Medical Imaging & Diagnostic Technologies
Diathermy: Surgical & Therapeutic Applications
High-frequency currents, apart from their usefulness for therapeutic applications, can also be used in operating rooms for surgical purposes involving cutting and coagulation. The frequency of currents used in surgical diathermy units is in the range of 1-3 MHz, in contrast to much higher frequencies employed in short-wave therapeutic diathermy machines. This frequency is quite high in comparison with that of the 50 Hz mains supply. This is necessary to avoid the intense muscle activity and the electrocution.
Computed Tomography (CT) Imaging
There are two main limitations of using conventional X-rays to examine internal structures of the body:
- Firstly, the superimposition of the 3-dimensional information onto a single plane makes diagnosis confusing and often difficult.
- Secondly, the photographic film usually used for making radiographs has a limited dynamic range and, therefore, only objects that have large variations in X-ray absorption relative to their surroundings will cause sufficient contrast differences on the film to be distinguished by the eyes.
In computed tomography (CT), the picture is made by viewing the patient via X-ray imaging from numerous angles, by mathematically reconstructing the detailed structures and displaying the reconstructed image on a video monitor. Computed tomography enabled radiologists to distinguish, for the first time, between different types of brain tissue.
Magnetic Resonance Imaging (MRI) & Nuclear Medicine
Clinical magnetic resonance imaging (Clinical MRI) is an imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body in both health and disease. MRI scanners use strong magnetic fields, radio waves, and field gradients to generate images of the organs in the body. MRI does not involve X-rays, which distinguishes it from computed tomography (CT or CAT). While the hazards of X-rays are now well-controlled in most medical contexts, MRI still may be seen as superior to CT in this regard. MRI is widely used in hospitals and clinics for medical diagnosis, staging of disease, and follow-up without exposing the body to ionizing radiation. MRI often may yield different diagnostic information compared to CT. There may be risks and discomfort associated with MRI scans.
Ultrasonic Imaging Systems
Ultrasound is a form of energy consisting of mechanical vibrations, the frequencies of which are so high that they are above the range of human hearing. Most biomedical applications of ultrasound employ frequencies in the range of 1 to 15 MHz. Velocities of ultrasound in soft tissues and bones are 1570 m/s and 3600 m/s, respectively.
Applications of Ultrasonography
- In neurology, to find any brain tumor.
- In ophthalmology, to find any foreign objects in the eye.
- In cardiology, to determine the cross-section of the heart and to determine heart rate.
Cardiac Pacemakers
A cardiac pacemaker is an electrical stimulator that produces periodic electrical pulses that are conducted to electrodes located on the surface of the heart, within heart muscles, or within the cavity or lining of the heart. It is used for the treatment of the following:
- Cardiac rhythm disorders.
- Abnormalities in the SA node, AV node, and Purkinje system.
Types of Pacemakers
- External Pacemaker
- Internal Pacemaker
External Pacemaker
They are used when heart block presents as an emergency and is expected to be present for a short time.
Internal Pacemaker
They are used in cases requiring long-term pacing because of permanent damage that prevents normal cardiac operations.
Modes of Operation
Asynchronous Mode
The fixed-rate impulses occur along with natural pacing impulses.
Synchronous Mode
They are programmed either in demand or synchronized mode.
Defibrillators
Cardiac Fibrillation
Cardiac fibrillation is a condition where the individual myocardial cells contract asynchronously, and an irregular cardiac rhythm is produced which causes the cardiac output to near zero. The fibrillation of atrial muscle is called atrial fibrillation, and that of the ventricles is called ventricular fibrillation. There are 3 basic types:
- AC Defibrillator
- Capacitive Discharge DC Defibrillator
- Capacitive Discharge Delay Line Defibrillator
Electroencephalogram (EEG)
Electroencephalography (encephalon = brain), or EEG, is the physiological method of choice for recording all electrical activity generated by the brain from electrodes placed on the scalp surface. The EEG has a very complex pattern, which is much more difficult to recognize than an ECG. The waveform varies greatly with the location of the measuring electrodes on the surface of the scalp.
Neuronal Communication
EEG Measurement: Types of Electrodes
Several types of electrodes may be used to record EEG. These include:
- Peel electrodes
- Stick electrodes
- Silver-plated cup electrodes
- Needle electrodes
EEG electrodes are smaller than ECG electrodes.
Placement of Electrodes in EEG
The 10/20 system or International 10/20 System is an internationally recognized method to describe the location of the scalp electrodes. The system is based on the relationship between the location of an electrode and the underlying area of cerebral cortex. The numbers ‘10’ and ‘20’ refer to the fact that the distance between adjacent electrodes are either 10% or 20% of the total front-back or right-left distance of the skull.
Electromyogram (EMG)
Electromyography is the science of recording and interpreting the electrical activity of muscle action potentials. The recording of peripheral nerve action potentials is called electroneurography. The electrical activity of the underlying muscle can be measured by placing surface electrodes on the skin. To record the action potentials of individual motor neurons in a muscle, the needle electrode is inserted into the muscle. Thus, EMG indicates the amount of activity of a given muscle or a group of muscles.
Nerve Conduction Velocity Measurements
The measurement of conduction velocity in motor nerves is used to indicate the location and type of nerve damage. Here, the nerve function is examined directly at the various segments of the nerve by stimulating it with a brief electrical shock (pulse duration of 0.2 to 0.5 milliseconds) and measuring the latencies. This allows calculation of the conduction velocity in that nerve. Latency is defined as the elapsed time between the stimulating impulse and the muscle action potential.
Respiratory Parameters & Lung Volumes
- Tidal Volume (TV): The volume of gas inspired or expired during normal quiet breathing.
- Minute Volume (MV): The volume of gas exchanged per minute during quiet breathing. It is equal to the tidal volume multiplied by the breathing rate.
- Alveolar Ventilation (AV): The volume of fresh air entering the alveoli with each breath. AV = (Breathing rate) x (Tidal volume – Dead space)
- Inspiratory Reserve Volume (IRV): The volume of gas that can be inspired from a normal end-tidal volume. IRV = VC – (TV + FRC)
- Expiratory Reserve Volume (ERV): The volume of gas remaining after a normal expiration less the volume remaining after a forced expiration. ERV = FRC – RV
- Residual Volume: The volume of gas remaining in the lungs after a forced expiration.
- Functional Residual Capacity (FRC): The volume of gas remaining in the lungs after normal expiration.
- Total Lung Capacity (TLC): The volume of gas in the lungs at the point of maximal inspiration. TLC = VC + RV
- Vital Capacity (VC): The greatest volume of gas that can be inspired by voluntary effort after maximal expiration, irrespective of time.
- Inspiratory Capacity (IC): The maximum volume that can be inspired from the resting end-expiratory position.
- Dead Space: Dead space is the functional volume of the lung that does not participate in gas exchange.
- Compliance (C): Change in lung volume resulting from unit change in trans-pulmonary pressure (PL).
- Chest-Wall Compliance (Ccw): Change in volume across the chest wall resulting from unit change in trans-chest wall pressure.
- Elastance (E): Reciprocal of compliance. Units are cmH2O/litre.
- Forced Vital Capacity (FVC): This is the total amount of air that can be forcibly expired as quickly as possible after taking the deepest possible breath.