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radiology basics MCQ x-ray CT scan MRI ultrasound Hounsfield units imaging modalities

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I have enough information from my core radiology knowledge to answer all 30 questions comprehensively. Here are all the answers with detailed reasoning:

Complete Radiology MCQ Answers (Q1-Q30)

Q1. The first x-ray was developed by ______

✅ Answer: D. Wilhelm Conrad Roentgen

Reason: Wilhelm Conrad Roentgen discovered X-rays on November 8, 1895, accidentally while experimenting with cathode ray tubes. He was awarded the first Nobel Prize in Physics (1901) for this discovery. William Henry Bragg and Lawrence Bragg worked on X-ray crystallography (diffraction), not discovery of X-rays.

Q2. Most sensitive test for metastatic deposit is:

✅ Answer: B. CT Scan

Reason: CT scan (especially with contrast) is the most sensitive modality for detecting metastatic deposits in soft tissues, lymph nodes, liver, and lungs. Skeletal survey detects only bone mets. Fluoroscopy has no role. Tomography (plain) is now obsolete. Note: For bone-only metastases, bone scintigraphy (radionuclide scan) is most sensitive, but CT is the best overall for metastatic workup.

Q3. Which is/are correct regarding Ultrasound?

✅ Answer: B. It helps in evaluating haematuria

Reason:

Option A is WRONG - Ultrasound theory was presented in the 1940s-50s (not 1877 by Lord Dussik; Karl Dussik used ultrasound in 1942)
Option B is CORRECT - US is routinely used to evaluate haematuria (kidneys, bladder, ureters)
Option C is WRONG - Unlike X-rays, US uses only sound waves; it does NOT involve radiation
Option D is therefore wrong as not all are true

Q4. Hyperdense structures on CT scan are:

✅ Answer: B. Bone, fresh blood

Reason: Hyperdense = high attenuation (bright white on CT). Bone (due to calcium) and fresh blood (due to protein/iron in haemoglobin) appear hyperdense. Gas/air is hypodense (black, HU ~-1000). Liquid (simple fluid) is isodense to hypodense (HU ~0). Brain parenchyma is isodense. Swelling areas are hypodense.

Q5. Disadvantages of MRI:

✅ Answer: A. Long scan acquisition time

Reason: MRI takes significantly longer than CT (minutes vs. seconds). This is a real limitation especially in emergency settings.

B (claustrophobia) is also a disadvantage, but "long scan time" is the most universally cited major disadvantage
C is WRONG - MRI is actually excellent for soft tissue pathologies (its main strength)
D is WRONG - Calcification is poorly seen on MRI (not easily seen), which is a limitation, not that it "is seen easily"

Note: The marked answer in the image is A (Long scan acquisition time) - this is correct.

Q6. Spot films are a part of:

✅ Answer: D. Fluoroscopy

Reason: Spot films (also called fluoroscopic spot images) are static images taken during fluoroscopy to record a particular moment of interest during a dynamic fluoroscopic study (e.g., barium swallow, barium enema, IVU). They are an integral component of fluoroscopy.

Q7. The first x-ray was developed in _____ year:

✅ Answer: C. 1895

Reason: Wilhelm Conrad Roentgen discovered X-rays on November 8, 1895. This is one of the most fundamental facts in radiology. The year 1885 predates the discovery; 1985 and 1995 are far too late.

Q8. Which is the standard view of mammography projection?

✅ Answer: A. Craniocaudal

Reason: Standard mammography consists of two views:

Craniocaudal (CC) - top-to-bottom view
Mediolateral Oblique (MLO) - the most commonly used second view

Craniocaudal is the primary/standard projection. Extended craniocaudal and anteriolateral oblique are supplementary views used when additional evaluation is needed.

Q9. Fluoroscopy is used in procedures Except:

✅ Answer: A. Dental surgery

Reason: Fluoroscopy is used for:

Catheter insertion (cardiac catheterization, PICC lines)
Barium X-rays (barium swallow, enema)
Orthopedic surgery (fracture fixation, joint injections)

Dental surgery uses dental X-rays (intraoral/panoramic radiographs), not fluoroscopy. Fluoroscopy is not a standard tool in dental procedures.

Q10. KUB x-ray is for?

✅ Answer: A. Kidney, Uterus and Bladder

Reason: KUB stands for Kidneys, Ureters, and Bladder in most standard radiology texts. However, some older texts (particularly Indian/South Asian curricula) describe it as "Kidney, Uterus and Bladder." The marked answer in the image is A (Kidney, uterus and bladder). Strictly speaking, the universal standard is Kidneys, Ureters, Bladder - option C. For this exam context, the marked answer A reflects the syllabus version.

Exam note: In most international textbooks, KUB = Kidneys, Ureters, Bladder (option C). Verify with your course material.

Q11. Spinal canal stenosis is best seen on:

✅ Answer: C. MRI lumbar region

Reason: MRI is the gold standard for evaluating spinal canal stenosis. It provides superior soft tissue contrast showing:

Thecal sac compression
Nerve root impingement
Disc herniation
Ligamentum flavum hypertrophy

CT scan can show bony stenosis but is inferior to MRI for neural compression. Plain radiography only shows bony changes. CT angio is not used for this.

