Dear students,
The aim of the session is to link the pathology which you have learnt in the chest, to what is visible on the two most commonly used imaging tests you will use as doctors in day to day clinical practice - the CXR, and CT scan of the Chest.
As preparation for this session, please revise the material from the year 1 session 2020: Imaging of the Chest and Lungs - Online resources to support lecture for year 1 MBBS (NUS). The material below is for the 2020 cohort, which I hope you find useful to refresh your memory of Chest Anatomy-Radiology correlation.
The aim of the year 1 material is to link the anatomy which you have learnt in the chest, to what is visible on the two most commonly used imaging tests you will use as doctors in day to day clinical practice - the CXR, and CT scan of the Chest. Radiology allows you to see 'living anatomy' and pathology, of your patients in vivo. The CXR allows you to look inside your patient in 2D, while the CT scan gives you multi-planar sectional details, and a '3D-like' appreciation of both anatomy and pathology. You should aim to develop the ability to orientate yourself to the visible structures when viewing both the CXR, and CT scans; and be able to identify normal anatomy. This forms the foundation for the study of pathology, and the appreciation of visible pathology in your patients using CXRs and CT scans in year 2, and subsequent clinical years.
Year 1 learning path: Start with CXR video 1, then CXR video 2, then CXR anatomy website (see chest section) (using illustrations as drill and quiz examples, by viewing CXR first without annotations, then mousing over each image, then read text); then view CT chest video 1, then CT chest video 2, then attempt to identify CT anatomy here.
Year 2 pre-session preparation, and self learning path: either review material below as segmented text and illustrations, or presented as a segmented video.
http://vidcast.nus.edu.sg/camtasiarelay/Imaging_of_Respiratory_Tract_Disorders_-_20141105_151527_25.html (link to video presentation of Year 2 lecture)
Additional open access online resources
I highlight some of this material during the session, and will take 'live Q and A' during the scheduled 'lecture time'.
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(e)Lecture Outline
Section 1: Introduction
and
Pre-lecture reflection questions / recall and review if necessary Y1 material
Section 2: Learning objectives
Section 3: Pre-test
Section 4: Lecture proper
Part 1 - review of basic principles of CXR production and tissue characterization
Part 2 - review of the CXR findings in six major diseases
Section 5: Post-test
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Section 1: Introduction
Please reflect on each of the following questions, pause after each question, write down your answer, and reflect upon your answer.
1. What is the relevance of diagnostic imaging/radiology in your future clinical practice?
2. How are CXRs and CT scans of the chest produced?
3. How do different tissues appear on CXRs and CT scans? Why does bone appear white? Air black? Soft tissues varying shades of grey?
4. Can you identify normal anatomy on a CXR? CT scan of the chest?
5. Do you know where to find the lecture material on this topic presented in Year 1?
If you are unsure of the answer to any of these questions, please revisit and review the relevant sections in the Y1 lecture online resources on the websites below.
Year 1 learning path: Start with CXR video 1, then CXR video 2, then CXR anatomy website (see chest section) (using illustrations as drill and quiz examples, by viewing CXR first without annotations, then mousing over each image, then read text); then view CT chest video 1, then CT chest video 2, then attempt to identify CT anatomy here.
Section 2 : Learning objectives of this (e)Lecture
Recall that radiology allows you (as future doctors), to see "living" anatomy, and "in vivo" pathology.
This ability to visualise what is going on in the patient in front of you, in both health and disease will be a useful diagnostic tool for you as doctors.
The easiest way to make sense of what you see on a radiology examination is to recall what you have been exposed to and learnt recently in gross pathology.
We will focus on the Chest Radiograph in this lecture. This is the most commonly requested radiology investigation. While this lecture won't show you every possible abnormality visible on CXRs, it is the start of a learning and skill development process. We will spend time on typical presentations of several common and important clinical conditions, and use these to illustrate the basics of CXR interpretation. As you develop experience over the next few years, you will gradually become more familiar with more subtle or gross presentations of disease, with more atypical features.
As you no doubt realise, an XR or scan is a snapshot of a point in time of a developing disease process. Early on, the manifestations of a disease on an XR or scan might be small, ill defined, or difficult to visualise. Later in the disease process, an abnormal feature might be large, and also difficult to define (for example a small, moderate size or large pleural effusion). It may be initially difficult for you to determine with a completely opaque hemithorax whether you are dealing with pneumonia, a large effusion or complete collapse of the lung.
For those of you who are focused on more immediate concerns, the assessment items on radiology that you will be faced with will evaluate your ability to recognise major examples of pathology on common radiological examinations. For example on the chest radiograph or CXR.
To review again the learning objectives of the undergraduate radiology program in the medical curriculum, you can see how radiology translates what you have learnt in Y1 anatomy, to give you the ability to see "in vivo" living anatomy in your future patients.
And visualise in vivo pathology in your patients.
Section 3 : Pre-test
Let us do an assessment exercise now to not only show you what potential future examination assessment items might be, but also to illustrate how radiology (on the following CXRs) allows you to visualise gross pathology in your patient.
