CT Angiography in the Emergency Department: Maximizing Contrast Enhancement and Image - PowerPoint Presentation

Slide1

CT Angiography in the Emergency Department: Maximizing Contrast Enhancement and Image Quality While Minimizing Radiation Dose and Contrast Material Volume

Toshimasa J. Clark, MDDepartment of Radiology, University of Colorado—Denver, 12401 E 17th Ave MS L954, Aurora, CO 80045Address correspondence to: T.J.C. (e-mail: toshimasa.clark@ucdenver.edu)Martin L. Gunn, MBChB, FRANZCRDepartment of Radiology, University of Washington, Seattle, Wash

M.L.G. receives research support from Philips Healthcare; spouse is a contractor to Athena Health and Elsevier.

Recipient of a Certificate of Merit award for an education exhibit at the RSNA 2015 Annual Meeting (exhibit ER157-ED-X).

Slide2

Introduction

Computed tomographic (CT) angiographic image quality is highly dependent on contrast material injection and image acquisition technique. To provide patient-centric care, one should tailor the technique to the clinical question and the characteristics of each patient. In this RadioGraphics Fundamentals presentation, we simplify the topics of intravenous contrast material dynamics and technique optimization and describe how dual-energy and fast CT angiography can allow for further image quality improvement while reducing the contrast material volume and radiation dose.

Slide3

Overview

Principles of contrast material flowContrast agents and timing techniquesFactors that influence iodine enhancement:Iodine flux (concentration, rate, viscosity) and patient factors (intravenous line, cardiac output, height, age, sex)Volume of contrast materialTiming of CT relative to arrival of iodinated agentKilovoltage peakLow contrast material dose CT angiography and dual-energy CT angiographyCT angiography with fast (wide detector array and dual-source) scanners

Slide4

Learning Objectives

At the end of this presentation, the reader should be able to:Recognize the principles of intravenous contrast material flow and the factors that influence contrast enhancement in the performance of CT angiography.Modify one’s local protocols to maximize contrast enhancement, while minimizing image quality variability, contrast material usage, and radiation dose.Recognize when patient factors will lead to suboptimal image quality if a standard CT angiographic protocol were to be used and know how to optimize the contrast material injection protocol and CT technique to accommodate these factors.Apply newer technologies, such as dual-energy CT and low contrast material dose CT angiography, to improve image quality.

Slide5

Principles of Contrast Material Flow

Slide6

Contrast Material Flow

Peripheral

Venous

Access

Right Side of

Heart

Pulmonary

Circulation

10–25 seconds

Left Side of

Heart

Systemic

Arteries

~25 seconds

Organ

Parenchyma

40–55 seconds

Liver

~65 seconds

Generally, contrast material flows through the body in a predefined circuit, beginning with a peripheral vein.

Slide7

Principles of Contrast Material Injection:

The Dual-Chamber Power InjectorMost sites now use dual-chamber injectors for iodinated intravenous contrast material delivery, with flow rates between 1 and 6 mL/sec.A saline chaser is usually used too, because it:Flushes contrast material from systemic veins and superior vena cavaReduces streak artifactReduces contrast material volumeCan increase peak aortic enhancement if injected rapidly

SALINE

CONTRAST MATERIAL

No-Chaser

Streak

Saline Chaser

Slide8

Dual-Chamber Injector:

Varying Injected Iodine ConcentrationBy using a dual-chamber injector, it is possible to vary the flow rate of the contrast material and saline infusion.The principle is simple—if 350 mg of iodine per milliliter is injected at the same flow rate as normal saline at the same time, the iodine concentration injected will be 175 mg of iodine per milliliter.Dynamically varying the mixture of saline has several applications. For example, it can reduce artifacts from highly concentrated contrast material in the venous structures and the right side of the heart at coronary CT angiography. It is also valuable for multi-rule-out CT angiography of the chest in the emergency department.

Slide9

Timing Contrast Material Arrival

Elapsed Time (sec)

5

10

15

20

25

30

0

0

25

50

75

100

125

Relative HU

150

175

Three principal contrast material timing techniques:

Empiric delay

Works well in most solid organs

Quick setup

Timing bolus

Reference scan depicting organ of interest is performed, and time-attenuation curve is generated after injection of a 2–20 mL bolus of contrast material by using a region of interest cursor (red circle in top right figure).

Contrast material arrival (transit) time

is then calculated (peak labeled with gray arrow in bottom right figure).

Main scan is then performed by using the full dose of contrast material, timed at the peak, plus a further delay that is used due to the longer injection duration, as explained later.

Bolus tracking

TIMING BOLUS

Slide10

Bolus Tracking or Triggering

Manufacturers have trade names for bolus tracking, including:

SmartPrep

(GE), CARE Bolus (Siemens

Healthineers

), Bolus Pro (Philips)

With a bolus tracking technique, a reference scan is performed.

