DEM Lecture - Basic Radiological Imaging for Musculoskeletal Medicine 2023.pptx

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Basic Radiological Imaging for Musculoskeletal Medicine Class Course Lecturer Date DEM Year 1 Body Movement and Function (BMF) Dr. Kenny Winser November 2023 Lecture Learning Outcomes • Discuss the fundamentals of medi...

RCSI Royal College of Surgeons in Ireland Coláiste Ríoga na Máinleá in Éirinn Basic Radiological Imaging for Musculoskeletal Medicine Class Course Lecturer Date DEM Year 1 Body Movement and Function (BMF) Dr. Kenny Winser November 2023 Lecture Learning Outcomes • Discuss the fundamentals of medical imaging • Define the meaning of ionizing radiation • Discuss different imaging modalities for musculoskeletal imaging Lecture Outline Two threads to the lecture: • What is a medical image? • Ionizing vs non-ionizing radiation Imaging modalities for MSK • X-rays (basic ‘shadow’ images, DEXA, CT) • MRI • Ultrasound The Body: Movement and Function – Case Slide The content from this lecture will feature in both of the clinical cases for this module, in that the basics of the imaging techniques used in assessing these cases will be outlined and specific examples illustrated. Digital images are stored as a series of pixels in a computers memory, where 1 pixel is represented by a square box whose intensity or brightness corresponds to the ‘brightness’ of the object at a specific location. Each pixel is filled with only ONE colour In general….. • A template or ‘computer generated grid’ is constructed, which consists of a 2-d or 3-d array. • Each element in this array corresponds to a specific physical location within the patient (in the region to be studied). • Each element of the array has a colour and/or brightness value assigned to it (defined by the user) which depends on the value of some measured parameter such as absorption, emission of radiation, etc.. All non-visual, or computerised medical imaging systems carry out the following simple procedure. Identify what is to be imaged. Apply the appropriate technology. Measure the received signal as a function of location. Each element of the matrix corresponds to a physical position within the region of study The numerical value of each matrix element represents the strength of the signal received at that position. A colour scale is defined by the user whereby colours (or grey scales) are assigned to each numerical value. Ionizing Radiation We can see from the diagram that radiation with a frequency > ~1 x 1015 Hz will actually break bonds and cause electrons to be freed from an atom. Note: Frequency is directly proportional to radiation energy. For biological tissue, this can occur for radiation with wavelengths less than ~ 300 nm (UV) or photon How does the body respond to different types of electromagnetic radiation? 2-d (traditional) x-ray images X-rays are not absorbed equally well by all materials. - Heavy elements such as calcium are very good absorbers of x-rays, while lighter elements are generally poor absorbers of x-rays. Since x-rays can be used to blacken photographic film, ‘shadow pictures’ can be produced when a body is placed between an x-ray source and Photographic film Soft tissues (i.e. fat, muscle, tumour etc.) all have similar absorption characteristics for x-rays, and are therefore difficult to distinguish in standard x-ray pictures. The attenuation of an x-ray beam is the reduction in its intensity due to the absorption and/or scattering of some of the x-ray photons out of the beam (note: lambert-Beer Law). X-rays can be attenuated in biological tissue by a number of different processes! 2-d x-rays are commonly used to detect bone fractures. DEXA (Dual Energy X-ray Absorptiometry) DEXA is a scanning technique that is used to determine the density of bone, and hence it’s mechanical strength. It is considered as the gold standard method for diagnosing osteoporosis (and fracture risk). DEXA is used to measure bone mineral density (BMD) & is most DEXA works by sending two low-dose X-rays which are absorbed differently by bones and soft tissues (different attenuation coefficients). The ratio of the attenuation coefficients from these X-ray beams are used to calculate bone mineral density (BMD). The lower the density, the greater the risk of fracture. DEXA is painless and takes about 10 minutes. The amount of radiation is Bone densities are often given to patients as a T-score or a Z-score. Your BMD is compared to 2 norms—healthy young adults (your T-score) and age-matched adults (your Z-score). Your BMD is compared with the BMD results from a large sample of healthy 25- to 35-yearold adults of your same sex and ethnicity. The T-score is given as the number of Standard Deviations (SD) between your BMD and the mean BMD of the sample group. Positive Tscores indicate the bone