17-12-2012, 05:42 PM
Advances in Medical Imaging
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Abstract
This paper starts with giving the medical imaging modalities
that are in practical use and lists several of the new medical
imaging modalities under development. The remainder of the
paper is concentrated on progress in MR imaging, ultrasound
imaging and x-ray CT imaging. These modalities are major radiological
imaging tools, which will have growing significance
in the next decade. They are surpassed only by ordinary x-ray
projection imaging, which is much more static in its development.
Particular attention is given to applications where image
processing and image analysis tasks are needed.
Introduction
Medical imaging has gone through a revolution since the advent
of x-ray computed tomography (CT) imaging in 1972. Before
that time, the only standard imaging modality in radiological
departments was the ordinary x-ray projection imaging. This
imaging modality dates back to R¨ontgen’s discovery of x-rays in
1895. Today, up to date radiological departments will use x-ray
projection imaging, x-ray CT including spiral scanning, ultrasound
imaging, magnetic resonance (MR) imaging and gamma
camera imaging.
Several medical imaging modalities are used only to a limited
extent in radiological practice or are still at the experimental
stage. Positron emission tomography (PET) and single
photon emission computed tomography (SPECT) belong to
the former group. Magnetic resonance spectroscopy, electrical
source imaging, electrical impedance tomography, magnetic
source imaging and medical optical imaging belong to the latter
group [7]. Some of these experimental modalities are likely to
come into practical use in the next decade.
MR imaging
The formation of the two-dimensional (2D) MR image takes
place in the 2D complex Fourier domain. Correspondingly, the
formation of 3D MR images takes place in the 3D complex
Fourier domain. In MR literature, the complex Fourier domain
is called k-space. The time-limiting factor in the formation of
an MR image is the process of filling up k-space with data. Usually,
the lines in k-space (the frequency encoding direction) are
filled in successively. The different rows in k-space are encoded
by different phases of the spinning magnetic nuclei (usually hydrogen).
The simplest pulse sequences such as spin echo sequences
fill in one line for each excitation pulse, but faster pulse sequences
fill in several lines for each pulse. Echo planar pulse
sequences are the fastest pulse sequences. They fill up all of
k-space after a single pulse, allowing recording time for an image
as short as 50 ms. With such short recording times, real
time imaging of the beating human heart is possible. The cost
is severe image blurring and geometrical distortions caused by
accumulating phase errors in k-space. In contrast, spin echo
pulse sequences have very little image blurring and geometrical
distortion, but the recording time is in the order of 2-10 minutes
for a single image. Fast gradient echo pulse sequences allow
the recording of a single image in the order of 10 sec, without
introducing much image blur and geometrical distortion.
Ultrasound imaging
Ultrasound imaging is an important real-time diagnostic tool
in several major clinical disciplines, and its significance is still
growing fast. Depending of the set up of the ultrasound scanner,
it can produce real-time tomographic images of ultrasound
scattering, real-time images of blood and tissue motion, elasticity
and tissue flow (perfusion). All these images are built
up line by line by sending ultrasound pulses into the tissue and
recording the reflected radiofrequency signal. These reflected
signals provide the necessary information to derive the various
ultrasound image types listed above.
A number of pulsed-echo scan image formats exist, but some
are mostly of historical interest. The A-scan belongs to this
latter category, while the real time techniques M-scan (motion
scan), M-scan with Doppler, B-scan and B-scan with Doppler
are all in common use. Very recently, also some off-line threedimensional
scans have gained clinical importance.
An M-scan is a pulsed-echo scan with the beam pointing in
one direction. In M-scans the scattering echoes from the structures
are gray scale plots against time to show the size of the
echoes and the spatial variations of their positions as a function
of time. A B-scan is a pulsed-echo scan in which the beam
sweeps across a tissue plane. The scattering echoes from the
tissue structures are displayed in gray scale at the sites corresponding
to the location of the structures in the scan plane
which produced the echoes. The gray scale intensities increase
with the size of the echoes.
X-ray CT imaging
Computed Tomography (CT) was the first non-invasive radiological
method allowing the generation of tomographic images
of all parts of the human body without superposition of neighboring
structures. CT imaging has gone through a radial improvement
since its introduction in 1972 (Table 1). Today, a CT
scanner is a crucial part of any real radiological department.
The image is formed by projecting many x-ray beams
through the object using a fan-beam geometry. The x-ray source
is moved in a circle around the object. At fixed angles of the
circle an x-ray fan-beam is emitted and recorded by an array of
x-ray detectors at the opposite side of the object. When the xray
attenuation of all projections are recorded, the tomography
image is reconstructed from these projections using the Radon
transform. To image a complete organ, parallel image slices of
the organ is recorded. Today the planar resolution in standard
CT is less than 1 mm, while the axial resolution (or slice thickness)
is several mm. This is one serious disadvantage of standard
CT. Another disadvantage with standard CT is that each
scan lasts about 2 sec, and the scans have to be separated by
about 6 sec delays to reorient the x-ray source detector assembly
within the gantry to prevent entanglement of cables. This
delay is so long that many organs can not be imaged during one
breath hold. Thus, some lesions may be skipped.