Effects of Administration Routes on MicroCT Imaging Characteristics
of a Long-Acting Vascular Contrast Agent in a Murine Model
GN Ton1, WC Dow2, DA Bakan2, JP Weichert1
1University of Wisconsin-Madison,WI and 2Alerion Biomedical Inc., San Diego, CA

Results and Discussion

ITG

Fig 3. VC-enhanced microCT vascular images of C57Bl/6 mice 2 hours
following a single injection of Fenestra VC (15 mL/kg) via various routes.

Fig 1. Structure of BP and ITG Particles

ROIV

H5m2h24h

IV inj.
LGBSRO inj.

LL

SPS

400300300200200

B100203040

Intravenous injection
Intraperitoneal injection
Retroorbital injection

050

01

2

Post-Injection Time (Hours)

345

6

Post-Injection Time (Hours)

Time-intensity profiles exhibited a rapid uptake of Fenestra VC by the liver
cells, resulting in the removal of the majority of the blood-pool agent from thecirculation after 24 hours following administration via all three routes (IV, IP andRO) as shown in Fig 5A. Liver intensities increased slightly after 3 hours as initialelimination of the contrast agent began.

HVL

10B

100

Intravenous injection
Intraperitoneal injection
Retroorbital injection

BPCoronal500

4000PVFig 6. VC-enhanced microCT vascular images of C57Bl/6 mice following
an IP injection of Fenestra VC (15 mL/kg) .

Mesenteric
Vasculature (MV)BPBaseline

2h6h

24h

L
SP

Axial, coronal and sagittal images of C57Bl/6 mice obtained 2 hours afteradministration of Fenestra VC (15 mL/kg) via IV, IP, and RO routes are shown in Fig
3. The images clearly demonstrated the blood-pool agent continues to produce
excellent vascular contrast enhancement 2 hours after injections via IV and ROroutes. For the IP route, vascular signal intensity became peaked after 2 hours,
although a relatively small portion of the injected dose was still visible in theintraperitoneal space. This administration route is not desirable for identifying bloodvessels in the abdominal area owing to the present of the contrast agent in theintraperitoneal space.

AGI tract

Following an RO injection, the blood-pool agent diffuses into the circulation
over a period of 20-30 minutes, producing excellent vascular contrast enhancement(Fig 2.). Coronal images of female mice acquired 5 minutes after RO and IV
injections of Fenestra VC depicted a comparable level of vascular contrast
intensitywas achieved. Hepatobiliary elimination of the agent is evident after 24hours as indicated by increased intensity in the gall bladder, liver, and intestines.

B

Materials and Methods

500

500

Intravenous injection
Intraperitoneal injection
Retroorbital injection

B

400

400

B

300

300

200

200

100

100000MV

LS

CoronalSBPSSGB
GI tract

BlFig 4. Normalized time-signal intensity profiles of Fenestra VC (15 mL/kg)
in the IVC following administrations via iv, ip and retro-orbital routes.
Normalized Signal Intentsity (HU/Voxel )Fig 2. VC-enhanced microCT vascular images of C57Bl/6 mice following
RO or IV injections of Fenestra VC (15 mL/kg) at various timepoints.
24hA

Axial view

The blood-pool contrast agent (Fenestra VC) was obtained from ART
Advanced Research Technologies (Montreal, Canada). The finale iodine
concentration and mean particle size of this lipid emulsion were 50 mg I/mL and 180
nm, respectively. Mice were placed on soft diet at least 1 day prior to the initiation of
the study for minimizing streak artifacts produced by rodent chow. Anesthetized
female C57Bl/6 mice (2-3 per group) were given a single 15 mL/kg dose of the BP
via three different routes ( i.v., i.p. and retro-orbital injections. The mice were
scanned using a GE eXplore Locus microCT system (80 kVp, 450 µA, 200
msec/view, 5 frames/view, 400 views/scan and 93 µm3 resolution) prior to and at
predetermined time intervals following administration .
Images were reconstructed with the EVSBeam software (GE Medical System)and subsequently displayed and analyzed using both Amira 3-D visualization
software (V3.1) and MicroView program. CT values of volumetric ROIs in theinferior vena cava (IVC) and liver were normalized to Hounsfield Units (HU).
Normalized signal intensity (HU/voxel) was obtained from the difference ofenhanced SI and the baseline value.

