Contrast agent pharmacokinetics revisited: I. Reformulation

A Metric for Quantification of Iodine Contrast Enhancement (Q-ICE) in Computed Tomography

BACKGROUND

Poor contrast enhancement is related to issues with examination execution, contrast prescription, computed tomography (CT) protocols, and patient conditions. Currently, our community has no metric to monitor true enhancement on routine single-phase examinations because this requires knowledge of both pre- and postcontrast CT number.

PURPOSE

We propose an automatable solution to quantifying contrast enhancement without requiring a dedicated noncontrast series.

METHODS

The difference in CT number between a target region in an enhanced and unenhanced image defines the metric "quantification of iodine contrast enhancement" (Q-ICE). Quantification of iodine contrast enhancement uses the noncontrast bolus tracking baseline image from routine abdominal examinations, which mitigates the need for a dedicated noncontrast series. We applied this method retrospectively to 312 patient livers from 2 sites between 2017 and 2020. Each site used a weight-based contrast injection protocol for weights 60 to 113 kg and a constant volume less than 60 kg and greater than 113 kg. Hypothesis testing was performed to compare Q-ICE between sites and detect Q-ICE dependence on weight and kilovoltage (kV).

RESULTS

Mean Q-ICE differed between sites (P = 0.004) by 4.96 Hounsfield unit with 95% confidence interval (1.63-8.28), albeit this difference was roughly 2 times smaller than the SD in Q-ICE across patients at a single site. For patients between 60 and 113 kg, we did not observe evidence of Q-ICE varying with patient weight (P = 0.920 and 0.064 for 120 and 140 kV, respectively). The Q-ICE did vary with patient weight for patients less than 60 kg (P = 0.003) and greater than 113 kg (P = 0.04). We observed a roughly 10 Hounsfield unit reduction in Q-ICE liver for patients scanned with 140 versus 120 kV. We observed several underenhancing examinations with an arterial phase appearance motivating our CT protocol optimization team to consider increasing the delay for slowly enhancing patients.

CONCLUSIONS

A quality metric for quantifying CT contrast enhancement was developed and suggested tangible opportunities for quality improvement and potential financial savings.

Use of high-concentration contrast media in multiple-detector-row CT: principles and rationale

Contrast-medium-enhanced multiple-detector-row CT (MDCT) is a powerful technique for vascular and hepatic imaging. With increasingly faster acquisition speeds, which have become possible with latest 8- and 16-channel scanner systems, contrast medium delivery is becoming increasingly difficult. This article reviews the pharmacokinetic and physiologic principles of vascular and hepatic enhancement following the intravenous injection of iodinated contrast medium. The effects of user-selectable injection parameters, such as the injection rate, the injection duration, and the contrast medium concentration on arterial and parenchymal enhancement are elucidated. Equipped with this knowledge, rational injection strategies for CT angiographic protocols for scanners with different acquisition speeds are derived. Furthermore, injection and timing protocols, optimized for hepatic MDCT during the early arterial, late arterial, and parenchymal phases, are developed.

Hepatocellular carcinomas: correlation between time to peak hepatocellular carcinomas enhancement and time to peak aortic enhancement

PURPOSE

To prospectively assess the relationship between the time to peak enhancement of hepatocellular carcinomas (HCC) and that of the aorta at 64-detector computed tomography (CT).

MATERIALS AND METHODS

The study prospectively included 43 patients with known HCC. All underwent abdominal CT imaging by using BodyPerfusion CT model. The CT data acquisition was initiated with a delay of 8-15s from the beginning of the contrast material administered. The time-density curves (TDC) of the HCC and the aorta were drawn. The times to peak enhancement of the HCC and the aorta were recorded and the correlation between the time to peak enhancement of the HCC and that of the aorta was analyzed.

RESULTS

There were three tendencies of TDC of the HCC enhancement, only 23.3% of them were similar to that of the aorta. The mean time to peak enhancement of the aorta and the HCC (86.1%) was 23.38 s and 30.04 s, respectively. The time to peak enhancement of most HCC was positively and linearly correlated with the time to peak aortic enhancement (r=0.662, P<0.05).

CONCLUSION

The result may potentially allow scan delay optimization at contrast material-enhanced CT image in the detection of HCC according to interindividual variability.

DETERMINATION OF GLOMERULAR FILTRATION RATE IN DOGS USING CONTRAST-ENHANCED COMPUTED TOMOGRAPHY

The purpose of this project was to establish a procedure and reference values for glomerular filtration rate (GFR) using contrast-enhanced computed tomography (CT) in eight healthy dogs. A single section of the kidney was scanned sequentially after bolus injection (3 ml/s) of iohexol (300 mg/kg). Time-attenuation curves were constructed and the GFR per volume of kidney was calculated using Patlak graphical analysis software. The GFR was then converted from contrast clearance per unit volume (ml/min/ml) to contrast clearance per body weight (ml/min/kg). Individual kidney and global GFR were calculated using both CT and nuclear scintigraphy. Global GFR for each dog was also determined by plasma iohexol clearance. Contrast-enhanced CT underestimated the global GFR compared with the other two methods. The average global GFR was 2.57 +/- 0.33 ml/ min/kg using functional CT and 4.06 +/- 0.37 ml/min/kg using plasma iohexol clearance. There was significant (P < 0.05) interobserver variability of CT GFR of the right kidney and total GFR. There was decreased interobserver variability for the left kidney. There was no difference in the intraobserver variability for CT-determined individual kidney and global GFR. There was no difference between the motion corrected and nonmotion corrected values for individual and global CT GFR. Nuclear scintigraphy produced a slightly higher coefficient of variation than contrast-enhanced CT, 2.9% and 1.0%, respectively. It is hypothesized that altered renal blood flow, hematocrit of the small vessels, and nephrotoxicity play a role in the underestimation of GFR by contrast-enhanced CT.

