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Review |
1 Both authors: The Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins School of Medicine, 601 N Caroline St., Rm. 3251, Baltimore, MD 21287.
Received December 15, 2004;
accepted after revision June 24, 2005.
Supported in part by an unrestricted grant from Amersham, which is now part
of GE Healthcare. Elliot K. Fishman is on the advisory board for GE
Healthcare.
Abstract
 Top Abstract IntroductionClinical Applications: Abdominal...Clinical Applications: CT...ConclusionReferences |
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CONCLUSION. Higher concentrations (350 mg I/mL or greater) may improve visualization of small abdominal arteries. However, preliminary data comparing 300 mg I/mL to higher concentrations for MDCT of hypervascular hepatocellular carcinoma and pancreatic cancer have shown that higher concentrations may not increase tumor conspicuity.
Keywords: contrast media • CT angiography • liver • MDCT • pancreatic neoplasms
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TopAbstractIntroductionClinical Applications: Abdominal...Clinical Applications: CT...ConclusionReferences |
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To supplement the review, this article also presents body MDCT IV contrast infusion protocols used in investigations that evaluated abdominal solid organ tumors and body CT angiography. These were extracted from recent major studies (not original reports or technical innovations) published in two well-known radiology journals, Radiology and the AJR, and compiled into tables. Although similar research has been published in many other outstanding journals, these two journals were used for this compilation to yield a cross section of the literature within the constraints of a review article. A literature search from January 2000 to December 2004 identified the studies, with the exclusion of those evaluating radiation dose reduction measures, pediatric studies, and cardiac investigations, because cardiac MDCT is relatively new. MDCT contrast infusion protocols were included in the tables if all subjects were evaluated with MDCT or if the purpose of the study was to compare MDCT with single-slice CT. At least three studies were required to include the application, resulting in eight tables of infusion protocols.
Clinical Applications: Abdominal Tumor Detection
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MDCT for hypervascular hepatic masses, pancreatic tumors (islet cell and adenocarcinoma), and renal cell carcinoma requires an early acquisition before the hepatic (portal) venous phase. When a lower concentration is used, a higher infusion rate may be required. These are some of the practical considerations when designing MDCT contrast infusion protocols. Equally important are data that show how concentration affects diagnostic efficacy. Several studies comparing the effect of different IV contrast concentrations on abdominal tumor MDCT have addressed specific applications, including conspicuity of hypervascular hepatocellular carcinoma (HCC), parenchymal liver enhancement, and biphasic imaging for pancreatic carcinoma [3, 5, 9–13].
Hypervascular Hepatic Masses
With respect to multiphasic hepatic MDCT, in 2000, Foley et al.
[18] published one of the
early studies, using a test bolus technique with 60% contrast material infused
at 5 mL/sec. This study redefined multiphase hepatic CT by elucidating that
the late arterial phase is the portal inflow phase. With these infusion
parameters, the early arterial phase (initiated at peak aortic enhancement,
18–19 sec on average) was optimal to evaluate the arterial vasculature
for angiography. During the late arterial phase (initiated 4 sec after the end
of the early arterial acquisition), hypervascular tumor-to-liver contrast was
maximized. The hepatic venous phase (beginning at 60 sec after initiating
infusion and corresponding to the portal venous phase on single-detector CT)
showed the highest hepatic parenchymal enhancement of the three phases
[18,
19]. Results from several
other recent studies support the assertion of Foley et al. that the late
arterial phase is optimal for hypervascular HCC. In 2001, Murakami et al.
[20] showed that after
infusion of 300 mg I/mL at 5 mL/sec, the late arterial phase was more
sensitive than the early arterial phase for hypervascular HCC detection;
nonetheless, those authors recommended both phases be used to increase lesion
detection. However, Francis et al.
[21] (300 mg I/mL at 4
mL/sec), Ichikawa et al. [22]
(350 mg I/mL at 3 mL/sec), and Laghi et al.
[23] (300 mg I/mL at 5 mL/sec)
have all evaluated the utility of early and late arterial phases for
hypervascular tumor conspicuity or detectability and concluded that no benefit
is achieved by adding an early arterial phase.
