Condon Lau

Quantitative spectroscopic imaging for non-invasive early cancer detection.

We report a fully quantitative spectroscopy imaging instrument for wide area detection of early cancer (dysplasia). This instrument provides quantitative maps of tissue biochemistry and morphology, making it a potentially powerful surveillance tool for objective early cancer detection. We describe the design, construction, calibration, and first clinical application of this new system. We demonstrate its accuracy using physical tissue models. We validate its diagnostic ability on a resected colon adenoma, and demonstrate feasibility of in vivo imaging in the oral cavity.

In vivo retinotopic mapping of superior colliculus using manganese-enhanced magnetic resonance imaging.

The superior colliculus (SC) is a dome-shaped subcortical laminar structure in the mammalian midbrain, whose superficial layers receive visual information from the retina in a topological order. Despite the increasing number of studies investigating retinotopic projection in visual brain development and disorders, in vivo, high-resolution 3D mapping of topographic organization in the subcortical visual nuclei has not yet been available. This study explores the capability of 3D manganese-enhanced MRI (MEMRI) at 200 μm isotropic resolution for in vivo retinotopic mapping of the rat SC upon partial transection of the intraorbital optic nerve. One day after intravitreal Mn(2+) injection into both eyes, animals with partial transection at the right superior intraorbital optic nerve in Group 1 (n=8) exhibited a significantly lower T1-weighted signal intensity in the lateral region of the left SC compared to the left medial SC and right control SC. Partial transection toward the temporal or nasal region of the right intraorbital optic nerve in Group 2 (n=7) led to T1-weighted hypointensity in the rostral or caudal region of the left SC, whereas a clear border was observed separating 2 halves of the left SC in all groups. Previous histological and electrophysiological studies showed that the retinal ganglion cell axons emanating from superior, inferior, nasal and temporal retina projected respectively to the contralateral lateral, medial, caudal and rostral SC in rodents. While this topological pattern is preserved in the intraorbital optic nerve, it was shown that partial transection of the superior intraorbital optic nerve led to primary injury predominantly in the superior but not inferior retina and optic nerve. The results of this study demonstrated the sensitivity of submillimeter-resolution MEMRI for in vivo, 3D mapping of the precise retinotopic projections in SC upon reduced anterograde axonal transport of Mn(2+) ions from localized regions of the anterior visual pathways to the subcortical midbrain nuclei. Future MEMRI studies are envisioned that measure the topographic changes in brain development, diseases, plasticity and regeneration therapies in a global and longitudinal setting.

BOLD fMRI investigation of the rat auditory pathway and tonotopic organization

Rodents share general anatomical, physiological and behavioral features in the central auditory system with humans. In this study, monaural broadband noise and pure tone sounds are presented to normal rats and the resulting hemodynamic responses are measured with blood oxygenation level-dependent (BOLD) fMRI using a standard spin-echo echo planar imaging sequence (without sparse temporal sampling). The cochlear nucleus (CN), superior olivary complex, lateral lemniscus, inferior colliculus (IC), medial geniculate body and primary auditory cortex, all major auditory structures, are activated by broadband stimulation. The CN and IC BOLD signal changes increase monotonically with sound pressure level. Pure tone stimulation with three distinct frequencies (7, 20 and 40 kHz) reveals the tonotopic organization of the IC. The activated regions shift from dorsolateral to ventromedial IC with increasing frequency. These results agree with electrophysiology and immunohistochemistry findings, indicating the feasibility of auditory fMRI in rats. This is the first fMRI study of the rodent ascending auditory pathway.

High fidelity tonotopic mapping using swept source functional magnetic resonance imaging

Tonotopy, the topographic encoding of sound frequency, is the fundamental property of the auditory system. Invasive techniques lack the spatial coverage or frequency resolution to rigorously investigate tonotopy. Conventional auditory fMRI is corrupted by significant image distortion, sporadic acoustic noise and inadequate frequency resolution. We developed an efficient and high fidelity auditory fMRI method that integrates continuous frequency sweeping stimulus, distortion free MRI sequence with stable scanner noise and Fourier analysis. We demonstrated this swept source imaging (SSI) in the rat inferior colliculus and obtained tonotopic maps with ~2 kHz resolution and 40 kHz bandwidth. The results were vastly superior to those obtained by conventional fMRI mapping approach and in excellent agreement with invasive findings. We applied SSI to examine tonotopic injury following developmental noise exposure and observed that the tonotopic organization was significantly disrupted. With SSI, we also observed the subtle effects of sound pressure level on tonotopic maps, reflecting the complex neuronal responses associated with asymmetric tuning curves. This in vivo and noninvasive technique will greatly facilitate future investigation of tonotopic plasticity and disorders and auditory information processing. SSI can also be adapted to study topographic organization in other sensory systems such as retinotopy and somatotopy.

