Lectures 1,2: Intro, Medical Data Sampling & Acquisition Flashcards
1.1 Describe the Hypothetico-Deductive Approach to patient treatment.
1.2 Name the 5 main types of medical data used in the practice of medicine and allied health sciences.
- narrative, textual data
- numerical measurements
- recorded signals
- drawings
- photographs, images
1.3 Describe “narrative data”
Verbal descriptions in the form of textual recordings (written, typed) made by the physician in response to his/her focused questions about a patient’s illness/symptoms. Stored as medical records, narrative data accounts for a large component of info that is gathered in the care of patients.
For example, shorthand code is often in specific fields and practices, such the notation “PERRLA” in optometry medical records, which, regarding a patient’s eye examination, indicates that “Pupils are Equal (in size), Round, and Reactive to Light and Accommodation”
1.4 Describe the data type: “numerical measurements”
1.5 Describe the data type: “recorded signals”
1.6 Describe the data type: “visual images”
1.7 Describe the data type: “drawings”
2.1 Describe in basic terms how Conventional X-ray Imaging works, compared with Digital X-Ray Imaging.
X-Ray photos are produced when electrons of high energy strike a heavy metal target, e.g. Tungsten. X-Ray imaging is useful in viewing the body as absorption is proportional to tissue density. X-rays require contrast differences to outline structure. Contrast media can be used to outline vessels, lumina etc.
Digital X-Ray imaging uses an X-Ray detector, instead of film, where X-Ray photons excite a phosphor screen. CCDs (Charge-Coupled Device) and TFTs (Thin Film Transistor) convert light from phosphor to digital signal. Thus, this is known as ‘filmless radiology’, and images can be digitally stored, transmitted & manipulated (contrast and resolution, 2k x 2k x 16 bits grey scale).
2.2 What are the differences between CT and X-Ray imaging?
X-ray images are in two-dimension (2D) format, and the images of different organs are usually super-positioned on top of each other. Thus, it is sometimes hard to distinguish different organ structures.
Computer Tomography (CT) involves utilising X-ray technology with advanced digital image processing and reconstruction techniques. X-ray CT scanning at different angles can generate a cross-sectional “slice” display of the body.
The various densities of body materials will absorb the radiation at different levels, measured by the Hounsfield Attenuation scale, where air has a value of -1000 HU and shows up dark (black) on a CT scan (e.g. in the lungs), whilst much denser bone has a value of > +400 HU and shows up light (white).
Detail:
A rotating X-ray source provides illumination and the portion of the sensors opposite the source collect the X-ray energy that pass through the object (body). This is the basis for computerised axial tomography (CAT).
The output of the sensors must be processed by image reconstruction algorithms which can transform the sensed data into meaningful cross-sectional images, i.e. the images are not obtained directly from the sensors but require extensive processing.
A 3D digital volume consisting of stacked images is generated since the object is moving in a direction perpendicular to the sensor ring.
2.3 Blood will not show up in a standard X-ray. Describe how the blood vessels may be imaged to circumvent this, and an example of its use.
As blood as the same radio-density as the surrounding tissues, a radio-contrast agent (which absorbs X-rays) is added to the blood to make angiography visualisation possible.
For example, cerebral angiograms may be obtained by injecting an iodine-containing fluid into the arteries. The contrast dye subsequently fills the cerebral arteries, capillaries and veins. This may be helpful in the detection of an aneurysm or saccular dilation of a cerebral artery.
2.4 Explain how MRI works
Quick
MRI uses a powerful magnetic field (MF) to align the magnetisation of hydrogen (H) atoms in the body. Radio waves are used to systematically alter the alignment of this magnetisation, causing the H atoms to emit a weak radio signal, which is amplified by the scanner. This signal can be manipulated by additional MFs to build up enough info to reconstruction an image of the body.
Longer
MRI modality forms images of objects by measuring the magnetic moments of protons using radio frequency (RF) and a strong magnetic field (MF).
When a person is in the scanner, the hydrogen nuclei (protons, abundant in the H2O of the body) align with the strong MF. A radio wave at just the right frequency for the protons to absorb energy pushes some of the protons out of alignment. The protons then snap back to alignment, producing a detectable rotating magnetic field as they do so. Since protons in different areas of the body (e.g. fat vs. muscle) realign at different speeds, the different structures of the body can be revealed.