In 2002, Awai et al. [3] evaluated the impact of contrast concentration on hypervascular HCC conspicuity with MDCT. The study evaluated 187 patients with hepatitis, of whom 58 were confirmed to have hypervascular HCC. Triphasic MDCT was performed with either 300 or 370 mg I/mL IV contrast material. The patients received the same iodine load per body weight (518 mg/kg). The contrast injection duration was fixed at 30 sec for the lower concentration and 25 sec for the higher concentration, but the injection rates per body weight were nearly identical (0.057 vs 0.056 mL/sec/kg). A timing bolus was performed and scans were initiated at 15, 30, and 105 sec after aortic contrast arrival time, corresponding to two consecutive arterial phase acquisitions and a delayed equilibrium phase acquisition, respectively. A hepatic venous phase acquisition was not performed. Tumor conspicuity for hypervascular HCCs, defined by tumor-to-liver contrast, was higher in the first arterial phase with 370 mg I/mL than with 300 mg I/mL, with no significant difference in conspicuity between the two concentrations in the second arterial phase. From these results, the authors concluded that the higher concentration may be more efficacious for imaging of HCC during the arterial phase.
However, Awai et al. [9] subsequently conducted a study comparing high (350 mg I/mL) and moderate (300 mg I/mL) concentrations for depiction of HCC during double arterial phase MDCT. The study included 186 patients with hepatitis, 67 of whom had hypervascular HCC. Subjects were administered equal iodine loads per body weight (518 mg/kg) over a 25-sec period, so that the mean infusion rates were 4 mL/sec for 300 mg I/mL and 3.6 mL/sec for 350 mg I/mL. Four acquisitions were performed after contrast infusion, with the early and late arterial phase acquisitions obtained at 10 and 20 sec after aortic attenuation reached 100 H over baseline, determined by bolus tracking. Therefore, both the infusion and acquisition protocols were modified in comparison with the previous study [3]. The results showed that using this fixed duration of contrast infusion and bolus tracking protocol, the 300 mg I/mL concentration produced higher tumor-to-liver contrast during the late arterial phase, the arterial phase of maximum tumor conspicuity determined by previous studies [18, 20–23]. The improved lesion conspicuity was attributed to retention of a smaller percentage of the iodine load in the peripheral vein. The authors also noted that the contrast infusion time for the 300 mg I/mL concentration was shortened from 30 sec in the first study [3] to 25 sec in the subsequent investigation. Reviewed in conjunction, these two studies show how dramatically infusion and acquisition timing affect contrast efficacy. These results apply to infusion protocols with the iodine load tailored to patient weight, fixed infusion duration, and timing determined by test bolus [3] or bolus tracking [9]. A review of studies published using multiphasic MDCT to evaluate hepatic lesions shows that the investigators have used various infusion protocols and concentrations [18, 20–28] (Table 1).
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Hypovascular Liver Tumors
For hypovascular hepatic lesions, the contrast infusion protocol and
acquisition timing are designed to image the liver during peak parenchymal
enhancement, and the hepatic venous acquisition is optimal for hypovascular
lesion detection [21]. The
rapid scanning afforded by MDCT has decreased concerns about imaging before
the onset of the equilibrium phase
[14]. Brink
[14] reviewed the topic of
contrast protocols in hepatic MDCT and reported that higher iodine
concentrations result in increased arterial enhancement; however, the total
iodine load infused is the primary factor that determines the maximum hepatic
enhancement. A number of older studies with CT have emphasized the importance
of iodine load for hepatic imaging. Heiken et al.
[15] suggested that 521 mg/kg
was needed using conventional CT, and Freeny et al.
[16] concluded that using 300
or 320 mg I/mL and decreasing contrast volume from 150 mL (45–48 g I) to
100 mL (30–32 g I) could potentially decrease hypovascular liver tumor
detection with single-detector helical CT.
