X-rays

Question 1

What are X-rays and why are they used?

Answer 1

• Ionising electromagnetic radiation
• Short wavelength and high frequency
• Penetrating
• Considered as packet of energy, called a ‘photon’
• X-rays interact with electrons of patient
• Fraction of X-rays can pass through tissue and can be detected to form an imagine –> the X-rays that pass-through produce black image; X-rays that are absorbed are seen as white on image
• Attenuation/absorption of X-rays increases with higher density and atomic number
• Bowel gas is black as it absorbs no photons; soft tissue appears grey; bone is white

X-ray tube projects a beam of X-rays through the patient to the detector which produces an image.

X-ray source –> Objecy —> detector —> image

Question 2

What is the X-ray source?

Answer 2

The X-ray tube is an electrical device consisting of cathode (-)(filament – causes emission of electrons (thermionic emission) and anode (+) (target – produces the X-rays). It converts electrical energy into X-rays and heat. Electrical current flows through from cathode at high voltage to anode which causes the electrons to lose energy to the positive nuclei there, which in turn results in the generation of X-rays (energy is taken from electrons and is converted into photons). The closer to the nucleus the electron passes, the more energy the electron loses, and it is deflected to continue moving in a different direction at a lower energy or stopped altogether.

The cathode and anode are contained within an envelope to provide vacuum and insulation. You need an electrical supply to the X-ray tube to provide energy. We need direct current (DC) to allow the electrons to flow from cathode to anode (and not the other way round) – it helps produce smooth and consistent X-rays. The quality and quantity of X-rays are controlled by adjusting the potential difference across the tube, tube current and exposure time.

Other structures in X-ray tube:

• Rotor (allows anode to rotate rapidly)
• Oil (for heat conduction)
• Focusing cup (concentrates electron beam towards focal spot of anode)

Question 3

What is attenuation?

Answer 3

• As X-rays pass through object/patient, the X-rays are absorbed (loses its energy to patients’ electrons) so the intensity of the beam decreases exponentially – this is attenuation. The deeper the X-ray has to penetrate, the greater the attenuation. The attenuation of X-ray varies between tissues, with bone attenuating more than muscle, muscle attenuating more than lung tissue. The attenuation increases with higher atomic number, a higher density and increased thickness of the material X-rays must pass through.
• The higher the density of the tissue, the higher the electron density is, hence more X-rays are scattered and absorbed so fewer photons reach the detector. The contrast we see in an image is mainly due to the density of the tissue.
• Image contrast is due to density through body
• Some X-rays are attenuated, whilst others pass straight through and reach the detector

Question 4

What is the detector?

Answer 4

-The detector turns X-rays into measurable energy (i.e., electricity) that can be reconstructed as an image.

There are different types of detectors:

Plain film hardcopy: a film processor with tanks of chemicals such as silver nitrate which turns black when exposed to X-rays. This provides a high-resolution image. It is an analogue method

Computed radiology computer copy: phosphor plate is used with a special laser scanner that reads and forms a digital image. The benefit of this is that the image can be digitally enhanced and measured. This is a digital method

Digital radiography: A flat panel detector is used with a fully digitised system

Question 5

How would you describe the image?

Answer 5

• Analogue = measure of brightness of colour on the film; it’s a fixed image that can be copied but may degrade as it gets copied.
• Digital = image information is stored as digital numbers (array of numbers) but converted to analogue. Can be stored in a computer; can be altered and copied without degradation. The image is made up of certain number of pixels along an x-axis and y-axis. Behind each pixel is a certain number of bits in depth (certain amount of storage space). Easy retrieval, easy analysis by altering brightness or contrast

Question 6

What is spatial resolution and contrast resolution?

Answer 6

Spatial resolution:
-Shortest distance between 2 objects when they can still be seen as 2 separate objects and not just one blob. Poor spatial resolution means the objects merge and we can’t identify them as being separate

Contrast resolution:

• Contrast resolution = signal/noise. Usually, a ratio of 2 is needed to detect objects (depends on object size, image brightness)
• Image contrast is due to the difference in image characteristics between 2 locations on the image.
• Contrast is the signal we are trying to detect, whilst the noise is the random signal produced due to the other tissues and imperfect nature of X-rays. The signal may not be detectable if the noise is too high. The image noise affects how well low contrast objects can be seen.

Question 7

What are the limitation of X-rays?

Answer 7

Planar X-ray images have poor sensitivity to low object contrast due to image being a 2D shadow-gram, so we get overlap of different tissues within the body as there is only a small difference in X-ray attenuation between tissues. However, we can see object contrast differences of a few %, but this depends on tissue location and size of the object

-Shadow imaging – reduced contrast in the image due to overlapping tissues

Question 8

What is geometric accuracy?

