Editorial
Combined Positron Emission Tomography and Magnetic Resonance Imaging: A Technical Challenge
Ashish Kumar Jha*
Department of Oncology, Tata Memorial Hospital, Mumbai, India
*Corresponding author: Ashish Kumar Jha, Department of Oncology, Tata Memorial Hospital, Mumbai, India
Published: 04 May, 2017
Cite this article as: Ashish Kumar Jha. Combined Positron
Emission Tomography and Magnetic
Resonance Imaging: A Technical
Challenge. Clin Oncol. 2017; 2: 1276.
Editorial
A clinical prototype of combined Combined Positron Emission Tomography (PET) / Computed Tomography (CT) system went under the clinical evaluation in 1998. After three years of research and clinical evaluation first clinical combined PET/CT system was installed in a clinic in 2001 [1-3]. Gradually clinical utilization of combined PET/CT increased tremendously particularly in oncological imaging [3-6]. The success of PET/CT as a medical diagnostic device led to the development of other combined modality like SPECT/CT. the superiority of Magnetic Resonance Imaging (MRI) over CT as anatomical imaging device in providing unmatched soft tissue details along with a reasonable array of functional information through techniques such as MRI spectroscopy and functional MRI imaging has led to development of combined PET/MRI system. The first PET/MRI combined modality introduced in nuclear medicine practice for brain imaging to diagnose the complex disease processes based on improved and reliable anatomical and functional information provided by this combined system and gradually whole body PET/MRI is developed for clinical use [7]. Combining Positron Emission Tomography (PET) and Magnetic Resonance Imaging (MRI) in a single device was a challenging task as both the imaging device faced lots of interference from each other resulting in many new developments and changes in both the system [7-14]. Hence it is very important to understand the real technical problem faced by two systems in the process of combining with each other.
Ways in Which PET Can Affect MRI
The introduction of PET detectors inside the gradient coil and magnet can lead to interference [7-10].
• PET Scintilation detector like GSO and LGSO may cause small differences in magnetic
susceptibility within the magnet bore may result in an in homogeneity in the main magnetic field
(B0) [8].
• Gradient of MRI may induce eddy current in shielding material and electronics of PET
scanner may lead to distortion and non linearity in MRI image [8].
• PET electronics and power cable interfere with RF detection and can interfere with FR
pulse detection [8].
Ways in Which MRI Can Affect PET
The high static magnetic field (B0), quickly changing gradient fields (B1), and radiofrequency
signals (RF) from the MRI scanner prevent the normal operation of photomultiplier tubes and
front-end electronics of PET detectors used in current generation PET/CT System [7-10].
• Main magnetic field: The magnetic field perturbs the paths of electrons moving from the
photocathode down the dynode chain to the anode resulting in a loss of gain [7-9].
• Gradient fields: Rapidly switching magnetic fields can induce eddy current loops in any
conductive components introduced into the magnet bore, including PET circuitry. In addition to
signal interference, these can lead to heating and mechanical vibration [7-8].
• RF interference: Any electronics situated within the magnet bore may be susceptible to
RF interference generated by the MRI transmit coil. This effect is responsible for the drop in PET
count rate that is observed in many MRI-compatible PET systems during MRI acquisition [7-8].
MRI Compatible PET Detectors
The most widely used PET scintillators LSO, LYSO and BGO, have a magnetic susceptibility similar to that of human tissue and have been demonstrated to have negligible effects on MRI. Scintillators containing gadolinium, such as GSO and LGSO, are not suitable; however these are less commonly used for PET nowadays [9].
MRI Compatible PET Photomultiplier
Electrical photomultiplier tube (PMT) can be replaced by solidstate photo detectors i.e. avalanche photodiode (APD) which is functionally stable under high magnetic field of MRI magnet and also doesn’t distort the magnetic field. The electronic multiplication in APD is much lower than that of PMT [10].
MRI Compatible PET Shielding
Conventionally used lead based shielding material is replaced with MRI compatible material like tungsten.