More recently, with a 4-MDCT scanner, Roos et al. [11] compared concentrations of 200 mg I/mL (30 g I), 250 mg I/mL (35.5 g I), 300 mg I/mL (45 g I), and 350 mg I/mL (52.5 g I) in 100 patients without liver disease. Each patient received 150 mL of contrast material infused at 3 mL/sec followed by a 30-mL saline flush, and the hepatic phase was initiated at an average of 72 sec using a bolus tracking technique. Although 350 mg I/mL resulted in significantly greater liver enhancement than did 300 mg I/mL, all patients in these two groups showed sufficient liver enhancement, defined as a mean change in parenchymal enhancement values of more than 40 H, which was not obtained by some patients in the lower-concentration groups [11]. These single-detector CT and MDCT studies showed that a larger volume of moderate iodine concentrations (300–320 mg I/mL) can produce adequate hepatic parenchymal enhancement.
The requirement for an adequate iodine load with hepatic CT has been recapitulated in recent MDCT studies of patients with liver disease. Furata et al. [5] recently published a study that concluded that 100 mL of 300 mg I/mL did not produce adequate hepatic parenchymal enhancement in all patients with hepatitis or cirrhosis who weighed more than 60 kg. Using 100 mL of 370 mg I/mL contrast material, overall image quality and liver parenchymal enhancement were significantly greater during 60- and 180-sec acquisitions, which are of importance for detecting hypovascular hepatocellular cancers. Results from the study of Tsurusaki et al. [12], in which 100 hepatitis patients were prospectively allocated to receive 100 mL of 300 or 370 mg I/mL, also revealed significantly greater hepatic parenchymal enhancement during 70- and 140-sec acquisitions with the higher concentration. However, in each of these studies, the total iodine load was lower with the 300 mg I/mL protocol, and tumor-to-liver contrast was not evaluated. Therefore, the results do not indicate whether a higher volume of the 300 mg I/mL concentration would produce studies of equal diagnostic efficacy to those using 370 mg I/mL.
The two studies by Awai et al. [3, 9] of patients with hepatitis compared equal iodine loads per body weight of 300 mg I/mL to either 350 mg I/mL [9] or 370 mg I/mL [3]. Results showed that 300 mg I/mL resulted in comparable hepatic parenchymal enhancement to 350 mg I/mL during the hepatic venous and equilibrium phases [9]. Liver parenchymal enhancement during a delayed equilibrium acquisition was not significantly different when 300 mg I/mL was compared with 370 mg I/mL in their earlier study, which did not include a hepatic venous phase [3]. Suzuki et al. [13] reported similar results in 66 patients with liver tumors imaged with MDCT. Subjects were prospectively allocated to receive 600 mg/kg of 300 mg I/mL or 370 mg I/mL infused over 30 sec. Results showed no significant difference in hepatic enhancement during hepatic venous or equilibrium phase acquisitions performed at 50 and 160 sec after aortic contrast arrival time (150 H), respectively [13]. Table 2 shows that 300 mg I/mL continues to be used for MDCT hepatic venous phase acquisitions [29–31].
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Pancreatic Cancer
For pancreatic cancer imaging, single-detector helical studies have
elucidated the effect of contrast infusion rate, volume, and concentration on
degree and timing of pancreatic parenchymal enhancement
[32,
33]. Recent MDCT studies
addressing tumor conspicuity and detectability have focused on determining the
optimal timing with the new technology
[34,
35]. These studies have
emphasized the importance of a pancreatic parenchymal phase acquisition,
occurring after the early arterial phase but before the hepatic venous phase,
for assessment during maximum pancreatic enhancement. Results have shown that
the pancreatic parenchymal phase and the portal venous phase (the hepatic
venous phase of Foley et al.
[18]) are both significantly
superior to the early arterial phase for maximum tumor-to-parenchymal contrast
and tumor detection, and that the early arterial phase is not necessary to
evaluate vascular encasement
[34,
35].
With respect to contrast concentration, an investigation comparing concentrations and infusion rates with single-detector CT showed that 1.5 mL/kg of 370 mg I/mL at 5 mL/sec resulted in significantly greater pancreatic enhancement than a slightly higher iodine load of 300 mg I/mL (2 mL/kg) at 3 mL/sec. The authors concluded that enhancement of the pancreatic parenchyma relies more on iodine dose delivered per second than the total iodine load [33].