Answer 8

• All X-rays beam originate from a focal spot in the X-ray tube, and they spread out towards the detector, passing through the patient en route
• The object size on the film depends on 2 distances = the focus to object distance and object to detector distance
• This means we must take the sizes of objects in a planar x-ray with caution. The image size is more representative in 3D imaging (e.g., CT)

Question 9

What is efficiency?

Answer 9

This is another measure of performance.
A high efficiency is a high output signal from a low input radiation. Efficiency is important as the aim is to reduce the radiation dose by increasing detection efficiency. Reduces radiation risk.

Question 10

What is the dynamic range?

Answer 10

-This is the difference between the minimal signal and the maximum signal a system can detect. A poor dynamic range was one of the major limitations of plain film images.

Question 11

What are the different uses of X-rays?

Answer 11

• Diagnostic X-rays for skeletal issues (e.g clavicle, radius/ulna, proximal phalanx)
• Dental X-rays (low dose radiation)
• Chest X-rays (e.g squamous cell lung cancer, large pulmonary embolism)
• Mammography (e.g benign microcalcifications –> needs excellent low concentration detection and good spatial resolution at low radiation dose
• Fluoroscopy – real time imaging. Catheter is fed inside an artery and radio-opaque dye is injected. It shows blood flow inside vessels and can be used to assist with interventions
• Coronary angiography – real time imaging using fluoroscopy. Cardiac catheter is fed inside aorta. Radio-opaque contrast agent is used to identify areas of occlusion. Treatment may be balloon angioplasty or stent

Question 12

How can X-rays cause tissue damage?

Answer 12

• Mainly damages tissue by indirect action. When X-rays hit water, it splits to form H+ and hydroxyl free radicals. These radicals are highly reactive and go on to form hydrogen peroxide or to directly damage cellular DNA causing a biological response (e.g death or cancer or something genetic passed on to the next generation)
• X-rays can directly damage cellular DNA causing mutations and biological responses

Question 13

What are contrast studies?

Answer 13

-Barium contrast agent = it has a high atomic number so absorbs a lot of X-rays. Used for GI imaging to help create better images of GIT (as it’s full of gas). They can help look for vascular disease, coronary disease and cerebral circulation, urethra, uterine tubes and ducts. Can be used intravenously and intra-arterially.

Question 14

What are the views of X-rays?

Answer 14

• Posteroanterior (standard): Patient faces receptor. Patient maximally inspiring
• Lateral: Left side facing receptor with arms raised at 90 degrees. Allows X-rays to travel from left to right
• Anteroposterior: Patient lies down on top of receptor; X-rays travels through patient from front to back. Used when patient can’t sit up of stand up. Less revealing than posteroanterior view. Enlarged appearance of heart and mediastinum

Question 15

What is the systematic analysis of X-rays?

Answer 15

Air, airway, apices
Bones
Cardiac shadow, cardiovascular system
Diaphragm
Edges, effusions, extra thoracic soft tissues
Foreign bodies
Gastric bubble, great vessels
Hilum
Impression

Question 16

What would you look for in airways and apices?

Answer 16

Air is dark in the lungs (not black as there’s still tissue there but it’s still quite dark) – normal

• Look for regions of density in the lungs - are they localised (e.g in one lobe, or in just the bases) or are they diffuse (throughout entire lung field). Do they obscure the diaphragm? If patient presented with cough, fever, shortness of breath and a lobe of one of the lungs appears dense (more white/grey in colour), the patient may have pneumonia.
• If there appears to be a density with well-defined borders in one of the lungs of a patient experiencing cough and weight loss, the mass lesion may represent a malignancy
• If the lungs demonstrate diffuse markings that appear to follow the vasculature and are visible all the way to the periphery of the lungs, the patient may have vascular congestion which can be due to heart failure.
• Look at fissures – they can demonstrate thickening or excess fluid
• Look for areas of lucency – is there a darker bubble in the apex (topmost portion) of lung, representing a ruptured bleb and subsequent pneumothorax (free air in the thoracic cavity) – if the pneumothorax is large enough, it can even cause lung to collapse
• Trachea – check its in the midline, if it’s not in the midline, see if there are any foreign bodies in the bronchi, midline shift of other mediastinal structures or atelectasis (collapse of lung)
• Hilar region (where trachea bifurcates) – see if there is bilateral hilar lymphadenopathy (enlarged lymph nodes on both left and right) – can see this in the X-ray

Question 17

What would you look for in bones?