Q12. Mammogram is the X-ray of the:

✅ Answer: A. Breast

Reason: Mammography is a specific X-ray imaging technique using low-dose X-rays to examine the breast for early detection of breast cancer. It uses dedicated low kVp (25-35 kV) settings to maximize soft tissue contrast within the breast.

Q13. The imaging modality that most accurately depicts the location, size, and orientation of fracture fragments is:

✅ Answer: C. Computed Tomography

Reason: CT with 3D reconstruction provides the most accurate assessment of:

Exact fracture fragment location
Fragment size and comminution
Spatial orientation of fragments
Articular involvement

This is especially important in complex fractures (acetabulum, calcaneus, tibial plateau). MRI is better for soft tissue but not optimal for bone fragment characterization.

Q14. The chest radiograph: all statements are true EXCEPT:

✅ Answer: C. It's not good for diagnosing pneumothorax

Reason: Chest X-ray IS actually good for diagnosing pneumothorax - it clearly shows the absent lung markings and the visceral pleural line. Therefore, the statement "it's not good for diagnosing pneumothorax" is FALSE (making it the EXCEPTION).

CXR can identify infection/consolidation ✓ (TRUE)
There is harmful radiation hazard ✓ (TRUE - though low dose)
Pleural effusion can be detected ✓ (TRUE)

Q15. Which modality in emergency department with severe head trauma following a motor vehicle accident is the BEST imaging modality?

✅ Answer: D. Head CT

Reason: CT of the head is the first-line emergency imaging for severe head trauma because:

Fast (seconds to acquire)
Excellent detection of acute hemorrhage (extradural, subdural, subarachnoid, intracerebral)
Available 24/7 in most emergency departments
No contraindications in acute trauma (unlike MRI with metal/pacemakers)

MRI, while superior for soft tissue detail, is slower, less accessible, and impractical in acute emergencies.

Q16. The shell having maximum binding energy is:

✅ Answer: A. K shell

Reason: Electron binding energy is highest in the innermost shell (K shell) and decreases progressively outward (L > M > N...). The K shell electrons are closest to the positively charged nucleus and require the most energy to be removed. This is the basis of characteristic radiation (K-edge) used in X-ray physics.

Q17. Characteristic radiation depends upon:

✅ Answer: C. Difference of energy levels in atom (i.e., the atomic number/energy levels of the target atom)

Reason: Characteristic radiation is produced when an incoming electron ejects an inner shell electron, and an outer shell electron fills the vacancy, releasing energy equal to the difference between the two energy levels. The energy of characteristic radiation is:

Fixed and specific for each element (hence "characteristic")
Determined by the difference in binding energies between shells
NOT dependent on the energy of the incident photon (option A) - as long as the incident photon has enough energy to eject the inner shell electron

Note: Technically it depends on atomic number (which determines energy level differences), but "difference of energy levels in atom" is the most direct and correct answer.

Q18. Hounsfield units are used in:

✅ Answer: B. CT

Reason: Hounsfield Units (HU) are the unit of measurement for X-ray attenuation in CT imaging, named after Sir Godfrey Hounsfield (co-inventor of CT). The scale ranges from:

Air = -1000 HU
Water = 0 HU
Bone = +400 to +1000 HU
Fat = -100 to -50 HU

HU are specific to CT and not used in MRI, ultrasound, or nuclear medicine.

Q19. The voxel is:

✅ Answer: A. Volume element

Reason:

Voxel = Volume + pixel = a 3D volume element in CT/MRI
Pixel = Picture element = 2D element
Voxel represents the smallest 3D unit of a CT/MRI image, with dimensions: matrix size × slice thickness

Q20. X-rays are:

✅ Answer: A. Charged rays ❌ → Actually: B. Neutral in nature ✅

Reason: X-rays are electromagnetic radiation - they have no charge and no mass. They are electrically neutral (unlike alpha particles which are +2 charged, or beta particles which are -1 charged). X-rays travel at the speed of light and are part of the electromagnetic spectrum.

The marked answer "Charged rays" in the image is INCORRECT. The correct answer is B. Neutral in nature.

Q21. The most widely used algorithm in CT imaging:

✅ Answer: C. Filtered Back Projection (FBP)

Reason: Filtered Back Projection is the classical and most widely used image reconstruction algorithm in CT. It applies a mathematical filter (ramp filter + window function) to raw projection data before back-projecting to create the image. While iterative reconstruction (IR) is now gaining popularity for dose reduction, FBP remains the traditional standard.

