Physics Flashcards
What causes motion artefact, and how can it be minimized in MRI and CT?
Motion artefact: Minimized by using faster sequences, breath-hold techniques, and patient immobilization.
Describe methods to reduce metal artefact in CT imaging.
Metal artefact (CT): Use metal artefact reduction software, increase kVp, or utilize dual-energy CT.
How would you address aliasing artefact in MRI, and what factors influence its occurrence?
Aliasing (MRI): Reduce by increasing field of view (FOV), adjusting phase encoding, or using anti-aliasing software.
Explain strategies to reduce patient dose in CT without compromising image quality.
Reducing CT dose: Apply AEC, lower mAs, and use iterative reconstruction to maintain image quality.
How does kVp affect contrast in CT imaging, and what adjustments can reduce beam-hardening artefact?
Effect of kVp on contrast: Lower kVp increases contrast but may increase noise; optimized by balancing with iterative reconstruction for beam-hardening artefact.
Describe how Automatic Exposure Control (AEC) works in digital radiography and how it impacts image quality.
AEC in digital radiography: Adjusts exposure to patient thickness, optimizing image quality and reducing overexposure.
How does field strength in MRI affect susceptibility artefacts, and in which scenarios is susceptibility beneficial or detrimental?
Field strength & susceptibility (MRI): Higher fields increase susceptibility artefacts; avoid in regions like the sinuses or lungs but useful for fMRI.
What is the impact of slice thickness on partial volume artefact in CT and MRI?
Slice thickness & partial volume artefact: Thinner slices reduce artefact by minimizing overlap of structures in each slice.
Discuss the pros and cons of using iterative reconstruction in CT.
Iterative reconstruction in CT: Reduces noise and allows lower radiation doses, though it may slow processing time.
Compare gradient-echo and spin-echo sequences regarding sensitivity to susceptibility artefacts.
Gradient-echo vs. spin-echo (susceptibility): Gradient-echo sequences are more prone to susceptibility artefacts; spin-echo is preferable in metal-rich areas.
Describe the benefits and limitations of using Dixon techniques for fat suppression in MRI.
Dixon fat suppression: Provides uniform fat suppression and is beneficial in body MRI, especially when chemical shift artefact is a concern.
What imaging parameters affect aliasing in MRI, and how can aliasing artefact be mitigated?
Aliasing factors in MRI: Managed by increasing FOV or using saturation bands, particularly in phase-encoding direction.
What safety considerations are needed for imaging a patient with a pacemaker in MRI?
Pacemaker safety (MRI): Follow MRI-conditional device protocols, monitor patients, and adjust scan parameters for low SAR (specific absorption rate).
How can you determine if a foreign object in a patient is MRI-safe, and what artefacts might be introduced by such an object?
Foreign object MRI safety: Screen using radiographs; MRI-compatible items avoid artefacts, but some materials cause susceptibility artefacts.
Discuss the protocol modifications needed for patients with metal implants to reduce artefacts in MRI and CT.
Metal implants (MRI/CT): Use metal artefact reduction algorithm (MAR), adjust field strengths, or consider CT for less artefact impact.
Explain the principles of dual-energy CT and how it can be used to reduce artefacts.
Dual-energy CT:
uses two separate x-ray photon energy spectra, allowing the interrogation of materials that have different attenuation properties at different energies.
In essence, dual-energy CT improves diagnostic quality by using distinct X-ray energy data to reduce artefacts, enhance material differentiation, and generate more accurate images for a range of clinical applications.
- Reduces Metal Artefact
- Reduces Beam Hardening Artefact
- Material-Specific Separation
Describe the use of fat suppression techniques in MRI and situations where different types (e.g., STIR vs. chemical shift fat saturation) may be preferred.
Fat suppression techniques: STIR is effective in high magnetic fields and metal-rich areas; chemical shift methods are ideal for uniform fat saturation.
What are the advantages of using T1- and T2-weighted imaging in MRI for different tissue characterization?
T1- and T2-weighted MRI: T1 is best for anatomy and post-contrast, while T2 highlights fluid and edema, useful in pathology characterization.
Approach to verbally aggressive patient during ultrasound:
Remain calm, ensuring personal safety and the safety of others. Politely explain the purpose of the exam, and address any concerns the patient may have. Offer a break if needed, and if aggression persists, consider involving security or a senior colleague.
Key elements of informed consent:
Voluntary consent, without coercion or misrepresentation.
