Chapter 4 Diagnostic imaging in emergency patients

The aim of this chapter is to explain briefly the need and usefulness of diagnostic imaging services in emergency situations. Many of these emergencies arise ‘after hours’ and staff in most emergency departments have no immediate access to radiologists. The chapter also outlines the various diagnostic imaging modalities available, the basic principles involved in each modality and some clues to interpreting some of the most obvious lesions.

IMAGING MODALITIES

Plain X-rays

Plain X-rays are a commonly used modality. An X-ray beam is passed through the body.

Different tissues of the body absorb different amounts of X-rays. Unabsorbed X-rays are recorded on a film placed on the opposite side. Bone absorbs the most, hence it looks white on film. Air absorbs almost none, hence it looks black on film. Other tissues are demonstrated in shades between black and white.

Plain X-rays are performed as the first line of emergency investigations in many instances, as in fractures and abdominal or chest pain.

Ultrasound

Transducers used in this examination pass ultrasound into the body and also receive the echoes. The intensity of the echoes depends on the degree of absorption of sound waves by various tissues. On the image, echogenic areas appear white and sonolucent areas (that transmit sound, e.g. fluid) appear black.

Immediate and instant demonstration of organs by real-time ultrasound imaging and the fact that it is non-invasive and harmless (no radiation) have made this tool very popular. It is used in the following:

• To find out whether a lesion or lump is solid or cystic.

• Abdominal and pelvic organs—liver, gall bladder, bile ducts, pancreas, spleen, kidneys, aorta, inferior vena cava (IVC), bladder, prostate, uterus and ovaries, renal stones, gall stones, aortic aneurysm, traumatic and other haematomas—abscess and abnormal fluid collections can be demonstrated.

• To examine small parts (thyroid, testes and breast) and neonatal heads for ventricular size etc.

• For obstetrical work-up, including study of fetus for early detection of abnormalities, and for emergencies like ectopic pregnancy, vaginal ultrasound is extremely useful.

• Ultrasound-guided biopsy and interventional procedures.

• Musculoskeletal investigations, e.g. shoulder, knee, muscle and tendon injuries and acute tenosynovitis.

• Duplex and colour Doppler ultrasound:

• With the advent of duplex and colour flow Doppler it is now easy to identify arteries and veins non-invasively. This modality should be the first line of imaging for the detection of deep vein thrombosis (DVT) in the limbs. (This study can be difficult in extremely swollen, oedematous lower legs.) In difficult cases venography can be performed. It should be remembered that ultrasound is non-invasive and can be performed repeatedly even in pregnant patients with suspected DVT.

• Duplex ultrasound is useful in identifying superficial thrombophlebitis.

• While excluding thrombosis, ultrasound study can also diagnose other causes of a swollen, tender calf such as ruptured Baker’s cyst, haematoma in calf muscle, e.g. a gastrocnemius tear, mass in groin, axilla.

• Doppler ultrasound is useful in detecting and localising acute arterial occlusion in the limbs and in the investigation of peripheral vascular disease and carotid arterial disease.

• It is also useful in assessing renal artery stenosis and renal parenchymal vascular disease in the investigation of hypertension.

Computerised tomography (CT)

The same principles are applied in CT as in plain X-rays, but there are two main modifications:

1. The X-ray tube is rotated around the body in an axial plane.

2. Instead of an X-ray film, detectors are used on the opposite side. Multiple fixed detectors are placed around the body to pick up the signals as the X-ray tube rotates around the body. Signals from the detectors are digitalised and the computer builds up an image which can be seen on a TV monitor or recorded on a film.

The resulting images are transverse sections of the part examined. Using the stored information of consecutive thin transverse sections, the images can be reconstructed in sagittal, coronal and oblique planes. CT is now a proven diagnostic tool which delivers valuable information to help in the early diagnosis of many lesions and disease. Its use is greatly appreciated in many emergencies such as head injuries and some chest and abdominal emergencies. It is also useful in demonstrating fractures that are not shown by plain X-rays.

