Technical Help for AuroVist™

Updated: March 13, 2007 On this page What are the best instrument and beam settings to use? Toxicity is worse than I expected Other Resources Nanoprobes e-Newsletter Try our e-mail newsletter for tips and tricks for better gold labeling, and to keep up with new applications. Guide to Gold Cluster Labeling Detailed description of gold cluster labeling, tips and tricks for successful conjugations, isolation of conjugates, and how to calculate labeling.

AuroVist™ is a new product, and therefore it has not been tested in all possible applications. We encourage you to tell us how well it works in your application, or if you encounter problems, let us know - that way, you can help contribute to the knowledge base on this reagent and its applications.

What are the best instrument and beam settings to use?

The absorption increases by a significant factor (jump ratio) above the gold L and K edges. The X-ray properties of gold, showing the jump ratios for these regions, are shown in the table below:

	Energy (keV)	µ/p(cm2/g)	jump ratio
	11.8	75.8
L3	11.9	187.0	2.5
	13.6	128.3
L2	13.7	176.4	1.4
	14.3	158.8
L1	14.4	183.0	1.2
	80.6	2.1
K	80.7	8.9	4.2

It is therefore advantageous to image using these absorptions, and the settings below are appropriate for the different instruments:

• Mammography: These instruments are suitable for small animal imaging. Use of lower kVp (e.g., 22 kVp) is recommended to take advantage of the L edge Au absorptions. Exposures are typically 1 second or less for a mouse, so live imaging is possible. Resolution can be < 0.1 mm.

• Clinical CT: 80 kVp gives the greatest attenuation, but higher voltages, particularly with filtering can make use of the Au K edge; beam energy can be tuned to just above gold's 80.7-keV K-edge. Imaging time is typically a few seconds, with resolution ~0.3 mm.

• MicroCT: Here the resolution is increased (to even 2 microns), but the tube power is typically ~100 times less than a clinical unit. Fine area 2D detectors mean that many tiny pixels must each receive enough counts. This then requires a much longer imaging time (e.g., 0.5 - 2 hours) than clinical CT. Many units also slow the tube rotation down such that only 1 revolution is done in the selected imaging time (e.g., 1 hour). If the animal moves during collection of this data set, the back projection 3D reconstruction will be errant. This places significant constraints for live animal imaging, and motion must be minimized, such as breathing and heartbeat (mouse = 600 beats/min). A simple solution is to kill the animal some time after injection and then image, but live imaging has been accomplished if the region can be gated or immobilized during the imaging time. Beam energy should be just above gold’s 12-14-keV L-edge.

Toxicity is worse than I expected

Certain strains of mice appear to be more tolerant of this gold. For Balb/C, the LD50 is ~ 3.2 g Au/kg. Nude mice and C3H mice also seem to respond about the same. Some outbred mice, however, appear to have a lower LD50 of about > 1.4 g Au/kg. If you are using outbred mice, or a different strain to those mentioned above, it is recommended that you use this lower value. The following suggestions may be helpful:

• Start with a moderate dose, such as 120 or 160 mg/mL. The value of 270 mg/mL reported in our paper in the British Journal of Radiology was the highest value tested; at high concentrations acute toxicities increase rapidly, and a modest reduction in dose can significantly reduce toxicity without compromising your results.

• Centrifuge and then filter the reconstituted reagent through a sterile filter immediately before injection to ensure that no aggregates or larger particles are present. Larger particles and aggregates can impact biocompatibility, and may also have reduced stability and hence a higher tendency to deposit in tissues and organs with negative consequences.

• Use one of the pure-bred mouse strains mentioned above which are known to have higher tolerance. If you are planning to conduct multiple experiments using a different strain, it may be worthwhile to test your strain first.

Any tips for other users? Things you'd like to see us make? Or, if you want to tell us your question or problem, E-mail us at tech@nanoprobes.com.