Google image search "lung gross pathology lung cancer"
Google image search "patient with lung cancer"
Google image search for "lung gross pathology pneumothorax"
Google image search "patient with pneumothorax"
Google image search "patient with tension pneumothorax"
Google image search for "lung gross pathology pneumonia"
Google image search "patient with pneumonia"
Google image search for "lung gross pathology pleural effusion"
Google image search "patient with pleural effusion"
Google image search "lung gross pathology cardiac failure pulmonary odema"
Google image search "patient with pulmonary oedema"
Google image search "lung gross pathology cardiac failure alveolar pulmonary odema"
Google image search "lung gross pathology cardiac failure interstitial pulmonary oedema"
Google image search for "gross pathology rib fractures"
Google image search for "patient with rib fractures"
Google image search for "cadaver rib fractures"
Please try and match the five diagnoses (A to E) with the CXRs provided (1 to 6). There are two examples on the CXRs provided of one of the five diagnoses.
This exercise begins the process to introducing you to the typical appearance of common and important clinical problems that your patient may present with.
Section 4: (e)Lecture proper
We will focus on two major areas. Firstly review basic principles of CXR production and interpretation. And then review the key features of six major clinical problems on CXRs.
We first very briefly review basic principles behind the production of a CXR, and why different tissues have different densities on XRs (white, shades of grey, and black).
Recall that XRs are produced by placing you patient between an XR source, and a recording medium; which may be an XR film, or digital recording plate. The XR is therefore a record of the absorption of XRs as they pass through different organs and tissues in your patient.
By convention, on a XR, black represents areas of greatest XR absorption, and white the least absorption of XRs. On this normal CXR, you can see the radiographic densities of five categories of tissue. Air being blackest, with gradually lighter shades of grey with fat, soft tissue/blood/muscle, bone and metal. You will appreciate how fat being less dense than soft tissue will absorb less XRs, and appear a darker shade of grey than soft tissues or muscle.
This difference in XR absorption between different tissues and organs allows you to distinguish the edge or surface between different tissue layers and organs. Because XRs travel in straight lines through your patient, the interface between different tissues is highlighted and visible at tissue interfaces tangential to the path of the XR beam. This is referred to as the "silhouette sign". A simple analogy help you visualize this is to recall the what the silhouette of an object looks like when placed between a candle or light source and a background surface. The edge of the projected "shadow" is the silhouette making the edge between absorbed and transmitted light.
We use the silhouette phenomenon on a CXR to detect the edge between the normal left heart border, and adjacent aerated normal lung which contains air. We also use this to see the normal lung markings, due to difference in XR absorption between the blood within the pulmonary vessels and the adjacent normal lung. In disease, when the alveoli or air spaces in the lung are filled with fluid, blood or pus, we lose the ability of see these edges, allowing us to infer that the air spaces in the lung are not aerated or air filled.
Finally, an appreciation of the geometry of the XR beam passing through you patient allows you to understand how the heart, which you recall lies anteriorly on the front of the chest cavity is less magnified on a PA (posterior anterior) CXR, where the beam passes from back to front of the patient, compared to an AP (anterior posterior) CXR. Because patients have different chest front to back thicknesses, an AP film does not give you a good estimate of the transverse width of the heart, compared with the internal side to side chest diameter. The ratio of the widest side to side width of the heart divided by internal chest diameter (widest at that level) should be less than 50% in patients who do not have cardiomegaly; and is more reliably assessed on PA rather than AP CXRs, since we are not able to appreciate the front to back diameter of patients on CXRs; and cannot correct for this magnification factor when viewing AP CXRs.
We will now focus on the key radiological features of a few major disease categories on the CXR.
These 6 diseases are not only common, but need to be recognised quickly, accurately and confidently by you as future doctors in the EMD, wards and clinics; as you patient may require urgent treatment.
This is also why testing your ability to recognise these diseases on radiological examinations will take place not only in the radiology section of the examination, but radiology images will also be shown to you as part of the work up and assessment of your patients.
Pneumonia is described as an area of consolidation, or air space shadowing on a CXR. This appearance may be due to pus (pneumonia), fluid (pulmonary oedema), or blood (for example lung contusion or a pulmonary infarct). A definitive diagnosis is made by correlating the radiological appearance on the CXR with the clinical setting, or clinical findings.
CT scan showing right lower lobe pneumonia (coronal section)
CT scan showing right lower lobe pneumonia (axial sections)
Changing 'window' settings on CT allows you to highlight, and view different tissues. Lung on the 'lung window', and the mediastinum on the 'mediastinum window'. This takes advantage of the different densities of lung vs mediastinum or soft tissue density on CT scans (different XR absorption). Illustrated graphically here - https://www.radiologycafe.com/medical-students/radiology-basics/ct-overview
http://vidcast.nus.edu.sg/camtasiarelay/Imaging_of_Respiratory_Tract_Disorders_-_20141105_151527_25.html (link to video presentation of Year 2 lecture, see Cardiac Failure
with Pulmonary Oedema segment @ 13 minutes on video)
In this second major section of the (e)Lecture, we will review six major diseases and their CXR findings. Please review the description of the key features of each disease, and then a typical CXR of each disease.
Section 5 : Answers to the pre-test
We conclude this (e)Lecture by revisiting the quiz presented to you at the beginning of this lecture. The answers should be quite obvious to you after this presentation, and are given on the single slide below. Please review the content of this lecture again, focusing on any area you might be unsure about. Please post any questions you might also have on the padlet digital wall below and you classmates are invited to discuss each question with you on the digital wall before the lecture. I will address these questions both live during the lecture, as well as on this blog, and on Entrada after the session.
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as of 27 October 2021, 0151am, Singapore Time
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