Magnetic resonance imaging (MRI)

In the past 20 years MRI has gradually become the technique of first choice in the investigation of many diseases. The physics involved in MRI is more complex than for any other radiological technique. However, the basic principles are indicated by the original terminology, nuclear magnetic resonance (NMR).

Magnetic

The hydrogen proton is a small, positively charged particle with associated angular momentum or spin. This situation represents a current loop and results in the formation of a magnetic field with north and south poles (dipoles). In other words, the protons behave as tiny magnets within the tissues. All nuclei used in MRI must have this property.

In the absence of influence from any external magnetic fields, the protons tend to orientate randomly in all directions. However, when a strong static external magnetic field is applied, these dipolar protons tend to align parallel to the direction of the external magnetic field (longitudinal plane).

The strong external magnetic field must be homogeneous over a volume large enough to contain the human body. This explains why the magnet tunnel is much longer than the CT gantry.

Resonance

This is a phenomenon whereby an object is exposed to an external oscillating disturbance that has a frequency similar to its own frequency of oscillation. Therefore, when a hydrogen proton is exposed to an external disturbance with a similar frequency to its own, the proton gains energy from the external disturbance. This is called resonance. This can happen only if the external disturbance is applied at 90° to the magnetic field of the proton. The oscillation frequency of the hydrogen proton in a static magnetic field of strength used in clinical MRI corresponds to the radiofrequency band (RF) in the electromagnetic spectrum.

Therefore, for resonance of hydrogen to take place, an RF pulse at the same frequency as the oscillation of the hydrogen proton must be applied at 90° to the magnetic field of the proton. The application of the RF pulse that causes resonance is called excitation, as it results in the nuclei gaining energy. This energy causes the magnetic field of the protons to change direction. Enough RF pulse energy is given to the proton to change the direction from the longitudinal to the transverse plane (flip angle of 90°). Now the protons are rotating in the transverse plane. According to the laws of electromagnetism, if a receiver coil is placed in the transverse plane, the transverse magnetisation produces a voltage in the coil. This voltage constitutes the MR signal.

When the RF pulse is turned off, the hydrogen protons return to their original orientations in the longitudinal plane. This is called relaxation. There are two main types of relaxation (T1 and T2). T1 is the return of net magnetisation to the longitudinal plane. T2 is the decay of magnetisation in the transverse plane. These two relaxations and their time variances are used to create imaging sequences. All relaxation times are based on fat and water. This is where most of the body’s hydrogen protons are.

T1 images are known for their anatomical details. In T1 images, fluid appears black and fat appears white.

T2 images are known for their contrast. In these images, fluid appears white and fat appears grey.

Proton density images are a combination of T1 and T2. These images specifically look at the concentration of hydrogen protons.

The above is a simple explanation of the basics, but more complex physics is involved in the formation of MR images which is beyond the scope of this chapter.

Advantages and uses of MRI

No ionising radiation.

Free of artefacts from adjacent bones and gas. Therefore it is excellent to demonstrate soft tissues adjacent to bones, e.g. base of brain and spinal cord.

Excellent resolution—even without contrast enhancement, MR is much more sensitive in detecting contrast differences between various tissues. This is due to the intrinsic differences in hydrogen proton density as well as T1 and T2 relaxation, magnetic susceptibility and motion in various tissues.

Uses:

• Brain

• Stroke—MRI can diagnose acute infarction within hours of onset when CT scan is still normal. This is done by using diffusion imaging which tracks the motion of water that becomes acutely restricted in the area of infarction. Once infarction has been diagnosed, an intravenous MRI angiogram can be performed at the same time to assess the calibre of the neck arteries.

• Haematoma—subdural and parenchymal haematoma and dural sinus thrombosis.

• Aneurysm and other vascular malformations—MRI angiography is very sensitive in finding small aneurysms up to about 2 mm and can also give some idea of the degree of any associated vasospasm.