A region of interest is placed on the target organ.

An

attenuation threshold

is programmed into the scanner.

The full volume of contrast material is injected.

When the attenuation reaches the threshold, or the CT technologist sees the contrast material arrive, the scanner is triggered to begin scanning after a

prescan

delay (usually 3–10 seconds).

ROI

Slide11

Factors That Influence Contrast Enhancement

Iodine flux (ie, iodine delivery per unit time)Iodine flux is dependent on three factors:Iodine concentration in contrast agentContrast material flow ratePatient factors (cardiac output, weight, intravenous line location, height, age, sex)Volume of contrast material used

Timing of CT relative to arrival of iodinated contrast materialTube potential

Slide12

Contrast Material Concentration and Iodine Flux

For a given contrast material viscosity, iodine flux is linearly proportional to the contrast material concentration.CT contrast material in the United States contains 300–370 mg of iodine per milliliter.For example, iohexol (Omnipaque) 300, iohexol 350, iodixanol (Visipaque) 320If injecting at 5 mL/sec:Iohexol 300: 1500 mg of iodine per secondIodixanol 320: 1600 mg of iodine per second

Iodine flux is increased 7% versus iohexol

300.

Iohexol

350: 1750 mg of iodine per second

Iodine flux is increased

17% versus

iohexol

300.

One would need to inject

iohexol

300 at

6 mL/sec

(1800 mg of iodine per second) to approximate iohexol 350 at 5 mL/sec.

Slide13

Contrast Material Injection Rate and Iodine Flux

The graphs above demonstrate the effect of 125 mL of contrast material injected at various flow rates on aortic (left) and liver (right) enhancement.Note that higher contrast material flow rates increase vascular enhancement substantially but have a small effect on the magnitude of parenchymal enhancement.Higher flow rates also shorten the time to peak enhancement, and the shorter injection duration results in decreased time of sustained near-peak enhancement. Compare the narrow green peak for 5 mL/sec in the aorta with the wider orange peak for 3 mL/sec. AORTA

Graphs inspired by Bae, Radiology 256(1), July 2010 and independently re-created in MATLAB (

MathWorks

, Natick, Mass).

LIVER

Slide14

Vascular Enhancement with Long Duration Injections

Time After Start of Injection (sec)

5

10

15

20

25

30

35

0

40

100

200

300

0

Vascular Enhancement (HU)

Contrast material injection duration

Hemodynamic

Perturbation

Recirculation

Due to an effect called

hemodynamic perturbation

(the flow of contrast material speeds venous return and shortens contrast material arrival time), the end of the contrast enhancement peak is earlier than would be otherwise expected given the contrast material arrival time plus injection duration (orange line and green arrow).

Due to

recirculation

, the peak enhancement is greatest near the end of the bolus.

Peak enhancement is

increased

but

shortened

due to recirculation and perturbation, respectively.

Graph adapted from Bae, Radiology 256(1), July 2010. Recirculation is greater than would typically be seen with CT for illustrative purposes.

Time (sec)

Slide15

Patient Factors

Larger patients generally have a larger blood volume, which leads to a greater dilution of contrast material.Enhancement is affected by patient weight for a given volume of contrast material:Generally, there is less enhancement with more weight.It is therefore standard practice to tailor contrast material volume to weight, although lean body mass and body surface area are more strongly correlated with blood volume.One can calculate volume by using formulas, injector software, or a chart (example below from the authors’ institution).Contrast material timing is largely unaffected by weight.

Body Weight (kg)

Body Weight (

lb

)

Volume (mL)

<55

<120

100

56–64

121–143

110

65–84

144–187

125

85–94

188–209

130

>95

>209

150

Slide16

Cardiac Output: Example—Pregnancy

Graph adapted from Robson et al. Serial study of factors influencing changes in cardiac output during human pregnancy. Am J Physiol 1989;256:H1060–5.

Pre

5

8

12

16

20

28

32

46

38

Post

24

5

6

7

8

Cardiac Output (L/min)

Gestation (weeks)

Baseline 4.8 L/min

Third Trimester 7.2 L/min

Cardiac output affects timing and magnitude of iodine enhancement.

Circulating concentration is proportionate to the volume of blood traveling per second and the iodine flux delivered per second.

In

low cardiac output states

(

eg

, heart disease), there is

delayed but higher vascular iodine enhancement.

In

higher cardiac output states

(

eg

, pregnancy), there is

earlier but lower vascular iodine enhancement.

In pregnancy, the cardiac output can increase by more than 50%. To achieve adequate vascular enhancement (

eg

, pulmonary embolism studies), the flow rate should be increased.