Balanced steady-state free precession fMRI with intravascular susceptibility contrast agent

One major challenge in echo planar imaging‐based functional MRI (fMRI) is the susceptibility‐induced image distortion. In this study, a new cerebral blood volume‐weighted fMRI technique using distortion‐free balanced steady‐state free precession (bSSFP) sequence was proposed and its feasibility was investigated in rat brain at 7 Tesla. After administration of intravascular susceptibility contrast agent (monocrystalline iron oxide nanoparticle [MION] at 15 mg/kg), unilateral visual stimulation was presented using a block‐design paradigm. With repetition time/echo time = 3.8/1.9 ms and α = 18°, bSSFP fMRI was performed and compared with the conventional cerebral blood volume‐weighted fMRI using post‐MION gradient echo and spin echo echo planar imaging. The results showed that post‐MION bSSFP fMRI provides comparable sensitivity but with no severe image distortion and signal dropout. Robust negative responses were observed during stimulation and activation patterns were in excellent agreement with known neuroanatomy. Furthermore, the post‐MION bSSFP signal was observed to decrease significantly during hypercapnia challenge, indicating its sensitivity to cerebral blood volume changes. These findings demonstrated that post‐MION bSSFP fMRI is a promising alternative to conventional cerebral blood volume‐weighted fMRI. This technique is particularly suited for fMRI investigation of animal models at high field. Magn Reson Med, 2012.

Re-evaluation of model-based light-scattering spectroscopy for tissue spectroscopy

Model-based light scattering spectroscopy (LSS) seemed a promising technique for in-vivo diagnosis of dysplasia in multiple organs. In the studies, the residual spectrum, the difference between the observed and modeled diffuse reflectance spectra, was attributed to single elastic light scattering from epithelial nuclei, and diagnostic information due to nuclear changes was extracted from it. We show that this picture is incorrect. The actual single scattering signal arising from epithelial nuclei is much smaller than the previously computed residual spectrum, and does not have the wavelength dependence characteristic of Mie scattering. Rather, the residual spectrum largely arises from assuming a uniform hemoglobin distribution. In fact, hemoglobin is packaged in blood vessels, which alters the reflectance. When we include vessel packaging, which accounts for an inhomogeneous hemoglobin distribution, in the diffuse reflectance model, the reflectance is modeled more accurately, greatly reducing the amplitude of the residual spectrum. These findings are verified via numerical estimates based on light propagation and Mie theory, tissue phantom experiments, and analysis of published data measured from Barretts esophagus. In future studies, vessel packaging should be included in the model of diffuse reflectance and use of model-based LSS should be discontinued.

Assessing epithelial cell nuclear morphology by using azimuthal light scattering spectroscopy

We describe azimuthal light scattering spectroscopy (phi/LSS), a novel technique for assessing epithelial-cell nuclear morphology. The difference between the spectra measured at azimuthal angles phi = 0 degrees and phi = 90 degrees preferentially isolates the single backscattering contribution due to large (approximately 10 microm) structures such as epithelial cell nuclei by discriminating against scattering from smaller organelles and diffusive background. We demonstrate the feasibility of using phi/LSS for cancer detection by showing that spectra from cancerous colon tissue exhibit significantly greater azimuthal asymmetry than spectra from normal colonic tissues.