Only one published study was identified that evaluates how contrast concentration affects pancreatic enhancement and tumor conspicuity on MDCT. This recent study in the British Journal of Radiology compared the effect of a high-concentration contrast material (400 mg I/mL) to a moderate concentration (300 mg I/mL) in 50 patients [10]. (A concentration of 400 mg I/mL has been used in studies from outside the United States but is not currently approved by the U.S. Food and Drug Administration for use in the United States.) Equal iodine loads (39 g) were infused at 5 mL/sec. Arterial phase imaging was initiated at the time to peak aortic enhancement (mean, 17 sec) as dictated by a test bolus, followed by an acquisition between 50 and 70 sec from the initiation of contrast infusion. The 50 patients were evenly divided between the two concentrations; six pancreatic cancers were included in the 300 mg I/mL group and five in the 400 mg I/mL group. Quantitative assessment showed that the higher concentration resulted in significantly higher pancreatic enhancement and pancreatic tumor enhancement in the arterial and hepatic venous phases. However, subjective evaluation revealed that the reviewers graded more studies as "good" or "excellent" for delineating tumor from surrounding tissue using the 300 mg I/mL concentration.
The improved delineation is likely the result of the fact that the mean attenuation difference of pancreatic parenchyma to pancreatic cancer (tumor-to-pancreas contrast) was slightly higher with the lower concentration during both the arterial phase (35 H with 300 mg I/mL vs 29 H with 400 mg I/mL) and the 50- to 70-sec phase, where the difference in mean tumor-to-pancreas contrast between the two contrast concentrations was even greater (39 H with 300 mg I/mL vs 17 H with 400 mg I/mL). Although these differences were not statistically significant, the significantly higher pancreatic parenchymal and pancreatic tumor enhancement from the 400 mg I/mL concentration did not result in improved tumor conspicuity [10].
The authors also reported that "concerning evaluability of organ infiltration next to the pancreas, both readers tended to assign group A patients (... 300 [mg I/mL]) to higher categories than group B patients (... 400 [mg I/mL])," reflecting the categorization of each study as excellent, good, sufficient, or insufficient for this criterion. This study [10] was limited by the very small number of patients. Larger studies comparing how concentration affects tumor conspicuity and the assessment of peripancreatic infiltration may be warranted. Table 3 lists five studies published between 2001 and 2004 that have used MDCT for pancreatic cancer, most of which used a concentration of 300 mg I/mL [34–38].
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Summary
In summary, optimization of liver and pancreatic tumor-to-organ contrast
requires infusion of an adequate iodine load coordinated with acquisition
timing to acquire data during the phase of maximum conspicuity unique to each
tumor type. The recent data from MDCT are limited in that only a few studies
evaluated tumor conspicuity. Nonetheless, some conclusions can be drawn,
elucidating the direction for future research. HCC imaging involves multiple
acquisitions, with a proven value in late arterial phase imaging
[18,
21–23],
supplemented by either hepatic venous phase or delayed equilibrium acquisition
[3]. A hepatic venous phase
acquisition, as part of an HCC protocol or for hypovascular metastases,
requires an adequate iodine load, feasible with any contrast material in the
range of 300–370 mg I/mL
[11,
13].
Patient weight must be considered [5, 15], which is why many investigators calculate volume by weight (Table 1). The survey of multiphasic hepatic CT infusion protocols in Table 1 provides valuable information about IV contrast volume, concentration, and infusion rates, elucidating that the higher concentrations (350–370 mg I/mL) were infused at 3–4 mL/sec, but most of the investigations using 300 mg I/mL were conducted with an infusion rate of 5 mL/sec [20, 22, 23, 25–28]. For practical purposes, selecting a higher concentration (350 mg I/mL or greater) for HCC imaging facilitates coupling of contrast infusion to the arterial phase acquisition with a lower infusion rate. Nonetheless, as Table 1 shows, various infusion protocols are being used, and diagnostic efficacy must be considered in selecting contrast concentration and infusion parameters. Using a fixed injection duration infusion protocol, 300 mg I/mL has been shown to yield higher hypervascular HCC conspicuity during the late arterial phase than 350 mg I/mL [9].