Specific to the procedure, with an understanding of benefits, risks, and alternatives.
Competent capacity, ensuring the patient understands the procedure and its implications.
Consent process for a non-English speaking patient for CT biopsy:
Engage a certified healthcare interpreter (not family or friends) to ensure clear communication and comprehension of risks, benefits, and alternatives, following the consent guidelines.
The CT radiographer comes to you asking for assistance with an anxious patient who does not speak English. The contrast consent and screening questionnaire has not been completed. How would you approach this? (3-4 marks)
Calmly reassure the patient, engage a certified interpreter to complete the contrast consent and screening questionnaire, and address any specific patient concerns related to the procedure.
What are three strategies that could be employed by a radiologist to assist in managing an anxious patient. (3 marks)
Employ clear, empathetic communication to reduce uncertainty.
Offer relaxation techniques or breathing exercises.
Allow time for questions, and ensure the patient feels supported.
A patient’s mother asks you whether the CT brain study for her 3 year son could cause cancer. How would you respond to this? (2-4 marks)
Acknowledge the parent’s concern, explain the low risk of radiation at diagnostic levels, and emphasize the necessity of the scan for accurate diagnosis, using understandable language.
Outline the role an Indigenous Health Worker may have in the care of an Indigenous patient undergoing a radiological procedure. (3-4 marks)
Indigenous Health Workers provide cultural support, facilitate communication, and ensure culturally safe care, particularly for Aboriginal and Torres Strait Islander patients undergoing medical imaging.
Approach to error in a colleague’s report:
Discreetly discuss the error directly with the radiologist to verify and correct it. If appropriate, escalate to a supervisor if correction is not pursued, prioritizing patient safety and maintaining professionalism.
Handling CTPA request without contrast:
Explain that a CTPA requires contrast for diagnostic accuracy. Offer to discuss alternative imaging with the senior clinician and educate the junior doctor on appropriate imaging protocols.
You have been asked to report an urgent prostate MRI, but do not feel you have adequate expertise in this area. How would you approach this? (2-3 marks)
Seek assistance from a colleague with prostate MRI expertise or suggest transferring the case to ensure accurate reporting and patient care.
Outline the evidence for the effectiveness of the (insert subspecialty) multidisciplinary team meeting and the potential impact on patient care. (3-4 marks)
MDTs improve diagnostic accuracy, facilitate comprehensive treatment planning, and enhance patient outcomes by integrating expert opinions across specialties.
A patient has imaging from an external private imaging provider that you have been asked to review for a multidisciplinary team meeting. How would you go about reviewing the imaging while maintaining patient privacy? (2-4 marks)
Ensure all images are de-identified, follow privacy protocols, and confirm that viewing and discussing them complies with institutional policies.
Describe the key elements of safe handover of care between medical imaging and clinical teams post-imaging guided procedures. (2-4 marks)
communicate findings, potential complications, and follow-up requirements clearly to the clinical team, ensuring a structured and documented handover.
Key elements of safe handover for clinically significant abnormality:
Ensure direct communication of critical findings to the responsible clinical team, confirming understanding and documenting the handover to guarantee timely follow-up.
What are the potential risks associated with gadolinium-based contrast agents (GBCAs)?
GBCAs carry a risk of nephrogenic systemic fibrosis (NSF) in patients with severe renal impairment and a very low risk of allergic reactions. Gadolinium deposition has also been observed in the brain, but no adverse effects have been conclusively linked to this.
How can the risk of nephrogenic systemic fibrosis (NSF) be minimized in MRI contrast administration?
Screen patients for renal impairment (e.g., eGFR <30 mL/min/1.73m²) before administering GBCAs, avoid using GBCAs in high-risk patients, and use lower-risk agents like macrocyclic GBCAs if necessary.
What is the protocol for managing a patient who reports a history of allergic reactions to MRI contrast?
Assess the severity and type of past reaction, consult an allergist if needed, and consider premedication with corticosteroids and antihistamines for mild reactions. For severe allergies, avoid GBCAs unless absolutely necessary.
What are macrocyclic and linear GBCAs, and how do they differ in terms of safety?
Macrocyclic GBCAs have a more stable structure that traps gadolinium ions, reducing the risk of dissociation and tissue deposition compared to linear agents. Macrocyclic agents are generally considered safer, particularly in patients needing multiple scans.
What precautions should be taken for pregnant patients requiring MRI contrast?