2h500

0IVC

L

Sagittal

Materialsand
andMethods
Methods
Materials

5mIP

Axial view

Fenestra VC is typicallyadministered to mice throughthe tail vein. However, being
highly proficient at giving tailvein injections can be quite
challenging. To alleviate thismatter, we set out to evaluate
the in vivo imaging efficacyof this long-acting vascular
contrast agent following
administrations through retroorbital (RO) sinus andintraperitoneal (IP) space asalternative injection routes .

BP

Fig 5. Normalized time-signal intensity profiles of Fenestra VC (15 mL/kg)
in the liver following administrations via IV, IP, and RO injections.
Normalized Signal Intentsity (HU/Voxel )Introduction
Introduction
High-resolution microCT systems capable of sub 20-micron isotropic spacial
resolution provide an attractive new approach for noninvasively studying models ofhuman diseases in live small animals.1,2 However, relatively long acquisition times
preclude the use of conventional water-soluble contrast media for imaging
pathological conditions associated with vascular abnormalities. Utilizing the ability of
PEG chains to modify surfaces of biological macromolecules, we have developed along-acting blood-pool contrast agent (BP) based on our hepatocyte-selective CT
contrast agent platform for vascular microCT imaging applications.3,4Macromolecular structures of BP and ITG particles are shown in Fig 1. Incorporationof PEG moieties into the phospholipid monolayer shell of the ITG vehicle interfereswith the association of Apo-E, which is believed to be essential for hepatocyte
recognition. Delaying the sequestration of ITG by liver cells results in prolonged
vascular residence time. The vascular contrast agent is now known as FenestraTM VC.

102030Post-Injection Time (Hours)

4050

Diffusion of Fenestra VC into the blood following an IP injection of 15 mL/kg
dose in C57Bl/6 mice took 4-6 hours, indicated by the present of the agent in theintraperitoneal cavity (Fig 6). Excellent vascular contrast enhancement and soft
tissue identification were observed at 6 hours post injection. Enhancement of thegall bladder and gastrointestinal tract was noticed after 24 hours, indicating fecal
elimination of the agent.

Conclusions
Intravenous, intraperitoneal, and retroorbital injections of Fenestra VC produced acomparable level of vascular contrast enhancement. Therefore, any of these administrationroutes can be used which may be advantagous to investigators without technical expertise intail vein injection.

Intravenous injectionIntraperitoneal injection
Retroorbital injection

0123456

Post-Injection Time (Hours)

Effects of administration routes on the vascular kinetics profiles of Fenestra
VC in female C57Bl/6 mice are shown in Fig 4. Vascular intensities obtained
following an IV or RO injection were insignificantly different. A 2-hour delay in thetime to achieve maximal vascular CT values was observed following IP
administration (Fig 4B). Regardless which route the blood pool contrast agent wasadministered, more than 75% of the injected dose was retained in the blood stream
after 6 hours, producing prolonged vascular enhancement contrast enhancement.

References[1] Paulus MJ, et. al, Neoplasisa (2000) 2:62-70.
[2] Paulus MJ, et. al, Lab Animals (2001) 30:1-10.
[3] Weichert JP, et.al, Academic Radiol. (1998) 5:S16-19.
[4] Weichert JP, et.al, Radiology, (2000) 216:865-871.

Supported in part by the UWCCC, UW-Radiology and Alerion Biomedical Inc.
gn.ton@hosp.wisc.edu, Poster #244