Multi-slice CT contrast enhancement regimens

Historically, the development of progressively faster Computed Tomography (CT) technology has dictated a recurrent need to re-examine intravenous contrast agent enhancement regimens. The most recently introduced development, the very fast, multi-slice helical/spiral systems, have raised the same issue yet again. It is possible, exploiting the technology to its maximum potential as regards speed, to perform an examination many times faster (depending on the number of detector rings from 4 to 64) even than with earlier single slice spiral instruments. In order to optimise image quality, such maximal speed gains will not usually be sought but, nevertheless, imaging time will generally be substantially reduced. It is natural that the question of a possible need to modify contrast agent enhancement protocols designed for an earlier generation of slower machines should again be considered. Using as a basis known contrast agent pharmacokinetics and results of modelling techniques, the matter is tackled in this paper.

Use of high concentration contrast media: principles and rationale-vascular district

Optimal contrast medium delivery remains a crucial issue in CT angiography and it will become even more critical with continuously evolving, faster CT scanner technology. This review article first explains the fundamentals of arterial enhancement using mathematical models of early contrast medium dynamics. The relationship of contrast medium volume, injection flow rates and injection duration are explicitly illustrated. Next, current techniques of contrast medium application are reviewed, with particular attention to methods of accurate timing of the scanning delay (test-bolus and automated bolus triggering), tools for automated saline-flushing of the veins (double-syringe power injectors) and the use of high-concentration contrast medium. From there, rational CT angiographic injection protocols for a wide range of selectable acquisition times for 4-, 8- and 16-channel MDCT are proposed.

Contrast agents in patients on dialysis

Contrast media are used to enhance the structural and functional information that is provided by imaging methods. They are used particularly in conventional radiographic (X-ray) investigations, but are also used in magnetic resonance imaging (MRI) and increasingly in ultrasound (US). The water-soluble, iodinated, intravascular radiographic agents are commonly used, and in large doses. They are excreted almost exclusively by the kidney and may have nephrotoxic effects. The use of these agents in patients with renal impairment may potentially be problematic. In this article the chemical composition and pharmacokinetics of these contrast agents are reviewed, as are their potentially toxic effects on the heart, neural tissues, kidney, and endothelium. Their safety for use in the patient with end-stage renal disease (ESRD) and those on dialysis is discussed. This aspect is also addressed briefly for the contrast agents used to augment MRI and US.

Improved uniformity of aortic enhancement with customized contrast medium injection protocols at CT angiography

PURPOSE

To compare the uniformity of aortoiliac opacification obtained from uniphasic contrast medium injections versus individualized biphasic injections at computed tomographic (CT) angiography.

MATERIALS AND METHODS

Thirty-two patients with an abdominal aortic aneurysm underwent CT angiography. In 16 patients (group 1), 120 mL of contrast material was administered at a flow rate of 4 mL/sec. In the other 16 patients (group 2), biphasic injection protocols were computed by using mathematic deconvolution of each patient's time-attenuation response to a standardized test injection. Attenuation uniformity was quantified as the "plateau deviation" of enhancement values, which were calculated as the SD of the time-contiguous attenuation values observed during the 30-second scanning period.

RESULTS

Group 2 patients received between 77 and 165 mL (mean, 115 mL) of contrast medium. Initial flow rates ranged from 4.1 to 10.0 mL/sec (mean, 6.8 mL/sec) for the first 4-6 seconds; continuing flow rates ranged from 2.0 to 4.8 mL/sec (mean, 3.1 mL/sec) for the remaining 24-26 seconds. The plateau deviation was significantly smaller in group 2 patients (19 HU) versus group 1 patients (38 HU, P <.001).

CONCLUSION

At CT angiography, tailored biphasic injections led to more uniform aortoiliac enhancement, compared with standard uniphasic injections of contrast medium.

Mathematical analysis of arterial enhancement and optimization of bolus geometry for CT angiography using the discrete fourier transform

PURPOSE

The goal of this work was to develop a clinically applicable mathematical algorithm to analyze and optimize individual arterial enhancement in CT angiography (CTA).

METHOD

Assuming a time-invariant linear system, the discrete Fourier transform was used to calculate the transfer function of the system ("patient function") from the arterial time-attenuation response to a test bolus. The patient function was subsequently used to predict aortic enhancement in five select patients and to calculate optimized biphasic injection protocols in two of these patients undergoing CTA of the abdominal aorta.

RESULTS

Arterial time-attenuation curves were accurately predicted in all patients. Optimized biphasic contrast agent injection protocols resulted in uniform aortic enhancement at the predefined level over the entire scanning period in both subjects despite markedly different contrast agent volumes and injection rates used.

CONCLUSION

Fourier analysis of the time-attenuation response to a test bolus is a simple and feasible approach to optimize arterial enhancement in CTA.

Contrast media as extracellular fluid space markers: adaptation of the central volume theorem

We present some theoretical observations on the pharmacokinetics of the conventional intravascular radiographic and gadolinium chelate MR contrast agents. These agents may be viewed from a physiological perspective as extracellular fluid (ECF) space markers. It can be shown that the passage of such agents through the body may be described by a variant of the "central volume theorem", if the central volume is taken to be that of the entire extracellular fluid. A consequence is that, in a manner akin to nuclear medicine techniques utilizing technetium-99m diethylene triamine pentacetic acid (DTPA), measures of glomerular filtration rate (GFR) related to total ECF volume rather than to the normal body surface area may be obtained from direct or indirect measurements of iodine or gadolinium plasma levels. Typical measurements are presented. A simple means is also described of generating curves of the fraction of contrast medium excreted from the body, applicable to both patients with normal and with abnormal renal function.