It is imperative to recognize that the results of these studies apply specifically to the infusion and acquisition parameters used in the investigation, and do not necessarily dictate the best contrast concentration for all protocols. In light of the acquisition speed, future investigations with 64-MDCT technology may be necessary to design infusion protocols that maximize hepatic and pancreatic cancer conspicuity during the increasingly narrow temporal window of data acquisition. For pancreatic cancer imaging, one article showed that despite a significant increase in pancreatic parenchymal enhancement, a higher concentration (not available in the United States) yielded lower tumor-to-pancreas contrast compared with 300 mg I/mL, although the difference was not significant [10]. Accordingly, to fully determine how pancreatic CT contrast infusion parameters such as concentration, volume, iodine dose, and rate affect pancreatic cancer imaging, tumor conspicuity must be evaluated in addition to pancreatic enhancement.
Clinical Applications: CT Angiography
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To evaluate the efficacy of a contrast concentration, studies correlating the attenuation measurements with subjective grading for vascular visualization or diagnostic accuracy provide a useful assessment. In the aorta, a number of MDCT investigators have used 200 H as a threshold for adequate enhancement [51–53]. For the smaller arteries, very few MDCT studies have performed quantitative and qualitative assessments. Tanikake et al. [4] noted that a minimum aortic density of 395 H was necessary to yield good or excellent 3D renderings of the small peripheral hepatic artery branches. Investigations aimed at determining the optimal attenuation values for evaluation of the branches of the abdominal aorta will be useful in guiding infusion protocols.
Abdominal Aorta
A number of MDCT investigations of the abdominal aorta have compared
contrast concentrations [3,
9,
11–13].
The study by Tsurusaki et al.
[12] comparing 100 mL of 300
mg I/mL with 370 mg I/mL at 4 mL/sec with a fixed delay of 20 sec showed
significantly higher aortic attenuation in the early arterial phase with the
higher concentration, which delivered a higher iodine load. However, the study
by Roos et al. [11] compared
150 mL of 300 mg I/mL with 350 mg I/mL infused at 3 mL/sec followed by a
saline flush and showed no significant difference in early arterial aortic
enhancement, timed with bolus tracking. In the study by Awai et al.
[3], in which equal iodine
loads of 300 and 370 mg I/mL were infused at the same rate per body weight,
370 mg I/mL provided significantly higher aortic attenuation during the first
of two arterial phase acquisitions; however, the aortic density was
significantly higher with 300 mg I/mL during the second acquisition. In their
subsequent study [9], when
equal iodine loads of 300 and 350 mg I/mL were administered over a fixed
infusion duration, so that the lower concentration was infused slightly
faster, 300 mg I/mL resulted in significantly greater aortic enhancement
during the early and late arterial phases. An aortic MDCT study by Awai et al.
[53] comparing fixed infusion
duration with a fixed rate with 300 mg I/mL has revealed benefits of the fixed
duration protocol. With contrast volume calculated by weight, the authors
noted that injection with a fixed rate results in variability of aortic peak
enhancement according to weight; however, infusion with a fixed duration
yields more constant time to aortic peak and degree of aortic enhancement
[53].
Despite the relatively higher aortic attenuation levels achieved with 370 mg I/mL compared with 300 mg I/mL in some studies [3, 12], other quantitative and qualitative MDCT assessments of the abdominal aorta suggest that 300 mg I/mL is adequate. Macari et al. [51] reported that 150 mL of 300 mg I/mL infused at 4 mL/sec after a standard delay of 25 sec resulted in adequate aortic enhancement (defined as mean attenuation of 200 H) and intense enhancement of the iliac and promixal femoral arteries in all patients, noting that one patient who weighed 147 kg had compromised image quality of volume-rendered and maximum-intensity-projection reconstructions. More recently, Ho et al. [52] have shown that the volume of contrast material can be reduced without significant reduction in aortic attenuation or subjective grading of aortic enhancement in patients with abdominal aortic aneurysms. Using 300 mg I/mL contrast material infused at 4 mL/sec and automated triggering, patients were administered approximately 150 mL, or the volume was reduced by terminating the infusion at the onset of the acquisition (mean reduced volume, 107 ± 20 mL). Table 5 is a summary of infusion protocols from MDCT studies of the aorta, most of which were conducted with 300 mg I/mL [1, 52–55].
