Magnetinc Resonance Image Modolaities

Published Date: 02 Nov 2017

Iman Eizadynejad

DIFUSSE OPTICAL TOMOGRAGHY AND MAGNETINC RESONANCE IMAGE MODOLAITIES

Advance Medical

Imaging

Submitted to DR. Nasser H. Kashou

DIFUSSE OPTICAL TOMOGRAGHY AND MAGNETINC RESONANCE IMAGE MODOLAITIES

ABSTRACT

In this article, we discuss about diffuse optical tomography (DOT) and magnetic resonance imaging (MRI) as two image modalities in biomedical field. The foundation of MRI, as a technology that used for study of disease, started in 1971, and DOT, as new technology in biomedical field, began during the year of 1991. DOT is an emerging modality that uses Near Infrared (NIR) light to reveal structural and functional information of deep biological tissue. If other imaging modalities are not available, DOT uses in diagnosis and monitoring, brain image, and small animal imaging to provide contrast mechanism for chemical, molecular, and anatomical imaging. DOT offers the potential to perform non-ionizing radiation and non-invasive three-dimensional reconstruction of optical properties of tissue given the measurements and forward model of photon propagation and the technology is fast and simple.

On the other hand, Magnetic Resonance Imaging, or MRI, stems from the application of nuclear magnetic resonance (NMR) to radiological imaging. The adjective 'magnetic' refers to the use of a range (usually 1.5 Tesla) of magnetic fields and 'resonance' refers to the need to match the radio frequency (RF) of an oscillating magnetic field to the 'precessional' frequency of the spins of some atomic nucleus in a tissue molecule. The principle advantage of MRI is its excellent contrast resolution, high soft tissue contrast, and non-invasive technology.

DIFFUSE OPTICAL TOMOGRAPHY

There are three basic types to optical tomography since the early 1980s: diffuse optical tomography (DOT), diffraction tomography, and optical coherence tomography. Optical methods are of particular importance in the medical field, because these techniques able to be safe and cheap and, in addition, offer a therapeutic potential. Tomographic techniques produce slice images of three-dimensional objects. Nowadays, the main applications of DOT are limb, breast, joint, and brain imaging.

The concept of DOT emerged in the early 1990s which is a biomedical modality that is based on near-infrared (NIR) optical technologies. In this technology, DOT uses NIR light to illuminate tissue and reconstruct optical scattering and absorption maps of the specific tissue being imaged by using trans-illuminated light collected at the surface of the tissue that provides 3-dimensional images of living tissue in bodies. These properties can be related to the concentration of tissue chromophores, namely oxy-hemoglobin, deoxy-hemoglobin, fat, and water. These concentrations, in turn can be related to tissue metabolism, making the information obtained through DOT functional information. By looking to infrared spectral region, light is extremely absorbed by hemoglobin and water within the tissues. In the region, the wavelength from 700 nm to 1000 nm, the blood and water absorb comparatively little light allowing this region to oblige as an "optical window" into the tissue.

Fig 1. Showing optical window

Their relative concentrations have previously been linked to cancer malignancy. For example, breast cancer has been one of the leading causes of dead for women in the United State. Therefore, a non-invasive DOT detector can be helpful to indicate the breast cancer. Optical contrast between unhealthy and normal tissues is high, DOT offers the ability to quantitatively image the high optical contrast that arises basically from molecular and cellular signal produced through the existing of blood, water, lipid, as well as cellular density along with phase contrast, which are the main transformations associated with malignancy.

DOT offers the capability to simultaneously quantify the tissue concentration of both oxy- (HbO) and deoxy-hemoglobin (Hb). In this technology, two or more near-infrared sources, with wavelengths specifically chosen to include the isosbestic point (related HbO and Hb have identical extinction coefficients) of the oxy/deoxy-hemoglobin absorption spectrum, illuminate the tissue at various locations.

