Medical Imaging
Background
Medical imaging is the process of creating an image of the human body, or parts of it, to reveal, diagnose or examine disease in clinical situations and/or to study normal anatomy and function in the medical science field.
Medical imaging constitutes a sub-discipline of many fields depending on the context for example:
Research and development in the instrumentation, image acquisition, modelling and quantification are usually found in biomedical engineering, medical physics and computer science fields;
Research into the application and interpretation of medical images is usually related to the medical sub-discipline relevant to medical condition (Radiology, neuroscience, cardiology, etc) under investigation.
The medical practitioner responsible for interpreting the images is a radiologist. Diagnostic radiography designates the technical aspects of medical imaging and in particular the acquisition of medical images. The radiographer or radiologic technologist is usually responsible for acquiring medical images of diagnostic quality, although some radiological interventions are performed by radiologists.
Net Resources
Medical Imaging Portal Diagnostic imaging topics - MRI, US, CT, Radiography, Bone Densitometry etc.
Radiology Journal
Medical Imaging News - gathers headlines daily on medical imaging and its major modalities.
In the following subsections, a brief introduction of some medical imaging modalities and their potential applications will be introduced.
Medical ultrasonography (sonography)
Medical ultrasonography as a diagnostic imaging technique is used to visualize muscles and internal organs, measure their size, structures and possible pathologies or lesions and in general it is effective for imaging soft tissues of the body.
The term "ultrasound" applies to all acoustic energy with a frequency above human hearing (20 KHz). Typical diagnostic sonographic scanners operate in the frequency range of 2 to 18 MHz. The choice of frequency is a trade-off between spatial resolution of the image and imaging depth: lower frequencies produce less resolution but image deeper into the body and vice versa. Superficial structures such as muscles, tendons, testes, breast and the neonatal brain are imaged at a higher frequency (7-18 MHz), which provides better axial and lateral resolution. Deeper structures such as liver and kidney are imaged at a lower frequency 1-6 MHz with lower axial and lateral resolution but greater penetration.
Medical sonography applications may include:
- Cardiology (echocardiography), Endocrinology, Gastroenterology, Gynaecology; Obstetrics; Ophthalmology; (A and B-scan ultrasonography), Urology, Musculoskeletal, Vascular, arteries and veins, Intravascular ultrasound , Ultrasound guided biopsy...
Ultrasound image formation
The formation of an ultrasound image takes three steps -
1- Producing a sound wave, 2- Receiving echoes (reflected or scattered signals), and 3- Interpreting those echoes.
A sound wave is usually produced by a piezoelectric transducer. Strong, short electrical pulses from the ultrasound machine make the transducer ring at the desired frequency. The sound is focused either mechanically by the shape of the transducer, or a lens in front of the transducer, or electronically using a complex set of control pulses from the ultrasound scanner machine. The wave travels into the body and comes into focus at a desired depth.
Impedance matching materials on the face of the transducer are usually used to enable the sound to be transmitted efficiently into the body. In addition, a water-based gel is placed between the patient's skin and the probe for efficient coupling between the transducer and the skin.
The generated sound wave is reflected at any density changes in the body: e.g. blood cells in blood plasma, small structures in organs, etc. Some of the reflections return to the transducer. The return sound wave vibrates the transducer, the transducer turns the vibrations into electrical pulses that travel to the ultrasonic scanner where they are processed and transformed into a digital image. The ultrasound machine determines the direction of the returned echo, its intensity and its time of flight. Then the software and/or hardware of the scanner use these information to form the image.
Net Resources
Electrical Impedance Tomography (EIT) (under construction)
The credit for the invention of EIT as a medical imaging technique is usually attributed to John G. Webster in around 1978.
Electrical impedance tomography, EIT, is a novel imaging technique with applications in medicine and process control. Compared with techniques like computerised x-ray tomography and positron emission tomography, EIT is about a thousand times cheaper, a thousand times smaller and requires no ionising radiation. Further, EIT can in principle produce thousands of images per second. Its major limitations are its low spatial resolution, and - in the medical field - large variability of images between subjects. Recordings are typically made by applying current to the body or system under test using a set of electrodes, and measuring the voltage developed between other electrodes. To obtain reasonable images, at least one hundred, and preferably several thousand, such measurements must be made.
In the medical field, the most studied applications for EIT are measurement of gastric emptying and lung function. In the industrial field typical applications are imaging the distribution of oil and water in a pipeline and imaging the flow of substances in a mixing vessel. In some ways industrial applications are more favourable for EIT because it is usually possible to use a rigid, fixed array of electrodes. The fixing of electrodes on the human body is one of the residual problems facing medical EIT.
Net Resources
Magnetic Resonance Imaging (MRI)
Magnetic resonance imaging is based on studies of nuclear magnetic resonance. Historically, MRI was referred to as nuclear magnetic resonance imaging (NMRI). So to prevent patients from making a negative association between MRI and ionizing radiation, the word nuclear has been almost universally removed.