BCAs are generally avoided during pregnancy due to limited safety data and theoretical risks to the fetus. Use of contrast is only considered if the potential benefits outweigh the risks, such as in critical clinical situations.
what are the types of dual energy scanners
- Dual-Source Dual-Energy CT:
- This type uses two X-ray tubes and two detectors mounted at 90 degrees from each other. Each tube operates at a different energy level, allowing simultaneous data acquisition. This setup is highly effective for rapid imaging, such as cardiac scans. - Single-Source Dual-Energy with Fast kVp Switching:
- In this type, a single X-ray tube alternates between high and low kilovoltage (kVp) settings very rapidly during each rotation. It’s commonly used for high temporal resolution applications and is efficient as it requires only one X-ray tube and detector. - Dual-Layer Detector Dual-Energy CT:
- This scanner has a single X-ray tube but uses a specialized detector with two layers. The top layer detects lower-energy photons, while the bottom layer detects higher-energy photons from the same X-ray beam. It allows simultaneous energy separation and is effective for routine clinical applications. - Sequential Dual-Energy CT (Dual kVp):
- This method involves taking two consecutive scans of the same area at different kVp settings. Though less efficient and potentially more prone to motion artifacts, it is an option for scanners that lack simultaneous dual-energy capabilities. - Dual-Spiral or Dual-Phase Acquisition:
- In this approach, two helical scans are performed at different energy levels with a single X-ray tube, which is adjusted between scans. It is generally used less frequently due to the longer scan times and higher radiation dose.
What is the INR threshold for low-risk IR procedures, and is withholding anticoagulants necessary?
Answer: For low-risk procedures (e.g., diagnostic angiography or non-vascular biopsies), an INR threshold of ≤1.8 is generally acceptable. Often, anticoagulants may not need to be withheld, but this depends on institutional guidelines and patient-specific factors. Consultation with the referring team may be warranted.
What INR level is recommended for intermediate-risk IR procedures, and how long should warfarin be withheld?
Answer: For intermediate-risk procedures (e.g., tunneled central venous catheter placement, lung biopsy), an INR ≤1.5 is typically recommended. Warfarin is usually withheld 3–5 days prior, allowing INR to fall to the acceptable level, with bridging therapy considered based on patient risk factors.
What is the INR requirement for high-risk IR procedures, and are there additional recommendations?
Answer: High-risk procedures (e.g., transjugular liver biopsy, renal biopsy) generally require an INR ≤1.5. Warfarin should be withheld for about 5 days, and bridging with low molecular weight heparin (LMWH) may be needed in high-thrombotic-risk patients. INR should be checked pre-procedure to confirm it is within the safe range.
How should antiplatelet agents like aspirin be managed before low-risk procedures?
Answer: For low-risk procedures, aspirin is usually not withheld, as the bleeding risk is minimal. However, if there is a specific contraindication or institutional preference, holding aspirin for about 5-7 days may be considered.
Should clopidogrel be withheld for intermediate-risk procedures? If so, for how long?
Answer: Yes, for intermediate-risk procedures, clopidogrel is typically withheld for 5 days to reduce bleeding risk. Consultation with the referring physician is advisable to assess thrombotic risk and to determine if bridging therapy is required.
What is the protocol if a patient on anticoagulation or antiplatelet therapy urgently requires an IR procedure?
Answer: In emergency situations, reversing anticoagulation with agents such as vitamin K or prothrombin complex concentrates (PCC) may be considered. The decision should weigh the urgency of the procedure against the bleeding risk, and consultation with hematology may be appropriate.
What is posterior acoustic enhancement, and in what situations is it commonly seen?
Answer: Posterior acoustic enhancement is an increased echo brightness seen distal to a fluid-filled structure, such as a cyst or bladder. It occurs because fluid allows sound waves to pass with minimal attenuation, resulting in stronger echoes from tissues behind it. This is often used as a diagnostic feature to differentiate cystic from solid structures.
What is the role of frequency in ultrasound imaging, and how does it affect resolution and depth?
Answer: Higher frequency transducers provide better resolution but have limited penetration, making them suitable for superficial structures (e.g., thyroid, breast). Lower frequencies penetrate deeper but offer lower resolution, which is more appropriate for abdominal or pelvic imaging.
Reverberation Artifact:
Cause: Caused by sound waves bouncing back and forth between two strong reflectors, such as the transducer and the tissue surface. It results in multiple equally spaced echoes, often seen with air or metallic objects.
