined phase (e.g., arterial phase for CTA) (1).

The latest generation of spiral computed tomography (CT) scanner, featuring up to 16-row MSCT technology, provides fast data acquisition, increasing the need for an accurate opti- mization of CM administration and synchronization tech- niques.

Scanning during the arterial phase in CTA, with low or no venous enhancement, enables optimal analysis of the acquired images and is critical for postprocessing techniques. Although axial images are commonly used for other conventional CT applications such as thoracic and abdominal imaging, for visualiza- tion and diagnosis of cardiac CT data sets, 2D multiplanar reformations (MPR) with standard and multiple intensity pro-

jection (MIP) algorithms, curved planar reformations (CPR), and 3D reconstructions are more important. Those techniques need a high vascular contrast in order to provide diagnostic images.

For all of these reasons, CM volume, concentration, rate, and ultimately the synchronization between CM material pas- sage and data acquisition, need to be optimized in order to exploit the potential of MSCT technique.

The bolus timing can be based on the knowledge of bolus geometry (e.g., demographics-based delay or fixed delay), but can better rely on synchronization techniques. An optimal tim- ing is achieved by predicting the veno-arterial transit time (e.g., test bolus), or synchronizing data acquisition with CM passage (e.g., bolus tracking).

BASICS OF BOLUS GEOMETRY

Bolus geometry is the pattern of enhancement plotted on a time(s)/attenuation (Hounsfield units—HU) curve, after intra- vascular injection of contrast material, and measured in a region of interest (ROI) (1). The value of enhancement is extracted by subtracting the attenuation value in an unenhanced baseline scan from the attenuation values in the enhanced scans (1).

Optimal bolus geometry, for the purpose of CTA, corresponds to an immediate rise to the maximum value of enhancement (high attenuation—HU) just before the start of the acquisition of CT data, and a steady state in which the enhancement does not alter during data acquisition (Fig. 1A). Actual bolus geom- etry is different. After intravascular injection of CM, there is a steady increase in enhancement; the peak of the curve will be reached after the end of contrast injection, followed by a steady decline in the enhancement. Normally, CTA will be performed during the upslope and downslope of the enhancement curve, and the peak of maximum enhancement (PME) will be inside the scan period (Fig. 1B).

The actual bolus geometry can be defined by the peak of maximum enhancement in HU (PME) and the time to reach that peak in seconds (tPME) (1).

PARAMETERS INFLUENCING BOLUS GEOMETRY Demographics

The time to peak (tPME) is not affected by age (2–4), weight

(2–5), height (3,4), body surface (3,4), blood pressure (3), heart

rate (3,5), and gender (2,3), and PME is not affected by age or gender (2). A heavier body weight is associated with a higher

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Fig. 1. Optimal and actual bolus geometry in CT angiography (CTA). (A) Optimal bolus geometry. The abrupt rise and the steady plateau of attenuation characterize the optimal pattern of bolus geometry. Ideally, the scan time tightly overlaps with the length of the plateau of enhancement in order to use the entire volume of contrast material (CM) administered for the acquisition. (B) Actual bolus geometry. The pattern of actual bolus geometry is quite different from the ideal one. Before and after the peak of maximum enhancement there are slopes of enhancement. Generally the up-slope is steeper than the down-slope, especially with CM administration protocols for CTA characterized by a high injection rate.

extracellular intravascular fluid volume. This results in a dilu- tion of CM with a lower iodine concentration in blood, and therefore a reduced PME.

Diseases

A reduced cardiac output produces a proportionally higher PME and longer tPME in the aortic bolus geometry (6). This is a result of the increase in circulation time and CM “pooling,”

which occur during reduced cardiac output conditions (6).

Injection Volume

A higher volume of CM shifts the time/attenuation curve upwards and rightwards (Fig. 2A) (1). This determines a higher PME and a longer tPME. The relation is independent of injec- tion rate and iodine concentration (7–9).

Injection Rate

Increasing the injection rate produces a proportionally higher PME and earlier tPME with a shift of the time/attenua- tion curve upwards and leftwards (Fig. 2B) (1). The relation-

ship is independent of iodine concentration and CM injection volume (2,6–8,10,11).

Iodine Concentration

Higher iodine concentration produces a higher PME (Fig.

2C) (18,12). Different iodine concentrations with constant rate and volume, do not affect the injection duration, and this means that the tPME remains unchanged.

