Scan delay

Fig. 5 Effect of varying relaxation delays between on- and off-resonance experiments in STD NMR experiments, a Experimental setnp for interleaved measnrements in STD NMR spectroscopy, n represents the nnmber of scans. The inter-scan delay Adi is varied while keeping on- and off-resonance freqnencies constant at -4 and -t300 ppm, respectively, b The resulting STD effects for the 0-methyl group of a-L-Fuc-O-methyl in the presence of RHDV VLPs. The equation that was used for non-linear least squares data fitting is based on the saturation recovery experiment [98], With Ti(iig) = 0.91 s as measured independently (unpublished results) and a Monte Carlo error estimation yields Ti(virus) = 10.06 0.41 s. This value does not directly correspond to a Tl relaxation time of the virus protons, because other factors also influence the observed relaxation [99]. According to these findings a relaxation delay Adi = 25 s was employed in all STD experiments. This results in a recovery of 92% of the virus resonance, and thereby reduces errors in epitope mapping that are introduced otherwise by non-homogeneous recovery of the binding site.

With OCT, a high power optical source is required to provide adequate illumination of the sample in a short amount of time. Also, a high speed scanning delay line is necessary in the reference arm of the interferometer in order to produce a high speed depth image acquisition. Finally, computation has to be optimized to acquire, process and display the images in real time ( 25 frames per second). 

Figure 8 Schematic of the optical system used to perform the Raman FID and echo experiments. P = Polarizer (D)BS = (dichroic) beamsplitter MD = manual delay line SD = computer-scanned delay line CSA = charge sensitive amplifier CH = chopper PH = pinhole S = sample F = bandpass and neutral density filters PD = photodiode A/D = analog-to-digital converter PC = computer PMT = photomultiplier X/2 = half-wave plate. (From Ref. 6.)...

Delays are an important part of pulse sequences and may be characterized by their duration and the effect they cause. Short delays in the micro-second region such as the pre-scan delay de or power switching delays, are not normally illustrated explicitly in pulse sequences and NMR-SIM simulations do not take these types of delay into consideration because an ideal spectrometer is assumed. Such delays may seem superfluous but they should nevertheless be considered and included sometimes in the interpretation of experimental artefacts or signal distortions. These effects are discussed in detail in sections 2.2.1 and 5.8.2. 

As an alternative to bolus tracking, a smartscan type function can be used, in which a test bolus is administered to determine the scan delay [64]. A region of interest is selected, typically in the proximal ICA, and 10 mL of contrast is injected. This region is scanned continuously using a low mAs/kVp technique, and the prep delay is chosen as the time corresponding to... 

Fig. 4.10 Limited evaluation of the vertebral artery due to streak artifact and poor choice of scan delay. In this example, there is a left C7 transverse process fracture through the foramen transversaiium. Fortunately, the ipsilateral vertebral artery, although poorly opacified, does not enter the foramen at this...

Shimizu T et al (2000) Helical CT of the liver with computer-assisted bolus-tracking technology scan delay of arterial phase scanning and effect of flow rates. J Comput Assist Tomogr 24(2) 219-223. 

Scanning on a 64-slice CT scanner requires only some minor adjustments of the scanning protocol. A continuous examination of the chest, abdomen, and pelvis may be beneficial in individual cases. When scanning from top to bottom, the scan delay for the chest should be between 50 and 55 s in order to scan the liver during the portal venous phase. However, with this delay, one will miss the phase of maximum enhancement of the pulmonary arteries. 

Schueller G, Schima W, Schueller-Weidekamm C et al (2006) Multidetector CT of the pancreas effects of contrast material flow rate and individualized scan delay on enhancement of pancreas and tumor contrast. Radiology 241 441-448... 

The optimal MR angiography datasets for virtual endoscopy are those obtained with the contrast-enhanced technique. A bolus of i.v. gadolinium (0.1-0.2 ml/kg at 2-3 ml/s) is administered to the patient, and in the same fashion as CT angiography, the acquisition starts with the proper scan delay for... 

Fig. 8.2. Effect of duration of injection on contrast accumulation and peak. Aortic and subsequently parenchymal peak attenuation accumulates with increasing duration or volume of injection. The peak occurs later requiring a modification of scan delays. Solid line 75 ml, dashed line 125 ml and dotted line 175 ml of CA volume. Modified according to Bae and Heiken (2000)...

In order to determine scan delays correctly, three factors have to be considered (1) contrast agent injection duration, (2) contrast arrival time (Carr) and (3) scan duration. 

In patients with normal cardiac output, peak arterial contrast is achieved shortly after termination of contrast agent injection (Bae et al. 1998c, 2003). A short, high-flow bolus will therefore result in an early Carr thus, a short scan delay should be selected for CTA. Conversely, a low flow, long duration bolus will result in delayed peak attenuation, and the scan start has to be delayed. Timing for such a bolus is more critical, and the first choice for CTA is therefore a compact, high-flow bolus. 

Fig. 8.4. Simulated aortic contrast attenuation curve with different scan delays (AT) designated for multiple scanner generations. AT has to be lengthened with shorter scan durations provided by a CT scanner with increasing detector width and rotational speed. A faster scanner may utilize peak contrast enhancement more efficiently than a slower one...

A volume that is routinely used for CTA of the entire aorta is sufficient for spinal artery imaging. Depending on scan time and venous access, the volume of contrast agent should be adjusted accordingly. To account for an additional scan delay after table movement, an appropriate amount of supplementary volume should be added. In the end, the technologist and the radiologist... 

The challenge for spinal artery CTA is to provide sufficient arterial enhancement but to scan before arrival of contrast medium in the venous system. An ROI of the bolus tracking system placed in the ascending aorta might be affected by inflow artifacts of the SVC and may result in a mistimed early scan. Therefore, placement of the ROI in the aortic arch or descending aorta is recommended. In the presence of aortic dissection, caution should be taken that the ROI is not too big or positioned in the false lumen or across the dissection membrane, respectively. In these cases, manual start of the scan should be considered. The Hounsfield unit threshold should be around 100 HU above baseline. Scan start is usually delayed by time for table movement (<3 s), which is usually right above the origin of the vertebral arteries. An additional scan delay of 3 s is recommended for scanners with equal to or more than 16 rows and rotation time equal or less than 0.4 s. Hounsfield unit values of attenuated blood in the thoracic aorta should never be lower than within the pulmonary trunk. 

In the helical CT era, a fixed scan delay for the pancreatic parenchymal phase of 40 s was recommended. With an acquisition time of approximately 20 s for a helical scan of the pancreas, good enhancement of the pancreas was guaranteed for most of the patients. However, with the evolution of MDCT scanners, which has considerably shorted the acquisition time of a pancreatic CT scan to not more than 3-5 s for modern scanners, a fixed scan delay may be suboptimal to catch the phase of maximum enhancement. Schueller et al. (2006) compared fixed scan delays of 40 s with an individual scan delay... 

Goshima S, Kanematsu M, Kondo H etal. (2006) Pancreas optimal scan delay for contrast-enhanced multi-detector rowCT. Radiology 241 167-174... 

The mass spectrometer was operated at a resolution of 1000 with an accelerating voltage of 2.5 kV, and an ion source temperature of 260 . Spectra were recorded with a V.G. Data System type 2040 which was set to acquire spectra repetitively throughout the elution of the chromatogram. A scan speed of 3 sec/decade with an inter-scan delay of 2 sec was employed, and data acquisition was started when the gas chromatographic column reached 190 . 

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