• Intracranial mass—MRI can accurately stage both primary and metastatic tumours. It is more sensitive than CT for metastases. Some masses like abscess and epidermoid can be differentiated from other tumours with certainty as they have restricted water diffusion. In certain cases spectroscopy can also help in the differential diagnosis. MRI is also superior to CT in detecting lesions closer to the skull base such as acoustic neuroma and pituitary tumour.

• Other uses—demyelination, congenital anormality, meningeal disease and investigation of epilepsy.

• Spine—MRI is the modality of choice for assessing cord contusion and haematoma in the acute stage, especially when there is an associated spinal fracture and neurological signs. It is also excellent in demonstrating cord compression, infarction, tumour, neuromas, metastases, myelopathy, radiculopathy and brachial plexus avulsion. It is also useful in post-laminectomy complications such as haematoma or infection as well as investigation of infective spondylodiscitis where early changes, such as fluid in the disc, paraspinal collections and end plate erosions, are seen.

• Musculoskeletal—MRI has the ability to simultaneously characterise soft tissues (muscles, ligaments, tendons and cartilage) as well as bone marrow. It is very useful in the investigation of musculotendinous injury and pathology, joint injury and pathology (e.g. rotator cuff injury in shoulder, meniscal and ligamentus injury in knee) and fractures not detected by X-ray (e.g. fracture in the neck of femur) as well as bone infection, avascular necrosis, marrow disorders, bone and soft tissue tumours.

• Chest—constrictive pericarditis, aortic and great vessel dissection, congenital cardiac and great vessel abnormality, lung cancers closer to the chest wall and apex (e.g. Pancoast’s tumour).

• Abdomen—adrenal mass using chemical shift imaging, cholangiography etc.

• Pelvis—uterine and cervical tumours and abnormalities, differentiation of adenomyosis from fibroids, endometriosis assessment, bladder tumour etc.

Contrast enhanced MRI

In spite of the excellent tissue contrast definition without intravenous contrast, contrast-enhanced MRI (gadolinium) provides even better imaging sensitivity and specificity.

The enhanced contrast shows subtle parenchymal as well as leptomengingeal lesions not otherwise visible, e.g. very small metastatic lesions, acoustic neuromas of 2–3 mm, pituitary microadenomas, differentiation of the actual size of tumour from the surrounding oedema and differentiation of more malignant areas from less malignant areas. These are helpful for the purpose of treatment and to select the exact site for biopsy.

Ongoing research into the development of new contrast agents targeted at specific organs, disease processes, cells or gene type is likely to result in considerable expansion of the use of MRI.

MR angiography (MRA)

Protons in flowing blood (moving protons) produce a high signal against the background of little or no signal from the surrounding stationary tissues—hence the development of the MR angiogram with no display of the background soft tissue.

Contraindications

Cardiac pacemakers, ferromagnetic intracranial aneurysm clips (except where MR-compatible clips have been used), cochlear implants, intravascular stainless steel stents inserted less than 6 weeks previously, surgical clips in chest and abdomen inserted less than 1 week previously, metal workers with metallic foreign body in the eye.

Disadvantages

Because of the length of the magnet (tunnel), some patients may experience claustrophobia. This can mostly be overcome by sedation. Scanners are now available with open gantry which will alleviate claustrophobia, but these scanners are dedicated and are used mainly for musculoskeletal and spinal work-up.

Contrast study

The main limitation in the use of plain X-rays is the superimposition of the shadows of various organs. In many instances this can be overcome by introducing contrast:

• Barium to demonstrate the gastrointestinal tract (GIT).

• Intravenous pyelogram (IVP)—IV contrast demonstrates kidneys, collecting system and bladder. Helpful to assess function (excretion) and to detect stones, obstruction, space occupying lesion, deformities of renal tract etc.

• Micturating cystourethrogram (MCU)—to demonstrate urethra, bladder and vesicoureteral reflux.