Comet Tail Artifact:
Cause: Similar to reverberation, but usually due to a small, highly reflective object like a metallic clip or tiny calcifications. It appears as a series of closely spaced echoes extending from the reflector.
What does T2 measure ?
a map of proton energy within fat and water based tissues of the body
What are T2 hypointense?
Anything with no fat or no water will be low on T2
- Hemosiderin
- Flow void
- Cortical bone
- Densely fibrotic tissue
- Calcium
what is the rationale behind a two stage revision of a total joint replacement performed for prosthetic joint infection ?
The rationale behind a two-stage revision for prosthetic joint infection (PJI) is to effectively eradicate the infection before implanting a new prosthesis, thus reducing the risk of recurrence. The procedure involves two stages:
First Stage: Removal of the infected prosthesis and placement of an antibiotic-loaded cement spacer. The removal of the infected hardware and thorough debridement helps eliminate biofilm and infected tissue, while the spacer maintains joint space and releases high local concentrations of antibiotics to treat the infection.
Second Stage: After confirming the infection has resolved (usually through negative cultures and clinical markers like CRP and ESR), a new prosthesis is implanted. This staged approach allows better control over infection, increasing the chances of a successful long-term outcome compared to single-stage revision.
What limitations are there to MRI in the setting of possible infected joint replacement, what can you do to minimise these limitations ?
MRI has several limitations in assessing possible infection in a joint replacement due to the presence of metal, which can cause artifacts that obscure surrounding soft tissue and bone detail. Here are the main limitations and strategies to minimize them:
Metal Artifacts: Metal in joint replacements causes signal voids, image distortion, and susceptibility artifacts that obscure adjacent tissues.
Solution: Use metal artifact reduction sequences (MARS), optimized MRI sequences (e.g., STIR or View Angle Tilting), and advanced techniques like MAVRIC or SEMAC to reduce artifact and improve visualization of the periprosthetic region.
Limited Sensitivity for Detecting Infection: MRI has reduced sensitivity for differentiating between inflammation due to infection versus other causes (e.g., aseptic loosening).
Solution: Complement MRI with clinical markers (e.g., CRP, ESR) and other imaging modalities such as nuclear medicine scans (e.g., white blood cell scintigraphy) or PET-CT to enhance diagnostic accuracy.
Challenges with Soft Tissue Visualization: Artifact may obscure important soft tissue details, such as abscesses or sinus tracts around the implant.
Solution: Position the patient to minimize artifact direction and select sequences that provide better contrast for soft tissues around the implant.
how does an infected joint replacement appear on a bone scan and labelled white cell scan ?
Bone Scan (typically a 3-phase bone scan):
Appearance: Increased uptake in all three phases (blood flow, blood pool, and delayed phase)
Distinguishing Features: While increased uptake is common in both infection and aseptic loosening, infection tends to show more intense and diffuse uptake in the delayed phase. However, bone scans alone cannot reliably differentiate infection from aseptic loosening.
Labeled White Cell Scan (e.g., with Indium-111 or Technetium-99m HMPAO):
Appearance: Increased uptake of labeled white cells around the prosthesis, particularly if the uptake persists or increases on delayed images (e.g., 24-hour images).
Distinguishing Features: In cases of infection, labeled white cell uptake will typically localize to areas of infection, while aseptic loosening usually shows little to no white cell uptake.
A combined labeled white cell and bone marrow scan can improve specificity, as matching uptake on both indicates aseptic loosening, whereas discordant uptake suggests infection.
how does diffusion restriction work ?
Diffusion restriction in MRI refers to the limited movement of water molecules in tissues, which can be visualized using diffusion-weighted imaging (DWI). It works based on the principle that water molecules naturally move, or “diffuse,” in tissues. In healthy tissues, water molecules can move more freely. However, in areas where cells are densely packed or there is cellular swelling, such as in tumors, abscesses, or acute ischemic strokes, water movement is restricted.
Patient has a cardiac pacemaker, how does a radiologist proceed with a MRI study ?
Verify Device Compatibility
Consult with Cardiology and Obtain Clearance
For MRI-conditional devices, follow the manufacturer’s guidelines and ensure all MRI conditions (e.g., specific field strength, limited body part) are met.
A qualified cardiology or EP professional should reprogram the pacemaker to MRI-safe mode
Implement Safety Monitoring
Use the lowest feasible MRI field strength (often 1.5T), minimize scan time, and apply MRI sequences that limit radiofrequency (RF) exposure and heating.
Post-Scan Evaluation by Cardiology.
