Low radiation dose in computed tomography: the role of iodine

Recent approaches to reducing radiation exposure during CT examinations typically utilize automated dose modulation strategies on the basis of lower tube voltage combined with iterative reconstruction and other dose-saving techniques. Less clearly appreciated is the potentially substantial role that iodinated contrast media (CM) can play in low-radiation-dose CT examinations. Herein we discuss the role of iodinated CM in low-radiation-dose examinations and describe approaches for the optimization of CM administration protocols to further reduce radiation dose and/or CM dose while maintaining image quality for accurate diagnosis. Similar to the higher iodine attenuation obtained at low-tube-voltage settings, high-iodine-signal protocols may permit radiation dose reduction by permitting a lowering of mAs while maintaining the signal-to-noise ratio. This is particularly feasible in first pass examinations where high iodine signal can be achieved by injecting iodine more rapidly. The combination of low kV and IR can also be used to reduce the iodine dose. Here, in optimum contrast injection protocols, the volume of CM administered rather than the iodine concentration should be reduced, since with high-iodine-concentration CM further reductions of iodine dose are achievable for modern first pass examinations. Moreover, higher concentrations of CM more readily allow reductions of both flow rate and volume, thereby improving the tolerability of contrast administration

Comparison of inversion recovery and contrast-enhanced T1-weighted fat-suppressed sequences for the staging of cervical lymphoma

In a retrospective analysis with two readers blinded to the clinical information, coronal short tau inversion recovery (STIR) images were compared to contrast-enhanced fat-saturated T1-weighted imaging (T1 CEfs) in 51 cases of cervical lymphoma. Interrater reliability was good to excellent. Although sensitivity and subjective quality of the STIR sequence were higher than those of the T1 CEfs sequence (sensitivity 85%/72%, respectively), specificity (82%/95%) as well as positive likelihood ratio (4.65/15.93) was much lower. Therefore, contrast-enhanced sequences should be included in the primary staging of lymphoma

Comparison of MR imaging sequences for liver and head and neck interventions: is there a single optimal sequence for all purposes?

To compare the appropriate pulse sequences for interventional device guidance during magnetic resonance (MR) imaging at 0.2 T and to evaluate the dependence of sequence selection on the anatomic region of the procedure. Using a C-arm 0.2 T system, four interventional MR sequences were applied in 23 liver cases and during MR-guided neck interventions in 13 patients. The imaging protocol consisted of: multislice turbo spin echo (TSE) T2w, sequential-slice fast imaging with steady precession (FISP), a time-reversed version of FISP (PSIF), and FISP with balanced gradients in all spatial directions (True-FISP) sequences. Vessel conspicuity was rated and contrast-to-noise ratio (CNR) was calculated for each sequence and a differential receiver operating characteristic was performed. Liver findings were detected in 96% using the TSE sequence. PSIF, FISP, and True-FISP imaging showed lesions in 91%, 61%, and 65%, respectively. The TSE sequence offered the best CNR, followed by PSIF imaging. Differential receiver operating characteristic analysis also rated TSE and PSIF to be the superior sequences. Lesions in the head and neck were detected in all cases by TSE and FISP, in 92% using True-FISP, and in 84% using PSIF. True-FISP offered the best CNR, followed by TSE imaging. Vessels appeared bright on FISP and True-FISP imaging and dark on the other sequences. In interventional MR imaging, no single sequence fits all purposes. Image guidance for interventional MR during liver procedures is best achieved by PSIF or TSE, whereas biopsies in the head and neck are best performed using FISP or True-FISP sequences

Multislice MR first-pass myocardial perfusion imaging: impact of the receiver coil array

To compare a new 12-element body phased-array coil with a conventional four-element surface receiver coil array to provide increased signal-to-noise ratios (SNRs) for cardiac steady state free precession (SSFP) perfusion imaging. Thirteen consecutive patients were included in the study. Patients were examined both with a four-element surface coil array and a 12-element body coil array. First-pass myocardial perfusion imaging using saturation recovery SSFP was acquired during antecubital injection of Gd-DTPA. Imaging parameters: TR 2.8 msec/TE 1.3 msec, flip angle 50 degrees , bandwidth 960 Hz/pixel and half-Fourier acquisition. SNR was calculated using six regions of interest (ROI) for the myocardial perfusion scans. Calculations of corresponding ROIs using the two different coil setups were compared using analysis of variance (ANOVA). Semiquantitative perfusion parameters were calculated for both groups. The mean SNR in myocardial perfusion imaging increased by 21% using the 12-element coil setup (P < 0.001) when compared to the four-element coil. ROI comparisons revealed an increased signal inhomogeneity with the 12-element coil when compared to four-element coil experiments. Absolute normal range values of semiquantitative perfusion parameters were consistently higher using the 12-element coil setup (P < 0.001). The 12-element coil array provides higher SNR, but these improvements come with trade-offs in image homogeneity. Increased SNR translates into higher semiquantitative perfusion values and offers the potential for improved detection of perfusion defects

Coronary MR imaging: navigator echo biofeedback increases navigator efficiency--initial experience

The aim of this study was to investigate whether a respiratory biofeedback system could increase navigator efficiency and maintain image quality compared to conventional respiratory-gated magnetic resonance coronary angiography (MRCA). Eighteen healthy volunteers underwent MRCA using three different respiratory-gating protocols. A conventional expiratory free-breathing (FB) sequence was compared to two approaches using navigator echo biofeedback (NEB), a midinspiratory approach (NEBin) and an expiratory approach (NEBex). Navigator data reflecting the position of the diaphragm relative to a 3-mm gating window were made available to the subject using a video projector in combination with a Plexiglas screen and mirror goggles. Image quality was graded by two radiologists in consensus using a visual score ranging from 1 (not visible) to 4 (excellent vessel depiction). The NEB approaches improved navigator efficiency (71.1% with NEBex and 68.0% with NEBin vs 42.2% with FB), thus reducing total imaging time. This difference was statistically significant (P(NEBin)=.007; P(NEBex)=.001). Image quality in the NEBex group was comparable to that in the FB group (median score, 2.44 vs 2.52), but it proved to be significantly lower (median score, 1.94 vs 2.52) for the right coronary artery and the left anterior descending coronary artery in the NEBin group. NEB maintains image quality and significantly increases navigator efficiency, thereby decreasing total imaging time by about 40% compared to a conventional FB acquisition strategy