What is a PET-MRI

The Biograph mMR scanner is a combination of positron emission tomography (PET) with magnetic resonance (MRI). It uses two separate phenomena for accurate diagnostics of neoplastic changes, as well as many individual diseases in the scope of oncology, cardiology, neurology and infectious diseases.

PET module examines the function of human body organs by tracing the metabolism and movement of chemoical radiolabel which is administered intravenously. The most commonly used substance is fluorodeoxyglucose (FDG) labeled with Fluorine 18 radioactive element, it accumulates in the changes with increased glucose metabolism.

MRI module uses radio waves and magnetic field to create detailed, more accurate than CT, images of internal body parts such as soft tissue, organs and bones, which allows to detect even small tumors, not visible while using other imaging methods. MRI scan also provides detailed information about the accumulation of the radiolabel.

By using this scanner it is possible to perform simultaneous PET and MRI using much lower doses of radiation than in PET-CT (computed tomography). It is a very modern diagnostic tool that allows to perform just one scan instead of two separate ones.

 

The course of the procedure:

The patient should appear for the registration on an empty stomach, first a medical history documentation is created, then the patient receives a questionnaire  to be filled in which contains questions about medication, concomitant diseases, allergies and any metal body parts.

Then the patient steps into doctor’s office, where physical examination and medical history are conducted. The patient is weighed and measured, a blood glucose level test is performed as well. The doctor calculates the isotope activity which is administered intravenously individually for each patient. Then the patient needs to wait in a still position for about 60 minutes in a dedicated room. The patient should empty the bladder shortly before the procedure.

After the specified time, the technician puts the patient on the bed and applies a special system of small tubes that allow to scan the whole body. The doctor supervising the procedure may order additional scans for different parts of the patient's body.

 Key information about the Biograph mMR scanner

The scanner is adapted to scan the entire body in a magnetic field of 3 T. The diameter size of the gantry is equal to 60 cm, and the length of the bed is equal to 199 cm. The system is equipped with 18 independent RF modules. PET detector contains 56 blocks of detectors in each of the 8 rings. The field of vision of the axis "z" is equal to 26 cm. The matrix of detector blocks is made of LSO crystal, and light pulses are recorded by avalanche photodiodes (APD).

 

 

MR-PET: Integracja

PET-MRI: Integration

Uzwojenie ekranujące magnesu

Magnet shield winding

Uzwojenie pierwotne magnesu

Magnet primary winding

Cewka gradientowa

Gradient coil

Detektor PET

PET detector

Cewka nadawczo-odbiorcza

Coil transceiver

Kriostat magnesu

Magnet cryostat

MR

MRI

PET

PET

Powietrze/próżnia

Air/vacuum

 

Biograph ™ mMR scanner is commonly used in clinical diagnostics because of its unique ability for molecular imaging which is useful especially in oncology, cardiology and neurology, it can also be used in paediatrics. Thanks to the high contrast in soft tissues which is obtained by means of the magnetic resonance imaging (MRI with the magnetic field size of 3T), it allows for the detection, characterization and monitoring of even the smallest changes in the soft tissue. MRI capabilities are greater than CT capabilties, especially in the differentiation of the nature of the soft tissue, functional imaging with the use of variety of signal excitation techniques such as diffusion sequences (DWI, DTI) spectroscopy, functional tests (BOLD).

This method is a source of information in assessing perfusion, angiogenesis, apoptosis, proliferation, etc., and the obtained PET and MRI images do not duplicate but complement each other, providing a lot more information than a PET-CT. The current MRI techniques make it possible to use this modality in gamma-ray-based attenuation correction which significantly improves image quality.

A fully integrated device is adapted to simultaneous examination of the whole body using two modalities of positron emission tomography (PET) and magnetic resonance. Biograph MMR is commonly known as the modality of choice for all kinds of tumor imaging by connecting information in the scope of morphology, function and metabolism. The combination of MRI and PET offers a more complete imaging and a way of better understanding of many diseases. Current research and development of new markers can make the tests which use this hybrid even more important.

In the brain, detailed structural information from the MRI combined with metabolic activity obtained from PET facilitate the early detection of the disease, precise planning of surgical intervention and provide an efficient evaluation method after treatment. In the musculoskeletal system, the combination of MRI and PET scans allows for an in-depth evaluation of soft tissue and bone marrow. In the trunk area, PET images provide detailed information on the metabolism of the organs in the chest and abdomen, while the MRI images supplement the picture with further functional and structural data. In addition, MRI images can be used to correct the motion artifacts in PET images, which improves the image quality significantly.

Except for PET-MRI, this device can be used to perform independent MRI scans in full scope. The scanner is equipped with a system of coils that enable accurate testing of soft tissue, breasts, spine, small and large joints and changes on the surface of the body .The anatomical, functional and metabolic information obtained in this way has a high resolution due to the high intensity of the constant magnetic field which is equal to 3T. The scanner can be used for early detection of changes which can be a crucial factor in selecting treatment plan and in therapy monitoring.

Physical fundamentals of PET imaging

Positron emission tomography the positively charged particles emitted by some elements - positrones (β+) produced in cyclotrons or generators. These include short lived isotopes of the basic elements required for human existence, ie. carbon, oxygen, nitrogen and fluorine, which is most commonly used.

The isotopes are administered intravenously, they accumulate in the selected organ and emit β+ radiation. The radiation intensity does not depend on the structure of an organ, but on its metabolic activity (function). Each dysfunction is projected as an area of increased or impaired metabolism. These elements disintegrate quickly (from several to tens of minutes), so the test is relatively safe.

After the isotope is administered, the patient waits for around 60 minutes for the distribution of the marker in the body, then the patient is laid in the field of view of the detectors recording gamma radiation. The emitted positrons are characterized by an initial energy of several MeV. Their movement in the body reduces the kinetic energy, mainly due to the interaction with electrons. After passing a specific route in the patient’s body of a few to several millimeters, an interaction of the positron with the electron that could cause their annihilation occurs.  It is important to remember that annihilation does not occur in the place of decay, and the greater the energy of the positron, the greater the maximum route that the positron can pass is. As a result of the annihilation, most frequently two photons of energy 511 keV traveling in opposite directions are created.

The photons are recorded by a ring of detectors placed in the gantry of the scanner, the registration of a photon by a given detector generates an electrical impulse. If a pulse generation occurs in a very short period of time in the two detectors that are located opposite each other in the ring of detectors, it can be concluded that they originate from the same source, and such system is called the coincidence registration system. The line connecting two detectors for each of the coincidence events is called the line of response (LOR). PET coincidence detection is determined by an electronic collimation method (which no longer requires a mechanical collimator) through which both the sensitivity and uniformity of response functions to the occurrence of a point source significantly increases, as for example compared to traditional SPECT systems.

 

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Due to the ongoing gamma rays scattering, gamma quanta registrations are random and repeated, and not all coincidences come from a line of response (LOR). The existence of various forms of false coincidences causes the degradation of the diagnostic information, reduction of contrast and increased information noise. To reduce the number of incorrect coincidences registration, a suitable time window is selected, i.e. time of registering the interaction of photons with the detector.