Single‐ vs. Double‐dose in contrast‐enhanced magnetic resonance angiography of the carotid arteries

Abstract

WE READ WITH great interest the article by Jourdan et al (1), published in the March 2007 issue of JMRI, in which the authors reported their results of dose comparison of single- vs. double-dose in contrast-enhanced magnetic resonance angiography (CE-MRA) of the carotid arteries. Their intraindividual crossover trial performed with 11 healthy volunteers confirms to a large extent our own conclusions after prospectively including 119 patients with suspected vascular occlusive disease in a similar study (2), stating that a single-dose of 0.1 mmol/kg body weight (BW) of an extracellular gadolinium agent may be the preferable dosage for the carotid arteries. Initially, we were surprised when reading that a double dose resulted in significantly (P < 0.05) higher signal intensity (SI), signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) compared to a single dose, while our patient study could not reveal such a difference. To the contrary, dosing did not influence our contrast enhancement when comparing double-dose (mean CNR = 70.01 ± 20.7) and single-dose (CNR = 69.12 ± 19.8). The most probable explanation for this difference seems to be the different contrast administration scheme—in our case both dosages were administered at 3.0 mL/second followed by a saline flush of 30 mL at the same speed. In the study of Jourdan et al (1), the infusion time was generally fixed for the gadolinium bolus to 10 seconds—followed by a 20-mL saline flush at the same rate. Overall, one can calculate a mean injection speed of 1.5 mL/second for single-dose and 3.0 mL/second for double-dose considering the mean weight of 71 ± 16 kg. Apparently, a higher injection speed results in higher CNR values, even though these have little impact on the diagnostic quality, as stated in both studies. It would have been interesting to retrieve some more information about the total acquisition time and the k-space filling techniques used by the authors, as these are of growing importance when evaluating and improving MRA techniques (3). In fact, centric-elliptic reordering or radial k-space techniques using segmented k-space filling should allow further dose decrease while improving spatial resolution and suppressing venous overlay. Using initial central k-space encoding and fixed acquisition times of 19 seconds, we could not observe a decreased coverage in case of single-dose or higher frequencies of venous overlay within the double-dose group. This confirms the high robustness of the MR angiographic technique we had implemented with little differences in our qualitative results despite the presence of all kinds of vascular pathologies in both dose groups. Mean CNR levels were considerably lower in the study of Jourdan et al (1), but this is probably attributed to the higher spatial resolution they report, and in part to the unequal formulas they used to calculate SNR and CNR. Nevertheless, the SNR penalty due to higher spatial resolution must perhaps be corrected by higher dosing? Furthermore, our study results could show that CNR is inversely related to BW both for single- and double-dose groups. Apparently, patients with higher BW present lower CNR values. This did not seem to lead to a decay of diagnostic quality. It remains to be clarified if patients with higher BW are rather underdosed, or the opposite, if patients with lower BW could benefit from a dose reduction even beyond single-dose. Influence of dosing on the degree of overstaging of stenosis in CE-MRA compared to digital subtraction angiography. Optimal dosing when segmented k-space techniques are implemented. (4) Dose influence on resolution or in other words on outer k-space trajectories. Adaptation of dosing to BW or better to body-surface of the patients. Dosing at high field. (3) Dosing when using parallel acquisition techniques with high acceleration factors. To conclude, data of both studies together are suggestive that single-dose, when ideally timed for central k-space would likely be optimal, both from the diagnostic and economic point of view for CE-MRA of the carotids. Johannes M. Froehlich PhD*, Martin Unterweger MD*, Rahel A. Kubik-Huch MD*, * Institute of Radiology, Cantonal Hospital Baden, Baden, Switzerland.