Slide17

CT Pulmonary Angiography in Pregnancy

100 mL of contrast material at 5 mL/sec  this yields a 20-second duration bolus.The cardiac output in 20 seconds is:Nonpregnant patient:4.8 L/min  1.6 L of blood in 20 seconds[Contrast material] = 100/1600 = 62.5 mL of contrast material per literPregnant patient:7.2 L/min 

2.4 L of blood in 20 seconds[Contrast material] = 100/2400 = 41.6 mL of contrast material per liter

50%

lower

concentration of contrast material in a

pregnant

patient with same injection protocol

, simply because of the different cardiac output

For pregnant patients:

Inject at a higher flow rate, or

Use higher concentration contrast material, or

Use a lower

kilovoltage

peak, or

Think about a different study (

eg

, ventilation-perfusion scan)

Slide18

Contrast Material Volume

Larger volumes of contrast material equate to increased injection duration at a given injection rate of 5 mL/sec.Larger volumes delay time to peak enhancement and increase peak enhancement in both the aorta and liver.AORTA

Graph adapted from Bae, Radiology 227(3), June 2003 and re-created independently in MATLAB.

Slide19

Time (sec)

10

20

30

40

50

60

70

0

80

100

200

300

0

Enhancement (HU)

64 (or higher)–detector row CT

90

100

120

AORTA

LIVER

16

detector row CT

 

64 (or higher)–detector row CT

Faster scanners (

eg

, 64 [or higher]–detector row CT) have shorter acquisition times, allowing one to achieve higher attenuation by injecting at a faster rate (increased iodine flux) and starting the acquisition later (delay relative to 16–detector row CT for aortic imaging shown by red arrow).

Scanner speed and perfect timing are less important for parenchymal organs (

eg

, liver) because the time-attenuation curve is broader and the peak is lower. This is why empiric scan delays are more successful for parenchymal organs (

eg

, venous phase abdomen) at CT than at angiography.

Timing CT Relative to Peak Enhancement

16–detector row CT

Time (sec)

Slide20

Low Contrast Material Dose CT Angiography and Dual-Energy CT Angiography

Slide21

There are widespread concerns about postcontrast

acute kidney injury.However, there is no conclusive evidence that the risk of acute kidney injury after administration of iodinated contrast material is dose related.Some studies have shown that the risk of postcontrast acute kidney injury (formerly referred to as contrast-induced nephropathy) may be negligible to nonexistent.Increasing iodinated contrast material dose may be associated with increasing toxicity, on the basis of general toxicology dose-response curves, but we do not know what part of the curve we are at when we use CT doses of contrast material.To be prudent, we can perform low contrast material dose CT angiography if we cannot perform MR angiography or ultrasonography.General Principles of Low Iodinated Contrast Material Dose Angiography

Slide22

Do not drop flow rate too low.Vascular enhancement is dependent on flow rate.

Volume (mL) = flow rate (mL/sec) x duration (sec)If one keeps the flow rate constant and reduces volume, the injection duration will decline proportionately. According to the principles in the earlier slides, this means that there will be:Earlier peak vascular enhancementShorter scan duration (because bolus will be shorter)Less overall enhancementTherefore, one must scan faster and earlier to maximize enhancement! One should also consider a lower kilovoltage peak technique.

General Principles of Low Iodinated Contrast Material Dose Angiography

Slide23

Be sure to get timing correct due to shorter duration (use timing bolus with 1–2 mL of contrast material or bolus triggering with manual trigger).

Decreasing scan range can also be used to decrease image acquisition time.Use a lower kilovoltage peak technique (eg, 80 or 70 kVp) to maximize iodine attenuation.Using dual-energy CT with low-energy reconstruction images will also increase iodine attenuation.Both low kilovoltage peak and dual-energy CT will increase artifacts (hardening, noise), especially in obese patients, so one needs to increase tube current–time product to compensate for noisy images.Tube current modulation will only go so far: one could max out the tube and generator output.Hence, more powerful tubes and generators are available on current CT scanners.

General Principles of Low Iodinated Contrast Material Dose Angiography

Slide24

CT: Polychromatic Photon Beam

120

Photon energy (

keV

)

kVp

Photon number

k-edge of I

33.2

80

kVp

100

kVp

120

kVp

CT scanners emit a polychromatic x-ray beam, described by the

kilovoltage

peak potential between the cathode and anode of the x-ray tube. A substantial proportion of the photons are lower energy than the

kilovoltage

peak.

Photons of lower

kiloelectron

-volt are attenuated more than those of higher

kiloelectron

-volt

, partly because of an increasing contribution of the photoelectric effect.

