S9: lung radiology & tuberculosis

Question 1

What is the silhouette sign?

Answer 1

Adjacent structures of differing density form a crisp silhouette
Heart next to lung = white next to black
Loss of this contour can locate pathology

Question 2

Describe x-ray findings of a mediastinal shift

Answer 2

Adequately centred image
Push = increase volume/pressure
Pull = decrease volume/pressure

Question 3

Describe the causes of lobar lung collapse

Answer 3

Volume loss within lung lobe
Causes:
1) Luminal – aspirated foreign material, mucous plugging, iatrogenic
2) Mural – bronchogenic carcinoma
3) Extrinsic – compression by adjacent mass

Question 4

Describe x-ray findings of lobar lung collapse

Answer 4

Elevation of the ipsilateral hemidiaphragm
Crowing of the ipsilateral ribs
Shift of the mediastinum towards the side of atelectasis
Crowding of pulmonary vessels

Question 5

What is consolidation?

Answer 5

Filling of small airways/alveoli with stuff

• pus = pneumonia
• blood = haemorrhage
• fluid = oedema
• cells = cancer

Question 6

Describe the causes of a space occupying lesion

Answer 6

Malignant – primary, metastases 
Benign mass lesion 
Inflammatory 
Congenital 
Mimics – bone lesion, cutaneous lesion, nipple shadow

Question 7

Describe the cardiac index

Answer 7

Normal <50% - must be a PA image

BEWARE OVERESTIMATION ON AP IMAGE

Question 8

Describe the microbiology of TB

Answer 8

Caused by bacteria belonging to the mycobacterium tuberculosis complex
Non-motile rod-shaped bacteria
Obligate aerobe
Long-chain fatty acids, complex waves & glycolipids in cell wall -> structural rigidity, resists decolourisation by acids during staining procedures (acid-fast)
Relatively slow-growing compared to other bacteria

Question 9

List groups at high risk of TB

Answer 9

Non-UK born/recent migrants 
HIV, other immunocompromised conditions 
Homelessness 
Drug use, prisoners 
Close contacts of TB
Young adults (also higher incidence in elderly)

Question 10

How is TB transmitted?

Answer 10

Spread by respiratory droplets – coughing, sneezing etc
Infectious dose 1-10 bacilli
Contagious, but not easy to acquire infection – prolong exposure facilitates transmission

Question 11

Describe the pathogenesis of TB

Answer 11

Inhaled aerosols
Engulfed by alveolar macrophages
Local lymph nodes -> primary complex (Ghon’s focus and draining LN)
Either progression to active disease (primary TB) or containment of the infection
Latent infection -> heals or reactivation to form post primary TB

Question 12

Describe the formation of tubercles in TB

Answer 12

Ingestion of MTB by macrophages causes a granulomatous reaction
Characteristic lesion of TB is a spherical granuloma with central caseation

Question 13

List the clinical features of TB

Answer 13

Onset of active TB is gradual over weeks or months – tiredness, malaise, weight loss, fever, sweats & cough (may be dry or productive)
May be no clinical signs on examination even when the CXR is abnormal

Question 14

Describe primary infection TB

Answer 14

First exposure to MTB in susceptible hosts
Ghon’s focus = development of a sub-pleural focus of tubercles
MTB bacilli drain from the primary focus into the hilar lymph nodes -> together these are known as the primary Gohn complex
Most primary infections will heal with/without calcification of the primary complex, but in some the bacteria enters the bloodstream and results in MTB in other parts of lung & body

Question 15

What is latent infection?

Answer 15

State when MTB can persist within the human host, without causing disease
Person remains well but the potential for reactivation at any site is always present
Reactivation occurs when the patient’s immune mechanisms wane or fail
Characterised by a positive ‘quantiFERON’ test or a positive tuberculin skin test

Question 16

Describe the tuberculin skin test

Answer 16

Tuberculin is injected intra-dermally
Presence of a skin reaction 48-72 hours later at the site indicates previous exposure to TB and is due to type IV hypersensitivity reaction to proteins
False positives – BCG, non-TB mycobacterium

Question 17

Describe post primary TB

Answer 17

Disease that arises in a previously infected and sensitised host – typically associated with weakened host resistance
Systemic signs and symptoms: malaise, weight loss, fatigue, low-grade fevers, productive cough & haemoptysis
Cavity formation
Haemorrhage
Spread to involve the rest of the lung
Pleural effusion

Question 18

Describe miliary pulmonary TB

Answer 18

Occurs when the MTB drainage through the lymphatic system enters the venous blood and circulates back to the lung
Some foci of infection are visible through the lungs (CXR)

Question 19

Describe extra pulmonary TB

Answer 19

Haematogenous spread of primary TB/reactivation of latent TB in sites other than the lung results in active TB at sites other than the lungs
Sites involved: lymph nodes, bones, CNS, joints, GI tract
Symptoms will depend on the organs seeded
NB: TB causing Addison’s disease is common

Question 20

Describe the diagnosis for active TB

Answer 20

Acid-fast smear and cultures of sputum
Sputum culture is the most sensitive and specific test & is required to provide drug susceptibility information – may take 1-3 weeks or 4-8 weeks depending on culture medium used

Question 21

Describe the diagnosis for latent TB

Answer 21

IGRA or tuberculin skin test
A positive IGRA or tuberculin skin test demonstrates T cell mediated immunity to mycobacterial organisms BUT does not differentiate between infection and active disease
Tuberculin skin tests do not differentiate between latent infection & people who have had BCG/infection by atypical mycobacterium
People with suppressed immune systems/active TB may have falsely negative IGRA/skin tests

Question 22

Describe the IGRA/QuantiFERON test

Answer 22

Based on the ability of some mycobacterium tuberculosis antigens to stimulate host production of interferon gamma
Lymphocytes from the patient’s blood are cultured with these antigens -> if patient has been exposed to TB before, T lymphocytes produced interferon gamma in response

Question 23

Describe the treatment for TB

Answer 23

Combination of antibiotics over several months
Drugs currently used: rifampicin, isoniazid (INAH), pyrazinamide & ethambutol
All 4 drugs for 2 months followed by rifampicin and INAH for a further 4 months

Question 24

Why are four drugs used to treat TB?

Answer 24

Wild MTB strain contains a small number of naturally drug resistant organisms arising through spontaneous mutations
Using a single drug allows selection of these resistant strains to emerge
Being resistant to all 4 drugs is unlikely

Question 25

Why is TB a leading cause of death in people with HIV/AIDS?

Answer 25

CD4 T cells are a key component of the immune response against MTB
HIV-infection-associated CD4 T-cell depletion greatly increases susceptibility to developing TB
NB: may have negative screening test due to co-infection

Question 26

Describe prevention of TB

Answer 26

Notifiable disease – must notify public health of a case
Infection control
Address risk factors
BCG vaccination – given to babies in high prevalence communities, 70-80% effectiveness in preventing severe childhood TB, little evidence protecting adults