Medical Physics

Medical physics is a branch of physics that applies the principles and techniques of physics to the field of medicine. It plays a crucial role in the diagnosis, treatment, and monitoring of various medical conditions. This set of notes will provide an overview of key concepts and applications of medical physics, including imaging techniques, radiation therapy, and biomedical instrumentation.

Imaging Techniques:

1.1 X-ray Imaging:

X-rays are electromagnetic waves used in medical imaging to produce images of bones and organs. X-ray machines emit X-rays that pass through the body, and the transmitted X-rays are detected by an image receptor (such as film or digital detectors). Different tissues absorb X-rays to varying degrees, resulting in contrasting images on the detector. X-ray images help diagnose fractures, lung diseases, tumors, and other conditions.

1.2 Computed Tomography (CT) Imaging:

CT scans provide detailed cross-sectional images of the body. X-ray beams rotate around the patient, and detectors measure the transmitted X-rays from different angles. A computer processes the measurements to create cross-sectional images. CT scans are widely used for identifying tumors, assessing trauma, and guiding radiation therapy.

1.3 Magnetic Resonance Imaging (MRI):

MRI uses a strong magnetic field and radio waves to generate detailed images of soft tissues. Hydrogen atoms in the body align with the magnetic field and emit radio waves when excited. These radio waves are detected and processed to construct high-resolution images. MRI is particularly useful for imaging the brain, spinal cord, joints, and abdominal organs.

Radiation Therapy:

2.1 External Beam Radiation Therapy:

Radiation therapy uses high-energy radiation to destroy cancer cells or shrink tumors. External beam radiation therapy delivers radiation from a machine outside the body. Precise targeting and dose calculation ensure maximum tumor destruction while minimizing damage to healthy tissues. Techniques such as intensity-modulated radiation therapy (IMRT) and stereotactic body radiation therapy (SBRT) improve treatment accuracy.

2.2 Brachytherapy:

Brachytherapy involves the placement of radioactive sources directly into or near the tumor. Radioactive seeds or wires emit radiation that delivers a high dose to the tumor while sparing surrounding healthy tissues. Brachytherapy is commonly used for treating prostate, cervical, and breast cancers.

Biomedical Instrumentation:

3.1 Electrocardiography (ECG):

ECG measures the electrical activity of the heart to diagnose heart conditions. Electrodes placed on the skin detect the electrical signals produced by the heart. These signals are recorded and displayed as a graph, showing the heart's rhythm and abnormalities.

3.2 Ultrasound Imaging:

Ultrasound uses high-frequency sound waves to produce images of organs and tissues. Sound waves are emitted by a transducer and reflected back from structures in the body. The reflected waves are detected and processed to create real-time images. Ultrasound is safe, non-invasive, and commonly used for prenatal imaging, abdominal scans, and cardiac evaluations.

The Physics of Scanning Techniques

Scanning techniques play a crucial role in various fields, including medicine, engineering, and scientific research. These techniques utilize fundamental principles of physics to generate detailed images and gather essential data. This set of notes will explore the physics behind three common scanning techniques: ultrasound, computed tomography (CT), and magnetic resonance imaging (MRI).

Ultrasound Imaging:

1.1 Principles of Ultrasound:

Ultrasound utilizes the properties of sound waves to create images of internal structures. Sound waves are mechanical waves that propagate through a medium, such as tissue or water. Ultrasound waves have frequencies above the audible range (>20,000 Hz) for human hearing. Transducers, which convert electrical energy into ultrasound waves, emit and receive these waves.

1.2 Reflection, Transmission, and Absorption:

When an ultrasound wave encounters a boundary between two different media (e.g., tissue and bone), some of the wave is reflected back, some is transmitted through, and some is absorbed. The reflected waves are detected by the transducer and used to create an image. The intensity of the reflected waves depends on the density and acoustic impedance of the tissues.

1.3 Doppler Effect in Ultrasound:

The Doppler effect is used in ultrasound to measure the velocity and direction of blood flow or motion within tissues. When there is relative motion between the ultrasound source (transducer) and the reflecting object (e.g., red blood cells), the frequency of the reflected waves changes. By analyzing this frequency shift, the Doppler effect allows the determination of blood flow direction and speed.

Computed Tomography (CT) Imaging:

2.1 X-ray Generation and Detection:

CT imaging employs X-rays to create detailed cross-sectional images of the body. X-ray tubes emit a beam of X-rays that pass through the body. Detectors measure the intensity of the transmitted X-rays, which provides information about the attenuation of X-rays within the tissues.

2.2 Data Acquisition and Image Reconstruction:

During a CT scan, the X-ray tube and detectors rotate around the patient. Multiple X-ray projections are acquired from different angles. Advanced algorithms, such as filtered back-projection or iterative reconstruction, are used to reconstruct cross-sectional images from these projections. The resulting images represent the X-ray attenuation characteristics of different tissues.

Magnetic Resonance Imaging (MRI):

3.1 Principles of MRI:

MRI uses a strong magnetic field and radio waves to generate detailed images of the body's internal structures. The body is placed in a powerful magnet that aligns the nuclear spins of hydrogen atoms. Radiofrequency pulses are then applied, perturbing the aligned spins. When the spins return to their original alignment, they emit radiofrequency signals that are detected and processed to create an image.

3.2 Tissue Contrast in MRI:

The relaxation times of nuclear spins in different tissues provide contrast in MRI images. T1 relaxation time determines the brightness of tissues in the resulting image. T2 relaxation time influences the contrast between different tissues. The manipulation of these relaxation times allows for differentiation between various structures.

In conclusion, medical physics plays a vital role in the field of medicine, combining the principles of physics with medical applications. Imaging techniques such as X-ray, CT, and MRI enable the visualization of internal structures, aiding in diagnosis. Radiation therapy techniques, including external beam radiation therapy and brachytherapy, contribute to cancer treatment. Biomedical instrumentation, such as ECG and ultrasound, provides valuable diagnostic information. Understanding these concepts is essential for physicists and healthcare professionals working together to improve patient care and treatment outcomes in the field of medical physics. Scanning techniques, including ultrasound, CT, and MRI, rely on the fundamental principles of physics to generate images and gather information about internal structures. Understanding the physics behind these techniques enables us to optimize imaging parameters, interpret images accurately, and develop advancements in the field. These scanning techniques have revolutionized medical diagnostics and continue to contribute to advancements in other disciplines, highlighting the significant role of physics in improving our understanding of the world around us.