Fig. 2. (right and page 240) Parameters affecting bolus geometry. (A) The influence of contrast material (CM) volume. Increasing the volume of injected CM produces an increase in peak of maxiumum enhancement (PME), and a delayed time to peak (tPME). (B) The influence of CM injection rate. Increasing the rate of injected CM produces an increase in PME, and an earlier tPME. (C) The influence of CM iodine concentration. Increasing the iodine concentration of the injected CM produces an increase in PME, without any influence on tPME.

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Bolus Chaser

A bolus chaser is a saline solution pushed through the injec- tion line immediately after the injection of the main bolus (1).

Advantages are reported from this technique applied in CT

(13–15). Bolus chaser is generally administered by a double-

head power injector system (13). With bolus chaser, less con- trast medium volume (up to 40% less) can be used for a CTA scan, without affecting arterial enhancement and diagnostic accuracy (15). The PME is higher and the tPME is longer when

a bolus chaser is added to the CM injection (16), while no increase in PME and tPME are expected if a bolus chaser is added to the injected volume with a concomitant decrease in the contrast volume, resulting in an unchanged total injection vol- ume (Fig. 2D).

Multiphasic Protocols

Multiphasic protocols are characterized by a decreasing of

the injection rate during CM administration (6,17–20). The aim

of multiphasic protocols is to create a steady plateau of enhance-

Fig. 2. (continued from page 238) (D) The influence of saline chaser. The saline chaser pushes the injected contrast medium through the veins of the forearm, providing a result similar to the injection of a larger contrast volume. The example shows the effect of a 50-mL saline chaser (thicker curve), using a bolus with the same volume, rate, and iodine concentration. Moreover, the saline chaser prevents the decrease of the CM in the arm veins, which may normally cause an increase in the CM concentration after the end of the contrast injection. (E) The influence of multiphasic protocols. Multiphasic protocols allow producing a longer plateau of enhancement. If the scan time is long (approx 30–35 s), then advantages are consistent, but with a shorter scan time, as with 16-row multislice CT, the importance of a long plateau of enhancement is reduced if compared to the impact of a very high PME.

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ment during the scan by means of a higher injection rate at the beginning of the injection and a lower rate in the second part (Fig. 2E) (17). The introduction of the last generation of MSCT scanners with shorter scan times reduced the impor- tance of multiphasic protocols. A higher PME can be far more important for the qualitative outcome of the CTA scan.

Injection Site

Contrast material can be administered through an antecu- bital vein that drains directly into the deep venous circulation of the arm (basilic vein), or a forearm vein that drains into the deep venous circulation of the forearm and subsequently in the deep venous circulation at the elbow joint or into the subclavian vein through the cephalic vein. Larger veins allow higher rates and more safety.

PREDICTION AND SYNCHRONIZATION OF BOLUS GEOMETRY

Patient Demographics

No or poor correlation was reported between a calculated tPME and the actual tPME (3,21). Nevertheless, it is not pos- sible to exclude that in the future a relationship between demo- graphics and bolus timing parameters will be found.

Fixed Delay

Fixed delay is a routinely applied technique for CTA. The increased scan speed of MSCT scanners needs a more careful CM administration. The risks of a fixed-delay technique are related to the fact that, in a percentage of patients, circulation time is quite different from the protocol applied. In those cases the scan could be successful if the circulation time is shorter than that fixed delay, even if a prominent venous enhancement can be observed; but when the circulation time is longer than fixed delay, the scan fails because of the lack of proper attenu- ation inside the coronary arteries (Fig. 3A).

Test Bolus

Test bolus technique entails that a ROI is plotted inside the lumen of an artery close to the region that needs to be studied.

A small amount of CM (10–15 mL, or around 10% of the main bolus) is injected at the same rate as the main bolus while a single level dynamic scan (e.g., monitoring scan) is performed at short intervals (approx 1–2 s). When CM arrives in the lumen of the artery at the level of the ROI, test bolus geometry is assessed and the time between the start of the test bolus injec- tion and a determined point of the time/attenuation curve of the test bolus is used as delay time for the injection of the main bolus (Fig. 3B) (1).

The use of test bolus is based on a relationship between the geometry of the test bolus and that of the main bolus. There is no or poor correlation between test bolus tPME and main bolus tPME (2,5,22), while there is a strong correlation between test bolus tPME and time to reach determined attenuation thresh- olds like T50, T100, T150, and T200 (2,5) (i.e., the time from the beginning of the injection to reach 50/100/150/200 HU in the ROI) in main bolus (Fig. 3B). The result of this is that with conventional test bolus technique the scan is safe but can be too early, especially if the vessels of interest are located at the beginning of the scan range.