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Abdominal Aortic Branches
For the abdominal arteries, a few investigations have shown that arterial
enhancement is higher in the arterial phase with 370 or 400 mg I/mL compared
with 300 mg I/mL, when patients received the same iodine load and infusion
rates [3,
7,
10]. The study of Awai et al.
[3] comparing 370 with 300 mg
I/mL with equal iodine loads and infusion rate/body weight showed
significantly improved depiction of the hepatic arteries with the higher
concentration. Squillaci et al.
[7] observed that compared with
300 mg I/mL (36 g I), 370 mg I/mL (37 g I) "yielded the best results for
evaluation of small vessels, such as the hepatic artery or of the vessels
developing along the x-axis, such as the renal arteries,"
although a small number of subjects (five per subgroup) was evaluated.
Table 6 lists studies using MDCT for the renal arteries that have evaluated anatomic variants, kidney donors, and renal artery stents. Several studies were performed with higher iodine concentrations [56–60]. As for other vascular territories, the pancreatic study by Fenchel et al. [10] in the British Journal of Radiology reported that 400 mg I/mL resulted in 63–88% of studies being graded as excellent for ability to evaluate the aorta, celiac artery, and superior mesenteric artery in the arterial phase compared with 32–77% of the studies performed with 300 mg I/mL, and quantitative assessment showed significantly higher arterial enhancement with 400 mg I/mL.
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With respect to the smaller arteries, the importance of a high infusion rate was shown in the multislice study of the hepatic artery performed by Tanikake et al. [4] with disparate iodine loads (100 mL of either 300 or 350 mg I/mL) infused at 4 or 5 mL/sec. The early arterial phase, triggered by bolus tracking 6 sec after the aortic attenuation reached 50 H over baseline, was volume-rendered and graded for degree of visualization of peripheral (subsegmental and sub-subsegmental) branches of the hepatic artery. Results of the visual analysis and aortic attenuation measurements showed no statistically significant differences between the groups given different concentrations of iodine at the same injection rate. Infusion at 5 mL/sec always resulted in higher-quality studies compared with 4 mL/sec, underscoring the importance of infusion rate. In fact, administration of 300 mg I/mL (30 g iodine) at 5 mL/sec resulted in significantly improved visualization of the peripheral hepatic arteries compared with 350 mg I/mL (35 g iodine) infused at 4 mL/sec. A close analysis of the results shows that the peripheral arterial visualization was rated as fair or poor in 26% of patients receiving 300 mg I/mL at 5 mL/sec, compared with 9% of patients who received 350 mg I/mL at 5 mL/sec, prompting the authors to conclude that for imaging the peripheral branches of the hepatic artery, "a contrast agent with an iodine concentration of 350 mg I/mL is preferable to increase the diagnostic capability" [4]. However, equal iodine loads were not administered in the study by Tanikake et al. [4], which would be necessary to fully assess the efficacy of the lower concentration. Investigative studies using MDCT to evaluate the hepatic vasculature have used a range of contrast concentrations, as shown in Table 7 [61–71].
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Lower Extremity Arteries
The peripheral vasculature is one anatomic region in which imaging has been
dramatically improved by the increased coverage, resolution, and contrast
efficiency afforded by MDCT [1,
72]. An early study of the
lower extremity inflow and runoff was conducted with an average of 184 mL of
300 mg I/mL infused over a mean duration of 53 sec. Results showed that
arterial attenuation reached greater than 150 H in 97% of the segments from
the aorta to the feet, with mean attenuation levels of 253 H in the mid
abdominal aorta progressively increasing to peak at a mean of 357 H in the
popliteal region [72]. In a
study evaluating peripheral arterial bypass grafts, MDCT was performed with
120 mL of 300 mg I/mL contrast agent followed by 30 mL of saline infused at 4
mL/sec. The opacification of the bypass graft segments was found to be
excellent by attenuation measurements (> 150 H) in 98% of segments, and
image quality was rated as excellent (79%) or good (19–20%) in most
patients. There were no nondiagnostic studies
[73]. A survey of recent
studies using MDCT for the lower extremity arteries shows that a range of
contrast concentrations has been used
[72–78]
(Table 8).