• Arthrography—e.g. shoulder, to detect rotator cuff tears. Sometimes this is combined with a CT scan, which helps to identify glenoid labral injury etc. (Arthrograms may not be required if there is access to high resolution ultrasound and MRI scan.)

• Herniography—contrast injected into the peritoneal cavity to detect clinically undetectable inguinal or femoral hernia that is producing symptoms.

• Myelography—contrast injected via lumbar puncture demonstrates the subarachnoid space around the spinal cord and nerve roots. With the advent of CT and MRI the need for this examination is almost nil.

• Cholangiography—in the past, oral cholangiograms were performed, but this is now replaced by CT cholangiography. This examination requires slow infusion of contrast (which is excreted by bile) followed by helical CT scan and 3D reconstruction of the biliary tree.

• Sialography—contrast injected into the salivary ducts (parotid or submandibular) to identify stones, strictures, sialectasis and lesions in the glands.

• Hysterosalpingogram—contrast is injected into the uterine cavity to demonstrate the uterine cavity, fallopian tubes and to detect patency of the tube in the investigation of infertility.

• Sinogram and fistulogram—contrast injection shows the cavities and communications.

• Venography—could detect clots, incompetent perforators and varicose veins (current trend is to do Doppler and colour Doppler ultrasound examination).

• Arteriography—contrast injected into various arteries using catheters demonstrates the arteries in various organs. It is used to demonstrate occlusions, narrowing of arteries, aneurysms, bleeding points, AV malformation and tumours. Digital subtraction angiography (DSA) has replaced the conventional angiograms. In this technique, the X-ray images are digitalised and stored and manipulated by computer. The image signal of an area or organ obtained before the injection of contrast is subtracted from the image signal obtained after injection of contrast into the vessels. This is done by the computer.

Advantages

The resulting image will demonstrate the vessels and branches without superimposition of bones and other soft tissue shadows.

Owing to the fact that the image signals can be intensified by the computer, contrast media of low concentration and volume can be used with thinner catheters.

Interventional radiology

Radiologists are able to perform certain therapeutic and interventional diagnostic procedures using various types of imaging equipment (fluoroscopy, ultrasound, CT and MR), needles, catheters and guide wires.

• Dilatation of narrowed arteries—transluminal angioplasty, insertion of vascular stents.

• Embolisation—of bleeding vessels, preoperatve tumour embolisation to assist surgery by reducing blood loss and reducing the duration of surgery, palliative embolisation of tumours, embolisation of some vascular abnormalities.

• Thrombolysis—using local low-dose intra-arterial injection of thrombolytic agents to relieve thromboembolism in certain vessels such as coronary artery and peripheral arteries.

• Inferior vena cava filter insertion to prevent pulmonary embolism caused by clots arising from pelvis and lower limbs.

• Percutaneous insertion of stents to relieve biliary or ureteric obstruction.

• Percutaneous drainage of cysts, abscesses, pleural effusion etc.

• Percutaneous removal of renal or biliary stones when other methods are contraindicated.

• Biopsy of deep and superficial lesions—lesions in liver, pancreas, kidneys, other abdominal masses, chest lesions, breast, thyroid, lymph node and other superficial and deep lesions.

Intravenous contrast reaction

Even though the incidence of fatality is much lower than in street accidents, it is a great worry to doctors. Some statistics show that about 1 in 80,000 patients developed severe or fatal reaction when ionic contrast was used. Incidence of mild reaction is probably about 5–15%, moderate reaction about 1–2% and severe reaction is probably about 0.2%. However, with the use of non-ionic contrast and taking good precautions, the incidence of reaction is said to have reduced to about one-third to one-quarter of the frequency.

Usually patients who develop severe reaction have some other aggravating disease as well.

The exact pathogenesis of the reaction is not very clear. However, the following are possible mechanisms:

• chemotoxic effect

• hyperosmolar reactions causing erythrocyte or endothelial damage, blood–brain barrier damage and vasodilatation

• vasomotor reactions with release of vaso-active substances such as histamine, serotonin and bradykinin

• vaso-vagal reaction with inhibition of enzymes such as cholinesterase.