Bolus Tracking

Bolus tracking technique is real-time bolus triggering tech- nique. It is based on an ROI that is plotted inside the lumen of

an artery close to the region which has to be studied, and a trigger attenuation value (threshold) that is arbitrarily chosen before starting the CTA data acquisition (1). A single level dynamic scan (e.g., monitoring scan) is performed at short intervals (1–2 s) during the injection of CM. When the CM arrives at the level of the ROI, the change in attenuation is detected and a CT scan is started after reaching the triggering threshold (Fig. 3C) (1).

Bolus tracking provides a better timing and allows the use of less CM with a higher injection rate. A pitfall of this technique occurs when the threshold is not reached. Even though it is a very rare event when the protocol is optimized, it is always possible to start the scan manually. In this case it is difficult to obtain a high-quality CTA because of the later start of the scan, the decreased amount of contrast medium inside the vessels, and the prominent venous enhancement.

PATTERNS OF ENHANCEMENT OF CORONARY ARTERIES

Coronary arteries originate at the root of the ascending aorta, and the intravenous CM arrives at that level already diluted, after pulmonary circulation and left-ventricle con- traction. Therefore, it can be assumed that the characteristics and the dynamics of bolus geometry inside coronary arteries are the same as in the ascending aorta. Differences in attenu- ation between ascending aorta and coronary arteries can be determined by stenosis or occlusions. In this case, in fact, there can be various degrees of attenuation inside coronary arteries and their branches depending on the flow through the stenotic vessel or on the backflow provided by collateral cir- culation. In a patent coronary artery, the flow speed guaran- tees an optimal enhancement in a few seconds. With stenosis and occlusions, the flow speed can be reduced. The MSCT scanners with 16 slices and approx 0.4-s rotation time provide a scan time of less than 20 s. With a good synchronization technique, the heart has at least 4–6 s before the smaller branches (diagonal and marginal) of the coronary arteries are acquired with a scan performed in the cranio-caudal direc- tion. This time span is generally enough to allow an arterial perfusion and even the collateral circulation to fill in the case of stenosis or occlusion of one or more vessels.

BOLUS TIMING TECHNIQUES WITH 16-ROW MSCT

For cardiac and coronary MSCT imaging, fixed delay, test bolus, and bolus tracking techniques have been applied

(23–28). There are no published data comparing bolus timing

techniques applied to coronary imaging in the 4-slice era. In fact, the modalities applied for bolus timing have been severely influenced by the speed of acquisition and by the hyperventi- lation performed just before the scan to allow a longer apnea.

In some cases, the use of oxygen to increase the apnea has been reported. For these reasons, bolus tracking was not possible with 4-row MSCT scanners.

With scanners that have 6 or more rows, the scan time can be reduced significantly, especially if there is a parallel reduc- tion in gantry rotation time (<500 ms). With those features, scan time can be reduced below 30 s. Apnea becomes more

“affordable” for a larger number of patients without prescan

hyperventilation or oxygen administration.

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Fig. 3. (left) Timing and synchronization techniques. (A) Fixed delay technique. The image shows the main pitfall of fixed delay technique.

The continuous curve shows the ideal situation when the fixed delay and bolus geometry correspond. In some cases the bolus geometry of the patient can be different from the expected one. If actual bolus geometry is faster than fixed delay (dotted curve), the enhancement of vascular structures will be still present but reduced, and prominent venous enhancement can be evident. If actual bolus geometry is slower than fixed delay (tittled curve), the risk is to scan the passage of contrast material (CM) only in the distal portion of the scan range with poor contrast enhancement of arteries. (B) Test bolus technique. The correlation between test bolus time to peak of maximum enhancement (tPME) and main bolus tPME is displayed. In this case, the test bolus tPME correlates with the time to reach 150 HU in main bolus. A main scan based on this information will be successful, even though not optimal as a CT angiography (CTA). In fact the vascular attenuation of 150 HU at the beginning of the scan is too low for optimal CTA. Worse results are obtained when the correlation is with the time to reach 50 HU or 100 HU. Test bolus actually has a different geometry than main bolus. The lack of injection power after the end of the injection of test bolus determines a pooling of the test bolus in the venous system without any vis a tergo. In other words, test bolus is left alone in the venous system of the arm, without the help of saline solution (bolus chaser) or adjunctive contrast media (main bolus) that pushes it forward. (C) Bolus tracking technique. The sequence for real-time bolus tracking is displayed. After the topogram is acquired, the monitoring scan is set at the level of aortic root and the region of interest (ROI) inside the lumen of ascending aorta. The trigger threshold is set at 100 HU. Then, CM administration and monitoring sequence are started at the same time, and when the attenuation in the ROI reaches a value greater than 100 HU at the triggering point (TP), a transition delay (TD—generally 4 s are enough for this procedure) starts while the table reaches its starting position and the patient receives breath-holding instructions.

