Radiodiagnosis
Imaging Modalities and Patient Preparation
Imaging modalities for MBBS and NEET-PG: choosing radiography, ultrasound, CT and MRI, contrast media and reactions, and patient preparation, mapped to NMC codes RD1.2 and RD1.11.
MedNext Academy | 3 min read
Imaging Modalities and Patient Preparation
Imaging modalities for MBBS and NEET-PG: choosing radiography, ultrasound, CT and MRI, contrast media and reactions, and patient preparation, mapped to NMC codes RD1.2 and RD1.11.
This chapter surveys the diagnostic imaging toolbox and teaches how to choose between modalities by their physical basis, strengths and safety profile. It covers contrast media, their reactions and contraindications, and the practical patient preparation, from fasting and a full bladder to allergy and renal screening, that makes each study safe and diagnostic.
High-yield: Imaging Modalities and Patient Preparation
- The main imaging modalities are radiography, fluoroscopy, ultrasound, computed tomography, magnetic resonance imaging and nuclear medicine, each with distinct strengths.
- Radiographs and CT use ionising X-rays, ultrasound uses high-frequency sound waves, and MRI uses a strong magnetic field with radiofrequency pulses.
- On a radiograph the five basic densities from black to white are air, fat, soft tissue or water, bone and metal.
- Ultrasound is the modality of choice in pregnancy and paediatrics because it is real time, portable and free of ionising radiation.
- CT is fast and excellent for trauma, acute stroke and chest imaging, while MRI gives superior soft-tissue contrast for brain, spine and joints.
- Iodinated contrast is used for CT and conventional angiography, while gadolinium-based agents are used for MRI.
- Iodinated contrast is nephrotoxic, so renal function should be checked and patients hydrated, and metformin may need to be withheld around the study.
- Gadolinium is contraindicated in severe renal impairment because of the risk of nephrogenic systemic fibrosis.
- A history of previous severe contrast reaction, asthma or atopy raises the risk of an anaphylactoid reaction to contrast.
- The absolute contraindications to MRI include a cardiac pacemaker, cochlear implant and intraocular ferromagnetic foreign body, though many modern devices are MRI conditional.
- Fasting for several hours is required before abdominal ultrasound and before contrast-enhanced CT to reduce aspiration risk and improve gallbladder assessment.
- A full bladder is needed for transabdominal pelvic ultrasound, as it provides an acoustic window and displaces bowel gas.
- Bowel preparation and oral or rectal contrast improve visualisation of the gastrointestinal tract on CT and fluoroscopic studies.
- Informed consent, allergy screening and renal function assessment are the key safety steps before any contrast study.
Modality selection and preparation
- **Ultrasound:** No radiation, real time. First choice in pregnancy and paediatrics. Full bladder for pelvis.
- **CT:** Fast, ideal for trauma and stroke. Iodinated contrast; check renal function and hydrate.
- **MRI:** Best soft-tissue contrast. Screen for pacemaker and metallic foreign body. Gadolinium contrast.
- **Contrast safety:** Iodinated agents are nephrotoxic; gadolinium avoided in severe renal failure.
NMC competencies in this chapter
- **RD1.2:** Evolution of radiodiagnosis and identification of current radiological equipment
- **RD1.11:** Preparation of the patient for common imaging procedures
Frequently Asked Questions
How do I choose between CT and MRI?
CT is fast and best for trauma, acute stroke, chest and bony detail, and uses ionising radiation. MRI gives superior soft-tissue contrast for brain, spine and joints without radiation, but is slower and has strict safety screening.
Why is a full bladder needed for pelvic ultrasound?
A full bladder pushes gas-filled bowel out of the pelvis and acts as an acoustic window, letting sound waves reach the uterus and ovaries clearly on transabdominal scanning.
What must be checked before giving iodinated contrast?
Renal function, a history of previous contrast reaction or severe allergy, and current metformin use. Patients should be hydrated, and consent taken, to reduce the risk of contrast-induced nephropathy and reactions.
Why is MRI unsafe for some patients?
The strong magnetic field can move ferromagnetic objects and disrupt implanted devices, so a cardiac pacemaker, cochlear implant or intraocular metallic foreign body are key contraindications unless the device is certified MRI conditional.
Continue reading
MBBSAll Radiodiagnosis chapters
Continue through the Radiodiagnosis chapter map.
Continue studying Radiodiagnosis
Explore clinician-written learning resources, structured revision and practice across the MedNext platform.
Open in the MedNext appSee plans