MRI Compatible PET Electronics
PET electronics pin were coated with nickel to avoid MRI interference to the PET electronics and vice versa.
PET/MRI Development Approach
Sequential v/s simultaneous imaging: PET/MRI system
development was based on two major approaches, namely, sequential
imaging and simultaneous imaging. Based on these two approach two
different models, separate gantry model and integrated gantry model
were developed [7-8,11-14].
Sequential imaging: In sequential imaging approach, both
the gantries were placed axially side by side like PET-CT. Due
to interference from MRI magnetic field to the PET component
and increased time of imaging has never allowed this concept to
evolve [8,10,11]. Another sequential approach has been utilized by
separating the PET and MRI gantries coaxially and patient table was
placed in between. Because of separation of gantry by large distance
allowed minimal interference from each other. This
approach requires no modification in the standalone PET and MRI
system except software [11,12]. Philips Ingenuity TF PET‐MRI is an
example of sequential imaging approach gantry separated coaxially
[11,12]. All PET and MR applications that are available on standalone
systems are available on the hybrid PET‐MR.
Simultaneous imaging: In simultaneous imaging approach, PET
gantry is integrated in MRI gantry (Figure 2). Several modifications
were required to develop such system. Inability of PMT to work
in side magnetic field was the major challenge to develop such a
system. Hence PMT was replaced with new generation of solid-state
photo detectors i.e. Avalanche Photodiode (APD) which do not get
influenced in magnetic field. Other electronics associated with PET
were shielded for magnetic and RF interference for proper operation
in the MRI gantry [7-14].
Siemens Biograph mMR and GE Healthcare SIGNA PET/MRI
are the truly integrated PET‐MRI system available in the market for
clinical use [13,14]. This PET/MRI system is capable of performing
simultaneous imaging. These two PET/MRI systems consist of a
dedicated whole‐body PET scanner built into a dedicated 3T MRI
scanner [13,14].
MRI based Attenuation Correction in PET
Since MRI signals do not have the same properties as gamma
photon and unable to produce attenuation value as CT scan, hence MRI based attenuation correction in PET was one of the major challenges in PET/MRI fusion. Two approaches have been extensively tested for MRI based attenuation correction of PET image i.e. Atlas
based approach and Segmentation based approach [15-19].
Atlas-based attenuation correction: A template MRI image is
created from multiple subjects (atlas) and the attenuation values on
it are assigned by using transmission or CT scans of the same subject.
These template images can then be co-registered with the MRI image
of subject, and the attenuation values can then be assigned to the PET
image. Atlas creation and co-registration is the main issue with this
method [15,16,19].
Segmentation based attenuation correction: Usually T1 weighted
images or two point Dixon sequence images are segmented into areas
with different attenuation values for air, lung, soft tissue, fat and
uniform attenuation value are assigned to one class of tissue and then
this attenuation map is applied to the PET images. Bone segmentation
is not possible in this method and assigned the attenuation valued of
soft tissue. In segmented based method differentiation between air
and bone is not possible hence quantification based error [17,19].
Methods to reconstruct the attenuation map from emission
data: This method exploit information about tissue attenuation
contained in PET emission data of time of flight imaging. Attenuation
map cannot be generated from non time of flight imaging data by
this method. Maximum likelihood reconstruction of activity and
attenuation (MLAA) algorithm are used to estimate the attenuation
map from TOF emission data [18,19]. Due to the simplicity of
segmentation based approach, this has been utilized in most of the
clinical PET/MRI scanners available commercially.
Clinical Utility of PET/MRI
Integrated PET‐MRI systems may potentially be used for a number of indications in oncology, neurology and cardiology but clinical utility over the PET/CT scanner yet to be established. Primarily this modality should be used for the functional assessment of cancer, in particular those cancers that are difficult to assess using CT, such as head and neck, brain and prostate cancer [20].
Conclusion
PET-MRI is a modality with tremendous potential to quantify the molecular pathways and pathology in vivo with excellent anatomical details can establish this modality in cardiology, neurology and oncology as a diagnostic tool of choice
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