For example, high overall hemoglobin concentration, with low oxy-hemoglobin concentration, has been positively correlated with cancer malignancy. As a result, the flux distribution at the tissue surface contains both spectral and spatial information about the subsurface of absorbers.

The basic function of DOT is measured photons absorbing and scattering in the tissue by estimating the concentration of different chromphores. The detectors calculate the measurement data by observing different absorbing and scattering wavelength.

Fig 2. Picture of absorbing and scattering wavelength in DOT

Base on the type of near-infrared laser source being used, DOT can use in three different types: time domain, frequency domain, and continuous-wave. The time and frequency domain methods are based on launching sinusoidal intensity modulated light and detecting the phase shift and amplitude of the reemitted light or are based on launching an incident impulse of light and detecting the broadened reemitted pulse.

Most of conventional time domain techniques utilize short pulse lasers for tissue illumination and time-correlated single photon counting devices for diffuse photon detection. The light is guided through optical fibers to various positions on the tissue. Pulses of light are emitted at frequency rate of 1-50 Megahertz with duration of 10-50 per second. The peak power touches less than 0.10 watts. The wavelengths are used usually between 780 nm and 830nm. These system architectures have system merits such as excellent dynamical range and temporal linearity. However, they still have system limitations.

Fig 3. The fundamental theory of DOT

For example, the long data acquisition time associated with the time-correlated single photon counting devices is not desired for clinical applications in which fast data acquisition is crucial. In the frequency method, instead of using of light pulses with short duration, the systems use sinusoidal wave of amplitude modulate light source. The rate frequency of this system is usually between 100-1000 Megahertz. The frequency domain technique consists of four parts: the light source, the light delivery systems from the laser to object, the light detection and collection, and the devices that measure the modulation and phase shift.

Fig 4. Time-domain Fig 5. Frequency-domain

(The slope of pulse is long-time = ) ()

The time domain and frequency domain methods require relatively expensive detection technique and complicated data acquisition software. But, Frequency domain systems require less expensive instrumentation compared to the time-resolved systems and have been successfully applied in imaging. Another type of instrumentation, continuous-wave, is simpler and cost efficiency optical components compared to time and frequency domain. Continuous wave systems normally use a low frequency laser source incident on the tissue, either directly or through an optical fiber. The amplitude of attenuation is measured directly by a detector or through an optical fiber interfaced to detector. It provides the highest signal-to-noise ratio (SNR) for image reconstruction.

There are advantage and disadvantage associated with DOT modality, which are listed on the table below:

DIFUSSE OPTICAL TOMOGRAPHY

ADVANTAGE

DISADVANTGE

Non-ionizing

Low resolution

Non-invasive

Hard to quantify

Deep penetration into the tissue

Potential of molecular sensitivity

Simple and Low cost

MAGNETIC RESONANCE IMAGING

Magnetic resonance imaging (MRI) is a technology that procedures a magnetic field, gradient coil, and radio frequency to make images of organs and structures inside the body. Magnetic resonance imaging is done for many reasons. It is used to find problems into soft tissues such as tumors, bleeding, injury, blood vessel diseases. It can determine if a shunt is operating and identify damage to the brain caused by an injury or a stroke. The part of the body to be imaged is positioned in a strong uniform magnetic field (1.5 to 3 Tesla).

The nuclear magnetic moments (spin states) of water proton (Hydrogen) in the soft tissue in the magnetic field are forced into one of the possible orientations, spin up or spin down. At this point, the anatomy to be imaged is illuminated by pulsed, polarized radio-frequency energy at the resonant frequency of these water protons. As the protons absorb these pulses of energy, their spin states are altered; however, between pulses, the spin sates snap back into alignment with the magnetic field. These transitions create a second radio frequency signal that is unique to the resonant frequency of the anatomical material being imaged, and these signals are detected a receiving coil. The tomographic slices are created by applying magnetic field gradients in such a way as to create a resonant condition only for those protons located in a predetermined slice. Two- dimensional Fourier data matrix contains each NMR signal from a single volume element (called voxel) within a tomographic slice. This matrix is then converted into a two-dimensional display that can be read by a radiologist.