Symptoms and signs

Most reactions occur within minutes of injection. However, delayed reactions have also been reported.

Mild reactions—hot flush, burning sensation, arm pain, dizziness, nausea, vomiting, headache and urticaria—are thought to be due to systemic effects as a result of histamine liberation. Usually reassurance and restoration of the patient’s confidence is all that is required, but sometimes oral antihistamine for urticaria, mild analgesics and sometimes tranquillisers for anxiety (5 mg benzodiazepam) may also be helpful.

Moderate reactions involve a slightly more serious manifestation of the above symptoms, with or without a moderate degree of hypotension and bronchospasm. They usually respond to reassurance and antihistamine (IM or IV), benzodiazepam 5 mg, salbutamol inhalation for bronchospasm, hydrocortisone (100–500 mg IM or IV) and occasionally adrenaline 0.3–1 mL of 1/1000 IM. Oxygen by mask is administered.

Severe reaction can be life-threatening and involve a severe form of the above reactions plus convulsion, unconsciousness, laryngeal oedema, bronchospasm, pulmonary oedema, arrhythmia, hypotension, cardiac arrest, anaphylatic shock. Severe reactions require urgent treatment (see treatment of anaphylaxis in Chapter 40, ‘Dermatological presentations to emergency’).

Pre-deposing factors—in the presence of these, the incidence of reaction can be about 2–10 times as severe:

• Previous adverse reaction to contrast.

• Significant allergic history including iodides.

• Asthma.

• Cardiac disease.

• Dehydration—all patients should be adequately hydrated as dehydration is dangerous, especially in patients with diabetes, renal impairment and multiple myeloma.

• Haematological and metabolic conditions such as sickle cell anaemia, patients with known phaeochromocytoma.

• Renal disease—patients with preexisting renal disease, especially patients with diabetes, have an increased risk of reaction as well as of renal failure. Metformin drugs used for diabetic treatment are excreted by the kidneys and in patients with renal impairment this has been known to cause lactic acidosis and lead to acute alteration of renal function. In view of this, all diabetic patients treated with metformin drugs must have serum urea and creatinine levels reviewed before the intravascular contrast injection (serum urea must be less than 6.7 mmol/L and serum creatinine must be less than 0.10 mmol/L). The metformin drugs should be discontinued from 48 hours before until 48 hours after the injection and should be recommenced only after checking the serum urea and creatinine. If required, another hypoglycaemic agent may be used during this period.

Prevention and precautions

1. Always try to use non-ionic contrast.

2. Like other drugs, contrast agents should be used only if indicated and should be used in the smallest possible dose and concentration that will result in adequate imaging.

3. Weigh the possible advantages of using contrast against the possible risk. In patients with a strong family history, contrast should be avoided. In many cases the contrast study may be replaced by another examination such as ultrasound, CT or MRI.

4. A test dose may not help. However, some authors advise a small test dose of about 1 mL IV in high-risk patients who definitely require contrast for diagnosis.

5. In some patients (e.g. asthmatic and allergic patients) premedication with oral corticosteroid and possibly antihistamines could be given during the 24 hours preceding the test. The patient should also be adequately hydrated.

IMAGING OF THE HEAD

Common emergencies are: trauma, severe headaches, collapse, syncope, seizures and stroke.

Trauma

Plain X-rays of skull

Views

• Include anteroposterior, lateral, Towne’s and basal views.

• ‘Shoot through’ lateral (patient in supine position and X-ray beam horizontal) is useful to demonstrate air–fluid levels, especially in sinuses.

Interpretation

Every doctor working in the accident and emergency department should try to be familiar with the appearance of normal skull X-rays. The best way to detect abnormality is to know the normal. (This applies to every modality of imaging.)