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An MDCT study of the infrarenal aorta and peripheral vasculature compared equal iodine loads (42 g) of 300, 350, and 400 mg I/mL concentrations [6]. This study included 66 subjects with peripheral arterial disease who were divided into subgroups of 10–12 patients. Subjects were either included in an evaluation using a fixed duration of infusion for all three concentrations or a second investigation in which each concentration was infused at the same rate. The arterial density from the aorta to the calf was measured in six locations. The results showed that with either infusion protocol, all three concentrations achieved mean density values of at least 300 H in the infrarenal aorta, progressively increasing to peak in the superficial femoral or popliteal region. With the infusion rate fixed at 3.5 mL/sec, the higher the concentration, the greater the attenuation measurement (up to 500 H in the popliteal region with 400 mg I/mL). With infusion duration fixed, the 300 and 350 mg I/mL concentrations resulted in higher density than 400 mg I/mL in the infrarenal aorta; however, from the common femoral artery distally, 400 mg I/mL yielded the highest mean density. The fixed infusion duration curves were more closely approximated for the three concentrations. These quantitative data were not analyzed statistically to determine whether differences were significant. What remains to be determined is whether these attenuation differences significantly affect the quality and diagnostic accuracy of 2D reconstructions and 3D renderings.
Because peripheral vascular disease and renal artery stenosis manifest with calcified plaque, optimization of infusion protocols may require balancing adequate vascular opacification and discrimination of contrast material from calcification, as has been suggested for CT angiography of coronary artery disease [8]. The cardiac literature has the unique advantage of containing investigations assessing the density of calcified coronary artery plaque using high-resolution unenhanced MDCT and research studies of contrast-enhanced MDCT angiography of the coronary arteries. One such study evaluating vessel density and image quality compared 140 mL of 300 or 400 mg I/mL concentrations infused at 2.5 or 3.5 mL/sec [8]. The authors suggested that attenuation levels of 250–300 H were optimal for the evaluation of coronary artery disease, and recommended that coronary artery CT angiography infusion protocols deliver 1 g of iodine per second to achieve these attenuation levels. They cautioned that high-concentration infusion protocols with a high infusion rate (400 mg I/mL at 3.5 mL/sec or 1.4 g/sec) resulting in density values greater than 350 H could potentially result in an underestimation of coronary artery stenosis because of obscuration of calcification. Future investigations that elucidate the attenuation of calcified plaque in other vascular regions, such as the lower extremity, may be helpful in determining the optimal vascular enhancement levels in that area.
Summary
For CT angiography, MDCT presents the challenge of coupling contrast
infusion with unprecedented acquisition speed. The use of higher
concentrations (350 mg I/mL or greater) of contrast material for abdominal CT
angiography is supported by practical considerations and published data.
Higher concentrations enable rapid infusion of the contrast bolus for an early
arterial phase acquisition, and the results from multiple studies reveal
improved arterial enhancement or visualization using concentrations of
350–400 mg I/mL when compared with both disparate and equal iodine loads
of 300 mg I/mL infused at the same rate
[3,
4,
6,
7,
10,
12]. However, each vascular
territory has different contrast infusion requirements, depending on the size
and location of the arteries. Abdominal aortic imaging can be performed with
300 mg I/mL [7,
9,
51–53],
but a few studies have reported improved visualization or enhancement of the
hepatic artery and renal arteries with concentrations of 350 or 370 mg I/mL
[3,
4,
7], and high infusion rates are
essential to depict small arterial structures
[4]. For evaluation of the
peripheral vasculature, there has been no consensus as to the optimal
concentration, with good results shown in studies using concentrations of 300
mg I/mL and higher
[72–78].
Most of the CT angiography studies reviewed here compared higher
concentrations (350–400 mg I/mL) with 300 mg I/mL, with no body of
evidence showing a significant difference in MDCT diagnostic efficacy among
the higher concentrations.
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