Animal model of metastatic pheochromocytoma: evaluation by MRI and PET

OBJECTIVE

The development of metastatic pheochromocytoma animal model provides a unique opportunity to study the physiology of these rare tumors and to evaluate experimental treatments. Here, we describe the use of small animal imaging techniques to detect, localize and characterize metastatic lesions in nude mice.

METHODS

Small animal positron emission tomography (PET) imaging and magnetic resonance imaging (MRI) were used to detect metastatic lesions in nude mice following intravenous injection of mouse pheochromocytoma cells. [18F]-6-fluoro-dopamine ([18F]-DA) and [18F]-L-6-fluoro-3,4-dihydroxyphenylalanine, which are commonly used for localization of pheochromocytoma lesions in clinical practice, were selected as radiotracers to monitor metastatic lesions by PET.

RESULTS

MRI was able to detect liver lesions as small as 0.5mm in diameter. Small animal PET imaging using [18F]-DA and [18F]-DOPA detected liver, adrenal gland, and ovarian lesions.

CONCLUSION

We conclude that MRI is a valuable technique for tumor growth monitoring from very early to late stages of tumor progression and that animal PET confirmed localization of metastatic pheochromocytoma in liver with both radiotracers.

Does vertical transmission contribute to the prevalence of toxoplasmosis?

Toxoplasma gondii is a ubiquitous parasite with a widespread distribution both in terms of geographical and host range. Although the definitive host is the cat, it is also a major health hazard to domestic animals and humans. Three routes of transmission are recognised (infection from the cat, carnivory and congenital transmission). We aimed to assess the relative importance of congenital transmission, using sheep as a model system, due to the lack of carnivory. We report, using PCR as a diagnostic tool, that congenital transmission occurs with high frequency (69%). If transmission from oocysts was important in sheep, we would expect sheep reared under the same environmental conditions (i.e. a single farm) to have a random distribution of Toxoplasma infection. Using breeding records in conjunction with PCR, some families were found to have high Toxoplasma prevalence and abortion while others were free of Toxoplasma infection and abortion (P < 0.01). This supports the notion that Toxoplasma may be transmitted vertically. In humans, we conducted a similar study and showed that Toxoplasma was transmitted from mother to baby in 19.8% of cases. Vertical transmission in Toxoplasma may be more important than previously thought and this knowledge should be considered in any eradication strategies.

MR hysterosalpingography: protocol development and refinement for simulating normal and abnormal fallopian tube patency--feasibility study with a phantom

PURPOSE

To develop and refine a pulse sequence and protocol for testing the feasibility of magnetic resonance (MR) hysterosalpingography in a phantom model.

MATERIALS AND METHODS

A phantom simulating the uterus, fallopian tubes, and surrounding pelvic cavity was constructed. T2-weighted acquisition strategies-breath-hold fast spin-echo, rapid acquisition with relaxation enhancement (RARE), and haff-Fourier RARE-were refined to acquire sequential 70-mm coronal imaging volumes. Contrast agent was injected into the introducing catheter entering the os of the simulated uterus. Interacquisition interval, type of contrast agent (eg, sterile saline solution or water), and quantity of contrast agent (eg, 1-5 mL per acquisition) were varied. Digital image subtraction was used to enhance image quality. Images were qualitatively analyzed and rated good, fair, or poor for temporal resolution, spatial resolution, fallopian tube conspicuity, and free spill conspicuity. Once the technique was refine, the phantom was reconfigured to simulate unilateral and bilateral hydrosalpinx.

RESULTS

The RARE sequence with an 8-second interacquisition interval and a 5-mL interacquisition of sterile water produced good images of the simulated fallopian tubes and free spill. Depiction of unilateral and bilateral hydrosalpinx was also reliably demonstrated.

CONCLUSION

This study with a phantom model demonstrates the feasibility of MR hysterosalpingography to depict normal and diseased fallopian tubes.

Abdominal MR imaging with a volumetric interpolated breath-hold examination

PURPOSE

To compare a T1-weighted, three-dimensional (3D), gradient-echo (GRE) sequence for magnetic resonance (MR) imaging of the body (volumetric interpolated breath-hold examination, or VIBE) with a two-dimensional (2D) GRE breath-hold equivalent.

MATERIALS AND METHODS

Twenty consecutive patients underwent 1.5-T MR imaging. The examinations included pre- and postcontrast (20 mL gadopentetate dimeglumine) fat-saturated 2D GRE breath-hold imaging and fat-saturated volumetric interpolated breath-hold imaging before, during (arterial phase), and after injection, with thin (2-mm source images) and thick (8-mm reconstruction images) sections. The three images were compared qualitatively and quantitatively (signal-to-noise ratio [SNR] and contrast-to-noise ratio [CNR]).