One should be familiar with the skull sutures, vascular markings, venous lakes along the inner table of the skull vault, appearance of normal sinuses and normal intracranial calcifications (e.g. pineal body, choroid plexus).

Fracture of the skull can be an undepressed crack fracture or a depressed fracture.

Do not misinterpret sutures and vascular markings as fractures. You can avoid this by knowing the normal positions of sutures and vascular markings. Sutures are usually serrated. Fracture lines are usually ‘blacker’ than the vascular markings.

Beware of metopic sutures—in some patients such a suture may persist throughout life and may look like a fracture.

In depressed fractures, one or more fragments may be depressed. In some cases, oblique or tangential views may be needed to demonstrate depression.

If pineal calcification is present, look for shift. This indicates a space-occupying lesion such as a haematoma. (Towne’s view is ideal for this.)

Look for air–fluid level in sinuses or air in ventricles in the horizontal beam lateral view.

Plain X-rays of the face

Most facial injuries can be evaluated clinically. However, X-rays are performed for:

• confirmation

• assessment of the degree of displacement or depression of fragments

• detecting fractures in the presence of extreme facial swelling which makes palpation difficult.

In some cases facial fractures are associated with other serious emergencies, such as intracranial, neck or chest injuries, which may require emergency management such as maintenance of airway. In these patients, X-rays of the face can be postponed to the latter part of the management and may even be deferred for a few days.

Views

• Routine facial views—posteroanterior (PA), occipitomental (OM) and lateral

• Special views—nose (AP, lateral and axial), zygoma (Towne’s or modified basal view for the arch), orbital, mandible (PA, lateral, oblique and sometimes orthopantomogram—OPG—and occlusal view for symphysis menti)

Classification

For convenience, facial fractures can be divided into three types: upper third, middle third and lower third.

2. Middle third. This includes the nose, zygoma, orbit (floor, lateral and medial walls) and maxillae (mid-facial bones):

• Nose. Nasal fracture and deviations are easily detectable clinically, but X-rays are needed in patients with severe swelling or for confirmation of the degree of damage. Fractures of nasal bones are better seen in lateral view (soft tissue exposure). Deviation is seen in axial view. PA view may demonstrate fracture of frontal process of maxilla and also deviation of fracture of the bony part of nasal septum. Associated fractures in severe cases are: bony nasal septum, cribriform plate, frontal process of maxilla, anterior nasal spine of maxilla.

• Zygoma. Fracture of the arch usually involves three sites causing depression of the arch. A commonly seen severe form of fracture is of the zygomaticomaxillary complex. One fracture line extends from the zygomaticofrontal suture across the orbital process of zygoma (lateral wall of orbit) up to the lateral aspect of the inferior orbital fissure. Another fracture extends from the lateral wall of maxilla to the inferior orbital margin (near the inferior orbital foramen) and extends along the floor of the orbit to the lateral aspect of the inferior orbital fissure, thus completing a circle. This fracture can cause separation of the zygomatic bone from the maxilla and orbit. (This fracture can be well understood if the reader takes a dry skull and follows the line of the fracture described above).

• Orbit

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Jun 14, 2016 | Posted by admin in EMERGENCY MEDICINE | Comments Off

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Fastest Anesthesia & Intensive Care & Emergency Medicine Insight Engine

Diagnostic imaging in emergency patients

IMAGING MODALITIES

Plain X-rays

Ultrasound

Computerised tomography (CT)

Helical CT scanning

Advantages

Multi-slice CT scans

Advantages and uses

Magnetic resonance imaging (MRI)

Nuclear

Magnetic

Resonance

Summary

Advantages and uses of MRI

Contrast enhanced MRI

MR angiography (MRA)

Contraindications

Disadvantages

Contrast study

Advantages

Interventional radiology

Intravenous contrast reaction

Symptoms and signs

Prevention and precautions

IMAGING OF THE HEAD

Trauma

Plain X-rays of skull

Indications

Views

Interpretation

Plain X-rays of the face

Views

Classification

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