Answer 17

• Symmetry: look for asymmetry between the bones on the left and right slides
• Look for lesions: are any areas patchy? Light and patchy could present an old break with healed, reformed sclerotic bone. A really dark or light spot on one or more of the bones could be a lytic or blastic lesion
• Look at edges of bone – bridge edges are the bone cortex which you should be able to follow all the way around the surface. If the cortex appears interrupted, it could signify the presence of a break, or even cancer (like osteosarcoma)

Question 18

What do you look for in cardiac silhoute and CVS?

Answer 18

• Heart should be in the left hemithorax – check for dextrocardia/situs invertus/Kartagener syndrome (presents with situs invertus, lung infections and infertility)
• Size of heart – width of cardiac shadow. It should be less than 50% of diameter of chest. If more, it could be cardiomegaly (make sure you know which view it is – PA or AP
• Look for vessels. There may be mass or aortic aneurysm or aortic dissection

Question 19

What do you look for in diaphgram?

Answer 19

• Shape. Usually curved; the right hemidiaphragm is higher than the left because it sits on top of the liver. If it looks flattened, the patient may have chronic asthma or COPD
• Look for free air under the diaphragm  can signify a gastric or intestinal perforation which requires emergency surgery

Question 20

What do you look for in effusions, edges and extra-thoracic tissues?

Answer 20

-Check for costophrenic angles at the peripheral edges of the diaphragm – blunt edges could signify pleural effusion (a fluid collection between the pleura and lung tissue)

Question 21

What do you look for in foreign bodies?

Answer 21

-e.g lines, nasogastric, orogastric tubes, cardiac monitor leads and others

Question 22

What do you look for in gastric bubble and great vessels?

Answer 22

-Observe gastric bubble (bubble of air in the stomach) on left side

Question 23

How do we detect pneumonia on X-rays?

Answer 23

• Pneumonia is a lung infection whereby there is swelling of the alveoli (filling of alveoli)
• Chest X-rays in a posteroanterior view should be carried out. It should be interpreted with the symptoms of the patient
• X-rays are useful here because they create contrast images (based on different densities of materials). In a patient with pneumonia, the lobes would appear white as there is greater attenuation of X-rays by the fluid in the alveoli compared to air in the lungs and lung parenchyma as the fluid has a higher density than air and lung parenchyma – this is lobar consolidation. On a chest X-ray you probably wouldn’t be able to tell what the fluid is but if we did a CT scan, it could help us identify the fluid (e.g blood, pus, vomit).
• However, we have to be wary as lobar consolidation could also be caused by other things such as lung scarring and congestive heart failure. When the diagnosis is not clear, we use CT.

Chest X-ray vs CT:

• CT gives 3D image; greater contrast; can identify the fluid
• Chest X-ray uses less ionising radiation

Question 24

How would we detect pneumothroax on X-rays?

Answer 24

• Pneumothorax occurs when air enters the space between the parietal and visceral pleura which reduces the surface tension and causes the lung to collapse.
• Serous fluid becomes replaced with air which has a lower density so there is less attenuation of X-rays by air, so it appears black. The pleural air displaces the lung so you get a hypodense region of just air and you can’t see the vessels. The pleural air pushes the vessels in the lung inwards and down (collapse of lung)
• Sometimes, you can have the silhouette sign which is the loss of the left or right border of the heart because it looks similar to the collapsed lung
• Sometimes, pneumothorax can be identified better in a chest X-ray where the patient is expiring as it may be easier to detect the air between the lung and pleural space.

Question 25

How can you detect dextocardia/situs inverstus?

Answer 25

-You can see the apex of the heart and the gastric bubble on the right-hand side (normally, both should be seen on the left). Under the left dome of the diaphragm, we can see the liver. You need to make sure that this isn’t just a mistake by the radiographer and it should be compared to previous imaging

Question 26

What is mammography?

Answer 26

-Using X-rays to detect breast cancer and other breast diseases

• search for high density regions or areas of unusual configuration that look different from normal tissue
• including cancerous tumors, non-cancerous masses called benign tumors, fibroadenomas, or complex cysts. Radiologists look at the size, shape, and contrast of an abnormal region, as well as the appearance of the edges or margins of such an area, all of which can indicate the possibility of malignancy (i.e. cancer).
• They also look for tiny bits of calcium, called microcalcifications, which show up as very bright specks on a mammogram. While usually benign, sites of microcalcifications may occasionally signal the presence of a specific type of cancer.
• compression of breast evens out the shape of the breast so that the x-rays can travel through a shorter path to reach the detector. This reduces the radiation dose and improves the quality of the x-ray image. , compression allows all the tissues to be visualized in a single plane so that small abnormalities are less likely to be obscured by overlying breast tissue.