is stronger than What does a T score tell us? World Health Organization definitions based on bone density levels Level Definition Normal Bone density is within 1 SD (+1 or −1) of the young adult mean. Low bone mass Bone density is between 1 and 2.5 SD below the young adult mean (−1 to −2.5 SD). Osteoporosis Bone density is 2.5 SD or more below the young adult mean (−2.5 SD or lower). Severe (established) osteoporosis Bone density is more than 2.5 SD below the young adult mean, and there have been one or more osteoporotic fractures. In general, the risk for bone fracture doubles with every SD below normal. Thus, a person with a BMD of 1 SD below normal (T-score of -1) has twice the risk for bone fracture as a person with a normal BMD. When this information is known, people with a high risk for bone fracture can be treated with the goal of preventing future fractures. Computerised Tomography (CT) The basic principle of CT is that the structure of an object can be ‘reconstructed’ from a series of x-ray projections taken uniformly around the Consider object. a thin pencil beam of x-rays Thin passing through a x-ray beam section of tissue from s many different directions so that all the beams have a common crossing Because all of the beams pass through the same ‘common volume’, a study of the transmission data for all of the beams will yield detailed information on the absorption within the common volume. Modern CT scanners use multiple x-ray projections to obtain detailed information on thousands of tiny volumes within a subject within a matter of seconds. When a thin x-ray beam is passed through a section of the body, the intensity of the beam on exit from the body depends on the linear attenuation coefficient (lac) – i.e. cumulative attenuation along the path of the x-ray beam – Lambert-Beer Law. CT scans consist of passing many thin x-ray beams strategically through sections of the body and determining the lac’s for each x-ray projection. These lac’s are mathematically manipulated using fast microprocessors to calculate the actual absorption as a function of position within the subject. In simple terms:  A 3-d array is constructed by the  Each elementcomputer. of the array corresponds to a specific location within the patient under assessment.  The numerical value assigned to each element corresponds to the calculated absorption at that location (using the lac’s from each projection).  A colour scheme is developed whereby a specific colour or greyscale is assigned to particular values of absorption.  The computer ‘paints by numbers’.  An image is ‘constructed’. In modern CT machines, the patient slides through a circular ‘gantry’ containing x-ray sources and detectors, creating a spiral motion, creating many x-ray projections. Liver CT scan showing significant detail. Ultrasound Ultrasound is a safe and painless way of using sound waves to look at parts of the human anatomy. The technology works on a principle similar to that used in sonar. We know the speed of the ultrasound, so measuring the time taken to detect a reflected ultrasound wave, allows us to calculate the distance to the reflective boundary. The intensity of the reflected ultrasound wave tells us about the density of the reflecting medium. Sound or pressure waves with a frequency > 20 kHz Wave Velocity (m/s) Frequency Waveleng (Hz) th (m) Sound in Air 344 20 – 20,000 20 – 0.02 Sound in Water 1500 20 – 20,000 100 – 0.1 Ultrasound in Tissue 1560 20 kHz – 20 MHz 0.1 – 0.00001 "Musculoskeletal ultrasound allows physicians to see, in high resolution, a person’s muscles, tendons, ligaments, nerves and joints,“ (Yale Medicine radiologist) Magnetic Resonance Imaging (MRI) The physics underlying MRI is complex. Protons within water molecules become aligned to the very strong magnetic field which exists within the ‘bore’ of the MRI An RF (Radio Frequency) pulse is transmitted and absorbed by the molecules causing the ‘spin’ of the protons to flip – they are now in a higher energy state. Also gives rise to a longitudinal magnetic field. In addition, all of the protons will precess in phase – gives rise to a measurable transverse magnetic field. When the RF pulse is removed, the protons deexcite, and return (exponential decay) to their lower energy, spin-up state. In doing this, they emit their excess energy as photons (in the form of radio waves). Also the longitudinal magnetic field decays. Protons from different tissues return to their equilibrium states at different rates – a difference that can be detected. Measurement of the recovery times (Spin-Lattice Relaxation or T1, and SpinSpin Relaxation or T2) allows us to distinguish (contrast) between different tissue types. By using a ‘gradient magnetic field’, we know from the energy of the emitted RF photons, precisely in which plane the MRI images of shoulder and cervical spine