Fig. 4. Examples of different contrast material administration protocols in 16-row multislice coronary CT angiography. Axial (A, B, and C) and sagittal (D, E, and F) multiplanar reformations of three different protocols of CM administration are displayed: 100 mL of 320 mgI/mL CM at 4 mL/s (A and D); 140 mL of 320 mgI/mL CM at 4 mL/s (B and E); 100 mL of 320 mgI/mL CM + 40 mL of saline chase at 4 mL/s (C and F). The images from the protocol with 100 mL and the images with 100 mL + 40 mL are similar, as expected, and show a decreasing cranio- caudal gradient of attenuation inside the pulmonary artery (D and F; black asterisks), that is not present in the protocol with 140 mL (E). The enhancement in the descending aorta is preserved in every protocol (D, E, and F; black +), as well as the enhancement in the left ventricle (A, B, and C; white +).

Bolus tracking is an optimal method for CM synchroniza- tion in noninvasive CTA (Fig. 4). Bolus tracking, in fact, is less time consuming than test bolus techniques (because of the calculation needed with test bolus), allows the use of less CM (the larger bolus needed to have a wider enhancement plateau with fixed delay, or the 20 mL needed for test bolus), and

prevents suboptimal synchronization (determined by inter-

individual veno-arterial circulation time with fixed delay, and

by the lack of reliable relationship between the tPME of test

bolus and main bolus). No significant differences are observed

regarding the attenuation reached at the level of aortic root,

whether an injection volume of 100 mL or 140 mL is used.

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attenuation value obtained, and concentrations of 320–350 mgI/mL are optimal for coronary angiography purposes.

Bolus chaser is always recommended because it allows using a reduced volume of CM more effectively.

GUIDELINES FOR CONTRAST MATERIAL ADMINISTRATION

CM administration technique for the purpose of nonin- vasive CTA with multislice scanners can be summarized as follows:

Choice of CM: use nonionic iodinated CM. Higher iodine concentrations will provide higher attenuation, and different molecules will not affect the final result significantly. The iodine concentration ranges between 300 mgI/mL and 400 mgI/mL.

Site of administration: start with the right antecubital access and then use the left one if there are difficulties. Use forearm or hand only if the antecubital accesses are not available. When using central IV lines, reduce the rate of infusion slightly to 2.5–3.0 mL/s.

Administration device: an 18-gage venflon (green) is suit- able in all patients. In women or younger patients, the access could be easier with a 20-gage venflon (pink), even though a reduced rate is recommended in this case (3.0 mL/s).

Volume: with the 16-row generation scanners, 100 mL of CM provides optimal enhancement if a bolus tracking tech- nique is properly applied. With a lower number of detector rows, an increased volume is needed, down to 150 mL of CM with 4-row generation scanners.

Rate: a 4 mL/s rate provides optimal results. Reduced injec- tion rates (3.0 mL/s) are needed with smaller venflons and/or in patients with small or fragile veins.

Bolus chaser: the use of bolus chaser, when possible, is strongly recommended to increase the amount of CM used during the scan and/or to reduce the overall amount of CM administered.

TIMING AND SYNCHRONIZATION TECHNIQUES

Fixed delay technique: a delay of 18 s allows a good scan in most of the patients. In patients with mildly or severely impaired cardiac function, a delay of 25 s is recommended.

Test bolus technique: calculate the tPME of the test bolus and then add 4 s for an optimal scan delay.

Bolus tracking technique: is safe and allows a tailored scan synchronization. The trigger threshold is set at 100 HU with a transition delay of 4 s needed to give breath-hold instructions to the patient. It requires proper training of the patient, and of the technician.

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