BOLD Temporal Dynamics of Rat Superior Colliculus and Lateral Geniculate Nucleus following Short Duration Visual Stimulation

Background The superior colliculus (SC) and lateral geniculate nucleus (LGN) are important subcortical structures for vision. Much of our understanding of vision was obtained using invasive and small field of view (FOV) techniques. In this study, we use non-invasive, large FOV blood oxygenation level-dependent (BOLD) fMRI to measure the SC and LGNs response temporal dynamics following short duration (1 s) visual stimulation. Methodology/Principal Findings Experiments are performed at 7 tesla on Sprague Dawley rats stimulated in one eye with flashing light. Gradient-echo and spin-echo sequences are used to provide complementary information. An anatomical image is acquired from one rat after injection of monocrystalline iron oxide nanoparticles (MION), a blood vessel contrast agent. BOLD responses are concentrated in the contralateral SC and LGN. The SC BOLD signal measured with gradient-echo rises to 50% of maximum amplitude (PEAK) 0.2±0.2 s before the LGN signal (p<0.05). The LGN signal returns to 50% of PEAK 1.4±1.2 s before the SC signal (p<0.05). These results indicate the SC signal rises faster than the LGN signal but settles slower. Spin-echo results support these findings. The post-MION image shows the SC and LGN lie beneath large blood vessels. This subcortical vasculature is similar to that in the cortex, which also lies beneath large vessels. The LGN lies closer to the large vessels than much of the SC. Conclusions/Significance The differences in response timing between SC and LGN are very similar to those between deep and shallow cortical layers following electrical stimulation, which are related to depth-dependent blood vessel dilation rates. This combined with the similarities in vasculature between subcortex and cortex suggest the SC and LGN timing differences are also related to depth-dependent dilation rates. This study shows for the first time that BOLD responses in the rat SC and LGN following short duration visual stimulation are temporally different.

Effect of anatomy on spectroscopic detection of cervical dysplasia

It has long been speculated that underlying variations in tissue anatomy affect in vivo spectroscopic measurements. We investigate the effects of cervical anatomy on reflectance and fluorescence spectroscopy to guide the development of a diagnostic algorithm for identifying high-grade squamous intraepithelial lesions (HSILs) free of the confounding effects of anatomy. We use spectroscopy in both contact probe and imaging modes to study patients undergoing either colposcopy or treatment for HSIL. Physical models of light propagation in tissue are used to extract parameters related to tissue morphology and biochemistry. Our results show that the transformation zone, the area in which the vast majority of HSILs are found, is spectroscopically distinct from the adjacent squamous mucosa, and that these anatomical differences can directly influence spectroscopic diagnostic parameters. Specifically, we demonstrate that performance of diagnostic algorithms for identifying HSILs is artificially enhanced when clinically normal squamous sites are included in the statistical analysis of the spectroscopic data. We conclude that underlying differences in tissue anatomy can have a confounding effect on diagnostic spectroscopic parameters and that the common practice of including clinically normal squamous sites in cervical spectroscopy results in artificially improved performance in distinguishing HSILs from clinically suspicious non-HSILs.

Functional magnetic resonance imaging of sound pressure level encoding in the rat central auditory system

Intensity is an important physical property of a sound wave and is customarily reported as sound pressure level (SPL). Invasive techniques such as electrical recordings, which typically examine one brain region at a time, have been used to study neuronal encoding of SPL throughout the central auditory system. Non-invasive functional magnetic resonance imaging (fMRI) with large field of view can simultaneously examine multiple auditory structures. We applied fMRI to measure the hemodynamic responses in the rat brain during sound stimulation at seven SPLs over a 72 dB range. This study used a sparse temporal sampling paradigm to reduce the adverse effects of scanner noise. Hemodynamic responses were measured from the central nucleus of the inferior colliculus (CIC), external cortex of the inferior colliculus (ECIC), lateral lemniscus (LL), medial geniculate body (MGB), and auditory cortex (AC). BOLD signal changes generally increase significantly (p<0.001) with SPL and the dependence is monotonic in CIC, ECIC, and LL. The ECIC has higher BOLD signal change than CIC and LL at high SPLs. The difference between BOLD signal changes at high and low SPLs is less in the MGB and AC. This suggests that the SPL dependences of the LL and IC are different from those in the MGB and AC and the SPL dependence of the CIC is different from that of the ECIC. These observations are likely related to earlier observations that neurons with firing rates that increase monotonically with SPL are dominant in the CIC, ECIC, and LL while non-monotonic neurons are dominant in the MGB and AC. Further, the ICs SPL dependence measured in this study is very similar to that measured in our earlier study using the continuous imaging method. Therefore, sparse temporal sampling may not be a prerequisite in auditory fMRI studies of the IC.