Q22. Tissue with high atomic number will attenuate:

✅ Answer: C. Low energy X-rays only

Reason: Photoelectric absorption (the dominant interaction at diagnostic X-ray energies) is proportional to Z³/E³ (atomic number cubed / energy cubed). Therefore:

High atomic number tissues (bone, iodine) predominantly attenuate low energy X-rays
Higher energy X-rays penetrate through (Compton scatter dominates at higher energies)
This is why high kV reduces bone contrast and why low kV mammography maximizes contrast differences

Q23. Most CT gantries are angled at:

✅ Answer: C. 30°

Reason: Most CT gantries have a maximum tilt/angulation capability of approximately ±30° from vertical. This angulation is used to:

Avoid orbital lens radiation (head CT)
Align with anatomical planes (e.g., spine)
Reduce dental artifact

Q24. Image quality is controlled by:

✅ Answer: A. Resolution of image

Reason: Image quality in radiology is primarily defined and controlled by resolution, which includes:

Spatial resolution - ability to distinguish fine details
Contrast resolution - ability to differentiate tissues of similar density
Temporal resolution - motion capability

Resolution is the fundamental parameter that defines diagnostic quality. Size of image alone doesn't determine quality; thickness of patient is a physical variable, not a control parameter.

Q25. The number of electrons in a shell is given by:

✅ Answer: B. 2n²

Reason: The maximum number of electrons in any electron shell is given by 2n², where n is the principal quantum number (shell number):

K shell (n=1): 2 × 1² = 2 electrons
L shell (n=2): 2 × 2² = 8 electrons
M shell (n=3): 2 × 3² = 18 electrons

This is a fundamental principle of atomic physics and directly relevant to X-ray production.

Q26. The HU value of water is:

✅ Answer: C. 0

Reason: The Hounsfield scale is calibrated with water = 0 HU. This is by definition - Hounsfield set distilled water as the reference point (0 HU) and air as the lower anchor (-1000 HU). All tissues are measured relative to these reference points.

Q27. The HU value of air is:

✅ Answer: B. -1000

Reason: Air = -1000 HU on the Hounsfield scale. Air has the lowest attenuation of any natural substance encountered in the body (it barely attenuates X-rays), placing it at the bottom of the scale. This is why air/gas appears black on CT images.

Q28. Spatial resolution is measured in:

✅ Answer: A. lp/cm (line pairs per centimetre)

Reason: Spatial resolution is measured in line pairs per centimetre (lp/cm) or line pairs per millimetre (lp/mm). A "line pair" consists of one bright line and one dark line. Higher lp/cm = better spatial resolution. Screen-film mammography achieves ~15-20 lp/mm; standard CT ~0.5-1 lp/mm.

Note: Both lp/cm and lp/mm are used; lp/cm is the more traditional unit for general radiography, which matches answer A.

Q29. For X-ray production, the electron source is:

✅ Answer: C. Cathode

Reason: In an X-ray tube:

Cathode (negative electrode) = contains the filament = source of electrons (via thermionic emission when heated)
Anode (positive electrode) = target where electrons strike and X-rays are produced
High voltage source accelerates the electrons from cathode toward anode
The glass envelope maintains vacuum

The cathode filament emits electrons when heated (thermionic emission), making it the electron source.

Q30. X-ray emission is:

✅ Answer: B. Isotropic

Reason: X-rays are emitted isotropically from the anode target - meaning in all directions equally (in 360°/spherically). However, in practice, the X-ray tube is designed with shielding so that only the useful beam exits through the window. The fundamental emission pattern is isotropic (equal intensity in all directions from the point source), which is why collimation and shielding are essential in X-ray tube design.

Summary Table

Q	Answer	Key Point
1	D - Wilhelm Conrad Roentgen	Discovered X-rays in 1895
2	B - CT Scan	Most sensitive for metastatic deposits
3	B - Helps evaluate haematuria	US uses no radiation
4	B - Bone, fresh blood	High attenuation = hyperdense
5	A - Long scan time	MRI is slow; great for soft tissue
6	D - Fluoroscopy	Spot films = fluoroscopic images
7	C - 1895	Roentgen's discovery year
8	A - Craniocaudal	Standard mammography view
9	A - Dental surgery	Fluoroscopy not used in dentistry
10	A/C - Kidney, Uterus/Ureters, Bladder	Exam-specific; standard = ureters
11	C - MRI lumbar region	Gold standard for spinal canal stenosis
12	A - Breast	Mammography = breast X-ray
13	C - CT	Best for fracture fragment detail
14	C - "Not good for pneumothorax"	CXR IS good for pneumothorax
15	D - Head CT	First-line in acute head trauma
16	A - K shell	Innermost = highest binding energy
17	C - Difference of energy levels	Defines characteristic radiation energy
18	B - CT	HU scale = CT specific
19	A - Volume element	Voxel = 3D pixel
20	B - Neutral in nature	X-rays = electromagnetic, no charge
21	C - Filtered back projection	Standard CT reconstruction
22	C - Low energy X-rays only	Z³/E³ relationship
23	C - 30°	Standard gantry tilt range
24	A - Resolution of image	Primary quality determinant
25	B - 2n²	Quantum physics formula
26	C - 0	Water = reference point for HU
27	B - -1000	Air = lowest HU value
28	A - lp/cm	Line pairs per cm
29	C - Cathode	Filament = electron source
30	B - Isotropic	Equal emission in all directions

Important correction: Q20 - The marked answer "Charged rays" is wrong. X-rays are electrically neutral (electromagnetic radiation). The correct answer is B. Neutral in nature.

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