RESULTS

Qualitatively, the 2-mm source images had poorer pancreatic edge definition on precontrast images compared with the other two data sets (P < .05). On gadolinium-enhanced images, scores for clarity of pancreatic edge, number of vessels visualized, and arterial ghosting were significantly lower for the postcontrast 2D GRE images. Quantitatively, SNR measurements in the liver, aorta, and renal cortex on pre- and postcontrast images were significantly higher for the 8-mm reconstruction images than for the 2D GRE or 2-mm source images (P < .05). Aorta-to-fat CNR was significantly higher on the 8-mm reconstruction images.

CONCLUSION

Fat-saturated volumetric interpolated breath-hold images have quality comparable to that of conventional fat-saturated 2D GRE images.

T2-weighted MRI of the uterus: fast spin echo vs. breath-hold fast spin echo

This study compared one routine T2-weighted fast spin echo (T2FSE) sequence with a breath-hold T2FSE (BH T2FSE) sequence of the female pelvis for image quality, uterine anatomy, lesion detection, and signal intensity measurements. Thirty-two consecutive women (mean age 41.7 years) were imaged at 1.5 T with one high-resolution routine T2FSE sequence and one BH T2FSE sequence in the sagittal plane as part of comprehensive pelvic magnetic resonance imaging. The different image sets were rated separately for imaging characteristics (overall image quality, uterine anatomy definition, lesion detection, and free fluid conspicuity) and then compared side by side. The image sets were also compared for artifacts (ghosting, blurring, pulsatility, and chemical shift misregistration). Signal-to-noise (S/N) and signal difference-to-noise (SD/N) ratios were calculated for the different uterine zones, uterine abnormalities, free fluid, rectus abdominis muscle, and bladder. Contrast-to-noise ratios (CNRs) were calculated for uterine abnormalities. Twenty-eight uterine abnormalities were detected in 20 patients and included leiomyomata (13 patients), adenomyosis (7 patients), benign endometrial polyps (6 patients), endometrial carcinoma (1 patient), and pregnancy (1 patient). BH T2FSE was superior or equivalent to T2FSE for overall image quality in 23/32 patients (71.8%), uterine anatomy definition in 19/32 patients (59.3%), and lesion detection in 13/20 patients (65%). BH T2FSE performed less well than T2FSE for free fluid conspicuity in 5/5 (100%) patients. BH T2FSE was equivalent to or less affected than T2FSE for ghosting artifact in 24/32 patients (75%) and blurring artifact in 29/32 patients (90.6%). Pulsatility and chemical shift artifacts were not problematic for either image set. S/N and SD/N were higher for all BH T2FSE determinations compared with T2FSE. For the endometrium, junctional zone, myometrium, and bladder, these differences were statistically significant. There were no statistically significant differences for CNR between the two image sets, although BH T2FSE values for leiomyomata, adenomyosis, and abnormal endometria were higher than those calculated for T2FSE. All pathology detected with T2FSE was detected on BH T2FSE despite the breath-hold sequence's inherently poorer spatial resolution compared with the non-breath-hold sequence. BH T2FSE may be able to replace T2FSE for some uterine applications with a substantial time savings.

HASTE MR imaging: description of technique and preliminary results in the abdomen

HASTE (Half fourier Single-shot Turbo spin-Echo) is a single-section T2-weighted sequence that acquires images in less than 1 second. Images are breathing independent and possess a variety of other features useful for imaging the abdomen. The design of this technique is described. Clinical studies of 38 consecutive patients were performed using this technique. HASTE images were considered good in 28 and fair in 10 patients, including five patients who could not suspend respiration. Definition of liver and bowel was particularly clear.

Phase-modulated binomial RF pulses for fast spectrally-selective musculoskeletal imaging

Two fat-suppressed three-dimensional gradient echo pulse sequences, FLASH and DESS (Double Echo in Steady-State), that have significantly reduced scan time compared with conventional chemical shift fat-suppression sequences are presented. This fat-suppression technique is based on selectively exciting water spins using a time-optimized binomial RF pulse pair at the water resonance frequency with a null in the excitation profile at the fat frequency. To minimize the total pulse length, the delay between the binomial components of the RF pulse is decreased from a standard jump-return implementation. To maintain the proper null frequency, the phase of the second RF pulse is shifted such that it returns the fat resonance back to its initial z position while further tipping the water spins. Nonselective phase-modulated RF pulse pairs can be implemented in 1.20 ms at 1.0 T, significantly reducing the minimum TR.

Optimization of a dual echo in the steady state (DESS) free-precession sequence for imaging cartilage

Three-dimensional (3D) MR imaging of the knee is useful to detect cartilage abnormalities, although the tissue contrast in 3D gradient-recalled echo (GRE) sequences such as gradient-recalled acquisition in the steady state (GRASS) or fast low-angle shot (FLASH) is poor. T2 contrast can be added to a GRASS sequence by combining the signals from the first and second gradient echoes, which form immediately after and immediately before each radio frequency (RF) pulse in a 3D GRE sequence. We have optimized a 3D dual echo in the steady state (DESS) sequence, which produces one averaged image from the two echoes, for use in the detection of articular cartilage abnormalities. In the optimization process, we examined the imaging parameters of flip angle (alpha), repetition time (TR), echo time (TE), and bandwidth to maximize the contrast between cartilage and joint fluid. A theoretical simulation of the sequence was confirmed with experiments conducted on phantoms with known T1 and T2. On the basis of theoretical predictions and experiments using healthy volunteers, we determined that an optimized sequence with a bandwidth of 98 Hz per pixel, TR of 30 msec, a TE of 7.1 msec, and an alpha of 60 degrees produced the highest contrast between cartilage and fluid within a defined acquisition time of 6 minutes. Additional contrast was obtained by filtering the second-echo image to eliminate noise before adding it to the first-echo image.