2.5 What are the advantages of using the MRI modality?
The MRI scanning mechanism is completely electronic, requiring no moving parts to perform a scan. IT reveals fine anatomical detail, yet is non-invasive and does not require ionising radiation such as X-rays, which is better for the health of the patient. Like X-ray CT, MRI imaging enables direct imaging and differentiation of soft tissue structures like liver, lung tissue and fat, without super-positioning. The allow physicians to obtain different angles of view, such as trans-axial view, sagittal view, coronal view and 3D rendering views.
It is a highly flexible technique so that contrast between one tissue and another in an image can be varied simply by varying the way the image is made. For example, a T1 weighted image depicts cerebrospinal fluid (CSF) as black, and in T2, white, with this contrast manipulated during image acquisition just by adjusting several parameters. These include the Repetition Time and the Echo Time (TR and TE respectively, milliseconds), which control the sensitivity of the signal to the local tissue relaxation times (T1, T2) and the local proton density.
Extra on T1/T2:
T1-weighted images or “anatomy scans” show most clearly the boundaries between different tissues as a result of the excellent contrast: fluids are very dark, water-based tissues are mid-grey and fat-based tissues are very bright.
T2-weighted images or “pathology scans” reveal collections of abnormal fluid bright against the darker normal tissue, thus are useful in obtaining images for diagnosis. On these scans, fluids have the greatest intensity (white) and water/fat-based tissues are mid-grey.
2.6 Explain & Describe how Ultrasound imaging works and its advantages/disadvantages.
Advantage: ultrasonic imaging is real-time, non-invasive, portable, and inexpensive compared with other clinical imaging modalities.
It is based on the reflection of ultrasound at tissue interfaces. Large, smooth surfaces in a body cause specular reflection, whereas rough surfaces and regions cause non-specular reflection or diffuse scatter.
It can be used to image muscle and soft tissue and is particularly useful for delineating the interfaces between solid and fluid-filled spaces, such as pregnancy check-ups. However, organs located under bone such as the brain cannot be imaged clearly.
It renders “live” images, where the operator can dynamically select the most useful section for diagnosing and documenting changes, often enabling rapid diagnoses.
2.7 Describe PET and SPECT and provide an example of their use.
Dynamic functional imaging techniques that allow the in vivo study of physiological processes: estimated by observing the behaviour of a small quantity of an administered substance (tracer) “tagged” with radioactive atoms (isotope). Images are formed by the external detection of gamma rays emitted from the patient when the radioactive atoms decay.
- The nucleus of the radioisotope (e.g. Fluorine-18) emits a positron (positive electron).
- This collides with an electron in the tissue and in the process converts mass to energy (E=mc2) in the form of 2 photons.
- The PET camera uses scintillation crystals placed around the subject to detect these photons.
- The crystals absorb the photons, producing light that is converted into an electrical signal.
Advantage: unlike anatomical imaging, functional PET/SPECT imaging can provide image wide quantification of physiological & biochemical processes within the body.
Example: cancer - malignant cells often exhibit elevated glucose metabolism (anaerobic glycolysis); the amount of abnormally increased glucose accumulating in tumour tissue signifies the degree of malignant aggressiveness. Any significant metabolic change within the tumour tissue during therapy can be established by comparing the uptake values between pre- and post-treatment PET scans. Therefore, whole-body PET imaging has become a standard procedure for imaging cancer.
2.8 Describe the differences between SPECT and X-ray CT
The main difference of image acquisition between SPECT and X-ray CT is the source of radiation used.
SPECT: radionuclide injected/inhaled into patient body is used as a source of an external X-ray energy. The signal in SPECT is the attenuation of gamma rays during their flight from the emitting nuclei to the detectors.
2.9 Describe how X-ray CT reconstructs images from projections using the Algebraic Reconstruction Technique (ART)
A 2D object is reconstructed from several 1D projections, each obtained via a parallel X-ray beam passing through the object. Planar projects are thus obtained by viewing the object form many different angles (rotating nature of the CT Scanner). Reconstruction algorithms using linear algebra derive an image of a thin axial slice of the object, giving an inside view otherwise unobtainable without performing extensive surgery.
ART is based on the very general premise that the resultant reconstruction should match the measured projections. See attached image for the defined iterative algorithm.
(Go through iterative example in either method [additive method or dividing by dimensions method])