Slide25

Kilovoltage

Peak and Iodine Attenuation

140

kVp

120

kVp

80

kVp

100

kVp

322 HU

390 HU

491 HU

652 HU

These images depict a bag of diluted iodinated contrast material (red circles) scanned at several

kilovoltage

peaks to illustrate dependence of measured attenuation on peak voltage.

The attenuation of iodine increases with reducing

kilovoltage

peak.

This can also be seen in vivo.

Slide26

40

keV

51

keV

68 keV

77 keV

993.3 HU ± 97.9

626.4 HU ± 45.8

332.1 HU ± 27.3

250.4 HU ± 37.8

(Images courtesy of Lee

Mitsumori

, MD)

Kilovoltage

Peak and Iodine Attenuation

These images depict increasing iodine attenuation with lower

kilovoltage

peak.

Slide27

Low Contrast Material Dose CT Angiography in Practice

Baseline CT AngiographyFollow-up CT Angiography130 mL100 kVp

20 mL

80

kVp

In this example, a standard

kilovoltage

peak and standard weight-based contrast material volume was used at baseline CT angiography in a 50-year-old woman.

At follow-up CT angiography, a low iodinated contrast material dose technique was used, enabled by a lower

kilovoltage

peak (for increased iodinated attenuation) and shortened scan duration (to allow for imaging of peak enhancement even during the shorter duration of the contrast material bolus).

Slide28

Dual-Energy CT

A dual-energy CT technique provides two characteristics that are beneficial when performing CT angiography in the emergency department:Virtual monochromatic images (as if from photons of a given, fixed kiloelectron-volt rather than a spectrum) can be created. This in turn is beneficial because of the increasing attenuation of iodine with decreasing kiloelectron-volt.Material decomposition by the atomic weight of elements is possible, with production of images that have only elements of a given atomic weight (eg, iodine map) or those that exclude elements of a given atomic weight (eg, virtual non–contrast-enhanced image).

Iodine map

Virtual non–contrast-

enhanced image

Slide29

Dual-Energy CT—Effect of

Kiloelectron-Volt at CT AngiographyAs this movie of dual-energy CT angiography plays, note the kiloelectron-volt in the lower left corner. As the kiloelectron-volt decreases (slider), the iodine attenuation appears to increase. This characteristic is fundamental to dual-energy CT angiography in the emergency department: Lower iodine doses can be used with adequate iodine enhancement.

Slide30

Readers can provide patient-centric care by optimizing contrast material injection and image acquisition technique for each of their patients.

This presentation provides an overview of intravenous contrast material dynamics and the relationship of contrast material injection rate, volume, and patient factors.For optimal vascular opacification, contrast material injection rate should be maximized, which requires precise timing of image acquisition relative to start of injection tailored to the patient’s blood volume and cardiac output.Low contrast material dose CT angiography requires further optimization of technique that can be achieved with low kilovoltage peak and dual-energy acquisition performed with fast multidetector scanners.Conclusions

Slide31

Bae KT. Intravenous contrast medium administration and scan timing at CT: considerations and approaches. Radiology 2010; 256(1):32–61.

Bae KT. Peak contrast enhancement in CT and MR angiography: when does it occur and why? Pharmacokinetic study in a porcine model. Radiology 2003; 227:809–816.Mourits MM, Nijhof WH, van Leuken MH, Jager GJ, Rutten MJ. Reducing contrast medium volume and tube voltage in CT angiography of the pulmonary artery. Clin Radiol 2016; 71(6):615.e7–615.e13.Schindera ST, Nelson RC, Howle L, et al. Effect of varying injection rates of a saline chaser on aortic enhancement in CT angiography: phantom study. Eur Radiol 2008;18(8):1683–1689.

Kim DJ, Kim TH, Kim SJ, et al. Saline flush effect for enhancement of aorta and coronary arteries at multidetector CT coronary angiography. Radiology 2008;246(1):110–115.Kerl JM,

Ravenel

JG, Nguyen SA, et al. Right heart: split-bolus injection of diluted contrast medium for visualization at coronary CT angiography. Radiology 2008:247(2):356–364.

Shahir

K, Goodman LR,

Tali

A, et al. Pulmonary embolism in pregnancy: CT pulmonary angiography versus perfusion scanning. Am J

Roentgenol

2010; 195(3):W214–220.

McDonald JS, McDonald RJ, Comin J, et al. Frequency of acute kidney injury following intravenous contrast medium administration: a systematic review and meta-analysis.

Radioogy

2013;267(1):119–128.

McDonald JS, McDonald RJ, Carter RE, et al. Risk of intravenous contrast material-mediated acute kidney injury: a propensity score-matched study stratified by baseline-estimated glomerular filtration rate. Radiology 2014;271(1):65–73.

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