Medical MRI relies on the relaxation properties of excited hydrogen nuclei in water and lipids. When the object to be imaged is placed in a powerful, uniform magnetic field, the spins of atomic nuclei with have to arrange in a particular manner with the applied magnetic field. Nuclei of hydrogen atoms (protons) have a simple spin 1/2 and therefore align either parallel or antiparallel to the magnetic field.
The spin polarization determines the MRI signal strength. For protons, it refers to the population difference of the two energy states that are associated with the parallel and antiparallel alignment of the proton spins in the magnetic field.
For example, in a 1.5 Tesla magnetic field (at room temperature) this difference refers to only about one in a million nuclei since the thermal energy far exceeds the energy difference between the parallel and antiparallel states. Yet the vast quantity of nuclei in a small volume sum to produce a detectable change in field. The magnetic dipole moment of the nuclei then precesses around the axial field. While the proportion is nearly equal, slightly more are oriented at the low energy angle. The frequency with which the dipole moments precess is called the Larmor frequency. The tissue is then briefly exposed to RF pulses in a plane perpendicular to the magnetic field, causing some of the magnetically aligned hydrogen nuclei to assume a temporary non-aligned high-energy state (i.e. the population difference of the two energy levels is altered). The frequency of the pulses is governed by the Larmor equation to match the required energy difference between the two spin states.
Image formation
In order to selectively image different voxels of the subject, orthogonal magnetic gradients are applied. All spatial encoding is obtained by applying magnetic field gradients which encode position within the phase of the signal. In one dimension, a linear phase with respect to position can be obtained by collecting data in the presence of a magnetic field gradient. In three dimensions, a plane can be defined by "slice selection", in which an RF pulse of defined bandwidth is applied in the presence of a magnetic field gradient in order to reduce spatial encoding to two dimensions (2D). Spatial encoding can then be applied in 2D after slice selection, or in 3D without slice selection. Spatially-encoded phases are recorded in a 2D or 3D matrix; this data represents the spatial frequencies of the image object. Images can be created from the matrix using the discrete Fourier transform (DFT). Typical medical resolution is about 1 mm³, while research models can exceed 1 µm³.
Scanner construction and operation
Magnet
The magnet is the largest and most expensive component of the scanner. Three types of magnet have been used:
- Permanent magnet: usually made from ferromagnetic materials. the magnet can weigh in excess of 100 tonnes, require little costly maintenance, field strength (usually < 0.4 T).
- Resistive electromagnet: A solenoid wound from copper wire, low cost, but field strength is limited, and stability is poor. This design is essentially obsolete.
- Superconducting electromagnet: produce extremely high field strengths, with very high stability. The construction of such magnets is extremely costly. However, despite its cost, helium cooled superconducting magnets are the most common type found in MRI scanners today.
1.0 - 1.5 T field strengths are a good compromise between cost and performance for general medical use. However, for certain specialist uses (e.g., brain imaging), field strengths up to 3.0 T may be desirable.
RF system
The RF transmission system consists of a RF synthesizer, power amplifier and transmitting coil. High-end scanners may have a peak output power of up to 35 kW, and be capable of sustaining average power of 1 kW. The receiver consists of the coil, pre-amplifier and signal processing system. A variety of coils are available which fit around parts of the body, e.g., the head, knee, wrist, or internally, e.g., the rectum.
Gradients
Magnetic gradients are generated by three orthogonal coils, oriented in the x, y and z directions of the scanner (usually producing gradients from 20 mT/m to 100 mT/m.
In order to understand MRI contrast, it is important to have some understanding of the time constants involved in relaxation processes that establish equilibrium following RF excitation. As the high-energy nuclei relax and realign they emit energy at rates which are recorded to provide information about the material they are in. The realignment of nuclear spins with the magnetic field is termed longitudinal relaxation and the time required for a certain percentage of the tissue's nuclei to realign is termed "Time 1" or T1, which is typically about 1 second. T2-weighted imaging relies upon local dephasing of spins following the application of the transverse energy pulse; the transverse relaxation time is termed "Time 2" or T2, typically < 100 ms for tissue.
Image contrast is created by using a selection of image acquisition parameters that weights signal by T1or T2, or no relaxation time ("proton-density images"). In the brain, T1-weighting causes the nerve connections of white matter to appear white, and the congregations of neurons of gray matter to appear gray, while cerebrospinal fluid appears dark. The contrast of "white matter," "gray matter'" and "cerebrospinal fluid" is reversed using T2 imaging, whereas proton-weighted imaging provides little contrast in normal subjects.
Net Resources
Nuclear Medicine (under construction)
Nuclear medicine is a branch of medicine and medical imaging that uses the nuclear properties of matter in diagnosis and therapy. Nuclear medicine differs from most other imaging modalities in that the tests primarily show the physiological function of the system being investigated as opposed to traditional anatomical imaging such as CT or MRI. In some centers, the nuclear medicine images can be superimposed, using software or hybrid cameras, on images from modalities such as CT or MRI to highlight which part of the body the radiopharmaceutical is concentrated in. This practice is often referred to as image fusion or co-registration.
Net resources