Hamidreza Saligherad

Assistant Professor of Medical Physics

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Email : h-salighehrad@tums.ac.ir

My major research interest falls in the area of advanced image acquisition and processing

techniques for Magnetic Resonance Imaging (MRI) and Spectroscopy in Medicine. As my

second interest, I conduct research in Wireless Communication Channel Modeling. These

include:

• Magnetic Resonance Imaging

- Clinical Application of Single-Voxel and Multi-Voxel MR Spectroscopy in Brain, Spine and

Musculoskeletal System

Magnetic resonance spectroscopy (MRS) allows the non-invasive measurement of selected

biological compounds in vivo. Feasibility was first demonstrated in humans in the mid-

1980s. Since that time, much experience has been accumulated with the use of MRS in both

research and clinical applications. Nearly all MRI scanners have the capability to perform

MRS, and MRS techniques still continue to improve, even after two decades of

development. MRS has been applied to the study of all major pathologies, particularly in

the brain, but has also found application in other organ systems as well, like musculoskeletal

and spine. In spite of this considerable research effort and the unique biochemical

information provided, only limited integration of MRS into clinical practice has occurred to

date. Here at the Tehran University of Medical Sciences, I have established close

collaborations among technical research as well as clinics to explore potentials of MRS as

an emerging technique into clinics, and in a few selected problems like quantification of softtissue

tumors (e.g. ewing- sarcoma and osteo-sarcoma), brain tumors and spinal injuries (e.g.

glial scar).

- MRI-Guided Attenuation and Scatter Correction Positron Emission Tomography (PET) in Brain

and Pelvis:

Reliable attenuation correction methods for quantitative 3D PET require accurate

delineation of both the organ contour and spatial distribution of attenuation coefficients. To

this end, it is necessary to carefully segment body organs into three different categories of

soft tissue (e.g. muscle and fat), hard tissue (e.g. bone) and air (e.g. lung cavity).

Conventional MRI (cMRI) techniques can easily segment out the soft-tissue, while they

usually confuse bone and air since they both do not enough visible signals in cMRI.

Therefore, advanced MR imaging techniques like UTE-based methods are required to

distinguish bone from air. Here at Tehran University of Medical Sciences, I have established

strong collaborations with our nuclear medicine center to conduct mutual research, starting

with PET/MR imaging in brain and pelvis.

- Ultra-Short Echo-Time (UTE) Imaging for Quantification of Bone Water in Cortical Bone:

A significant portion of the bones mechanical competence is attributed to the composition

and micro-architecture of cortical bone. Although the age-related thinning of the cortex is

partially offset by periosteal expansion, this process is accompanied by other remodeling

changes that adversely affect bone quality.

Key among these is an age-related increase in cortical porosity that is exacerbated in

osteoporosis. Although the mechanism for this process is poorly understood, there is

evidence from histomorphometry that in the femoral neck of hip fracture patients, Haversian

canals are expanded as a result of formation of “composite” osteonal systems in which a

single canal is surrounded by multiple packets of osteonal bone. In this study, we proposed

a new 3D hybrid-radial variable-TE sequence with half-pulse selective excitation to image

the micro-structure of the cortical bone and to accurately quantify the bone water and bone

porosity, by inference, in cortical bone, specifically in the tibial shaft. The experiment was

designed at 123MHz on a TIM Trio MR system (Siemens Medical Solutions). In this

project, I developed all imaging pulse sequences, reconstruction codes and analysis

techniques to precisely capture and to analyze the acquired bone water signal. I managed a

clinical study to analyze sixty five subjects in different age ranges in order to establish a baseline

for the new introduced metric of bone water concentration.

- Motion Correction for High-Resolution Trabecular Bone Imaging Employing Projection Navigators:

High-resolution magnetic resonance imaging (micro-MRI) captures structural details of

trabecular bone (TB) that are reflective of metabolic bone disease and treatment. Necessary

resolutions to resolve TB are in the order of the structure’s thickness: ∼100-200μm.

Three-dimensional imaging of TB is hindered by involuntary rigid body motion as it causes

significant blurring of the TB structure. Motion correction techniques that are developed to

correct for different types of motion (translation, rotation, etc), and to provide high precision

on the order of a half voxel dimension, still suffer from: 1) not enough signal-to-noise ratio

(SNR) for high-precision detection, and 2) not enough degrees of freedom for both

translation and rotation detection. In this study, I proposed a novel motion

detection/correction technique based on acquiring high SNR low-resolution full images as

navigators using advanced SNR enhancement techniques. I also developed all required

pulse sequences as well as reconstruction codes in MATLAB in order to implement it on a

Siemens 1.5T MAGNETOM Sonata scanner.

- Compressed Sensing MRI:

MRI requires a relatively long scan time compared to other biomedical imaging modalities,

which makes it also expensive. MRI data are collected in the spatial-frequency domain,

denoted by k-space. MRI data acquisition can be accelerated by undersampling k -space

from the Nyquist rate. As a new project in the field, I have introduced high performance

compressed sensing algorithms to accelerate the current time-consuming micro-MRI of

trabecular as well as whole body MR Angiography (MRA).

- High Field (7T) In vivo Phosphorus (31P) Imaging of Cortical Bone:

Phosphorus concentration of bone plays an important role in the maintenance of bone

strength in disorders such as osteomalacia which is characterized by hypomineralization of

bone. 31P MRI potentially provides a noninvasive method to evaluate the degree of bone

mineralization, however it is difficult to obtain sufficient SNR at desirable resolutions due

to the extremely short T2* (∼220μsec) and long T1 (∼50sec) of bone phosphorus, as well as

its relatively small gyromagnetic ratio. We estimate that 31P SNR at 7T is intrinsically about

1000 times less than that of muscle water. For this reason, optimization of the imaging

hardware, pulse sequence and post processing is particularly important. In this study, we

constructed small surface coils for use in a 7T whole-body MRI scanner and developed a 3D

radial concentric-cone imaging sequence to image 31P of tibial cortical bone in vivo.

- Large Flip Angle RF Pulse Design with Inverse Scattering Transform

Magnetization transfer (MT) contrast can be used in MRI as an endogenous tissue contrast.

Large flip angle, off-resonance RF pulses are commonly used for MT contrast generation.

Small tip angle approximation and SLR techniques are two robust and efficient methods

in design of low flip angle pulses, however they are not practical in high flip angle (>180)

pulse design. In this study, I investigated the use of inverse scattering transform (IST) as an

alternative strategy for high flip angle pulse design.

• Wireless Channel Modeling

- Space-Time-Frequency Characterization of Wireless Channels:

Wave propagation in two- dimensional and three-dimensional random scattering media,

MIMO multi-carrier wireless channels, narrowband and wideband wireless channel

characterization, straight and non-straight movements of the mobile station (MS), random

movements of MS and/or scatterers, fading statistics of Rayleigh channels, modeling and

evaluation of MIMO relay channels (Ph.D. work and individual works to advise master

students)

- Capacity Evaluation of Wireless Channels:

Information theoretic approaches to antenna design in MIMO systems, capacity analysis of

MIMO systems employing realistic channel correlation models (individual works to advise

master students)

• Space-Time Coding Techniques

- Space-Time Coding Ambiguities:

Identification of space-time coding ambiguities in joint adaptive channel estimation and

detection

• Image Processing

Inverse Imaging:

Theoretical methods in inverse imaging and 3D image reconstruction