Fig 6. MRI scanner

There are three types magnetic of MRI that used in medical field. They are including permanent magnets, resistive magnets, and superconducting magnets. There are pro and cons associated with each magnet, which are listed on the table below:

Type of magnet

Permanent

Resistive

Superconductor

Pros

No cryogen

Small fringe field

Low operating cost

Light weight

Can be shut off

High signal to noise ratio

Low power consumption

Fast scanning

Cons

Limited magnetic resonant (<0.3 T)

Very heavy

Always on

Limited magnetic resonant (<0.2 T)

Water cooling required

Large fringe field

Acoustic noise

Motion artifact

High cryogen costs

Combination of DOT and MRI

The combination of the DOT and MRI measurements into the same tomographic system yields additional aids that cannot be realized simply by obtaining MR and DOT images as stand-alone procedures. The functional information from MRI can significantly increase the quantification precision of the optical process by constraining the DOT inverse problem (The formation of an image for the optical properties of the tissue from a series of boundary measurements is the inverse problem). Therefore, tissue chromophores and fluorochromes related with function can be measured with high accuracy and resolution compared to other modalities in biomedical field. Nowadays, several DOT laboratories explore the idea of multimodality approaches. The DOT and MR examinations are fully friendly and, given the cost efficiency of optical systems.

Fig 7. Principle of DOT and MRI

The data provided from DOT_MIR is acquired simultaneously by both schemes. Once the sample positioned at the middle of the fiber optic interface, the patient couch that holds the fiber interface is slid to position the interface at the center of the magnet bore. The imaging process includes three stages:

localization of the magnetic resonance slice that matches with the optical imaging plane and starting of DOT data acquisition

acquisition of T2-weighted anatomic MRI image

calibration process for optical equipment

At the first step, the fast spin echo sagittal and coronal localizer pulse systems are used to pinpoint the investigations. At the same time, the diffuse optical tomography data acquisition is started. Later, an axial slice is selected using the data found by the localizer sequences and verified for the coverage of all of the fiber probes in this 5 mm slice. In the next step, T2-weighted images using a fast spin echo (FSE). After the DOT and the MRI acquisitions are finished, the sample is exchanged with the homogeneous solid phantom and calibration measurements for the optical imaging system are done.

Fig 8: (a)–(c) Photographs of a MRI- optical tomographic scanner prototype (d) Axial MRI slices were acquired and (e) segmented according to tissue type. The spatial priors were incorporated into the optical tomographic reconstruction algorithms using a spatially varying regularization scheme, and (f) the total hemoglobin concentration was calculated.

SUMMARY

In this research, we discussed about diffuse optical tomography (DOT) and magnetic resonance imaging (MRI) and combination of these methods as two image modalities in biomedical field. DOT is an emerging modality that uses Near Infrared (NIR) light to reveal structural and functional information of deep biological tissue. DOT offers the potential to perform non-ionizing radiation and non-invasive three-dimensional reconstruction of optical properties of tissue given the measurements and forward model of photon propagation and the technology is fast and simple. MRI stems from the application of nuclear magnetic resonance (NMR) to radiological imaging. It is used to find problems into soft tissues such as tumors, bleeding, injury, blood vessel diseases. The combinations of the DOT and MRI are fully friendly and, given the cost efficiency of optical systems. The functional information from MRI can significantly increase the quantification precision of the optical process by constraining the DOT inverse problem. The summarize table of pros and cons of DOT and MRI:

DOT

MRI

Pros

Provide physiological image of hemodynamics,

Non-ionizing,

Inexpensive

High resolution image

Non-ionizing

Cons

Resolution is poor

Expensive

The summarize table of pros and cons for DOT and MRI

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