Quantitation of the susceptibility difference between trabecular bone and bone marrow: experimental studies

In this study we quantify the effects of different relaxation mechanisms on the signal intensity in gradient-echo images of tissue such as bone marrow in the presence of trabecular bone. The susceptibility difference between trabecular bone and soft tissue produces distortions in the magnetic lines of force which induce strong inhomogeneities in the static magnetic field. Diffusion of tissue protons in such magnetic field gradients produce a shortening of the transverse relaxation time T2, while the dephasing of the transverse magnetization due to susceptibility differences produces a shortening of the apparent relaxation time T2* as demonstrated in gradient-echo images. We have used specimens of dried human vertebrae with different bone densities immersed in either saline to simulate tissue water or an emulsion of oil and water to simulate bone marrow to quantify these relaxation mechanisms in vitro. We have measured the MR relaxation times T1, T2, and T2* of protons within the trabecular spaces and correlated their variations with trabecular bone density. We have found that in vitro, at 1.5 T, the relaxation times T1 and T2 do not show significant variations with bone density and there are no significant contributions to the transverse relaxation rate due to the diffusion of tissue water in the magnetic field gradients. However, the relaxation rate, 1/T2*, of saline in the presence of trabecular bone increases at a rate of 0.2 s-1/mg/cc due to the dephasing of the transverse magnetization in the magnetic field inhomogeneities. Similar bone density-related T2* variations were observed for fat protons within the trabeculae where the chemical-shift-induced modulations of signal intensity in an oil-water emulsion have been separated from the susceptibility-induced relaxation effects. In addition, we have verified these effects in vivo and quantified in vivo variations in fat and water relaxation rates of bone marrow in the epiphysis and diaphysis in the appendicular skeleton of normal volunteers and found that both fat and water T2* are shorter in the epiphysis compared to the diaphysis, which correlates well with previous observations.

Renal disease profoundly alters cortical interstitial cell function

Interstitial cells were cultured from explants of the unilaterally hydronephrotic, contralateral, and normal kidneys. Two types of cells were identified in culture, macrophages, and cells which were tentatively identified as fibroblasts. Cells grew at a significantly faster rate in hydronephrotic compared to contralateral or normal kidneys. Cells from the hydronephrotic kidney increased prostaglandin (PG)E2 production in response to bradykinin. Cells from contralateral and normal renal cortex did not increase PGE2 production in response to bradykinin. These results indicate hydronephrosis induces functional changes in interstitial cells cultured from the cortex of hydronephrotic compared to contralateral and normal kidneys. The induction of increased PGE2 synthesis and bradykinin responsiveness in hydronephrotic cortex could be related to the exaggerated prostaglandin synthesis known to occur in hydronephrotic cortex. In hydronephrosis, cortical interstitial cells elaborate increased amounts of substances such as prostaglandins which have the capacity to modulate important parameters of renal function.

Error and corrections with scintigraphic measurement of gastric emptying of solid foods

Previous methods for correction of depth used geometric means of simultaneously obtained anterior and posterior counts. The present study compares this method with a new one that uses computations of depth based on peak-to-scatter (P:S) ratios. Six normal volunteers were fed a meal of beef stew, water, and chicken liver that had been labeled in vivo with both In-113m and Tc-99m. Gastric emptying was followed at short intervals with anterior counts of peak and scattered radiation for each nuclide, as well as posteriorly collected peak counts from the gastric ROI. Depth of the nuclides was estimated by the P:S method as well as the older method. Both gave similar results. Errors from septal penetration or scatter proved to be a significantly larger problem than errors from changes in depth.

Validation of corrections for errors in collimation during measurement of gastric emptying of nuclide-labeled meals

The study was undertaken to validate phantom-derived corrections for errors in collimation due to septal penetration or scatter, which vary with the size of the gastric region of interest (ROI). Six volunteers received 495 ml of 20% glucose labeled with both In-113m DTPA and Tc-99m DTPA. Gastric emptying of each nuclide was monitored by gamma camera as well as by periodic removal and reinstillation of the meal through a gastric tube. Serial aspirates from the gastric tube confirmed parallel emptying of In-113m and Tc-99m, but analyses of gamma-camera data yielded parallel emptying only when adequate corrections were made for errors in collimation. Analyses of ratios of gastric counts from anterior to posterior, as well as analyses of peak-to-scatter ratios, revealed only small, insignificant anteroposterior movement of the tracers within the stomach during emptying. Accordingly, there was no significant improvement in the camera data when corrections were made for attenuation with intragastric depth.