MRCOG Part 1 · Biophysics · X-rays, Computed Tomography (CT), and Magnetic Resonance Imaging (MRI)

X-rays: MRCOG Part 1 questions and key facts

In obstetrics and gynaecology, clinical applications of X-rays and related physical principles centre on dosimetry units, hysterosalpingography, and dual-energy X-ray absorptiometry. Understanding core radiological units is essential for quantifying absorbed energy and biological risk from ionising radiation. In clinical practice, hysterosalpingography uses fluoroscopic projection radiography with radio-opaque contrast to evaluate uterine architecture and tubal patency in subfertility workups. Dual-energy X-ray absorptiometry uses low-dose X-ray beams to quantify bone mineral density and assess fracture risk. Candidates sitting the MRCOG Part 1 must understand the definitions distinguishing absorbed and equivalent doses, recognise that pregnancy contraindicates hysterosalpingography, differentiate ionising from non-ionising imaging techniques, and interpret dual-energy X-ray absorptiometry diagnostic scores.

The Uterio bank has 10 questions on this topic. 2 arebelow in full; the syllabus detail is in the Biophysics study guide.

How the exam tests X-rays

  • Distinguishing the Gray, measuring absorbed physical dose, from the Sievert, measuring equivalent biological dose and effective dose.
  • Recognising that pregnancy contraindicates hysterosalpingography, whilst previous ectopic pregnancy is not an absolute contraindication, and endometriosis or fibroids do not contraindicate it.
  • Interpreting dual-energy X-ray absorptiometry diagnostic thresholds, including T-score and Z-score definitions.
  • Categorising medical imaging modalities into ionising and non-ionising techniques.

High-yield facts

  1. The Gray is the standard unit of absorbed radiation dose, defined as one joule of radiation energy deposited per kilogram of matter.
  2. The Sievert measures equivalent dose, weighted for radiation type and biological effect, and effective dose, further weighted for tissue radiosensitivity.
  3. The Becquerel is the standard unit of radioactivity, defined as the number of nuclear decays occurring per second.
  4. Hysterosalpingography is contraindicated in pregnancy; other recognised contraindications include active pelvic infection and allergy to iodinated contrast medium.
  5. Previous ectopic pregnancy is not an absolute contraindication to hysterosalpingography, whilst previous caesarean section, endometriosis, and fibroids do not contraindicate it.
  6. Hysterosalpingography assesses tubal patency and uterine cavity morphology by injecting radio-opaque contrast medium through the cervical canal under radiographic screening.
  7. Dual-energy X-ray absorptiometry T-scores compare bone mineral density to that of a healthy young adult of the same gender at peak bone mass.
  8. Dual-energy X-ray absorptiometry Z-scores compare bone mineral density to individuals matched for age, gender, and ethnicity.
  9. In postmenopausal women and men aged 50 or over, a T-score at or above minus 1.0 is normal, and at or below minus 2.5 defines osteoporosis.
  10. In postmenopausal women, a T-score below minus 1.0 and above minus 2.5 defines osteopaenia, whereas Z-scores are used in premenopausal women.
  11. Positron emission tomography uses ionising radiation, whereas magnetic resonance imaging, ultrasound, optical coherence tomography, and thermography are non-ionising imaging modalities.

Common pitfalls

  • Confusing the Gray, which quantifies absorbed physical dose, with the Sievert, which measures biologically weighted equivalent and effective dose.
  • Assuming previous ectopic pregnancy, endometriosis or fibroids contraindicates hysterosalpingography, whereas pregnancy is the contraindication the exam tests.
  • Confusing DEXA T-scores, referenced against young adults at peak bone mass, with Z-scores, which compare to age-matched controls.

Practice questions: X-rays

Single best answer, exactly as the exam asks. Commit to an option before opening the explanation.

Question 1

Biophysics

Hysterosalpingography is contraindicated in which of the following conditions?

  1. APrevious ectopic pregnancy
  2. BPrevious caesarean section
  3. CPregnancy
  4. DEndometriosis
  5. EFibroids
Show answer and explanation

Correct answer: C. Pregnancy

Key takeaway

Hysterosalpingography (HSG) is absolutely contraindicated in pregnancy due to the risk of radiation exposure to the foetus and potential disruption of the pregnancy.

Hysterosalpingography (HSG) is a radiological procedure used to evaluate the uterine cavity and fallopian tube patency, typically in the investigation of infertility.

Pregnancy is an absolute contraindication for HSG. The procedure involves instilling contrast medium into the uterus and taking X-ray images. This exposes the developing foetus to ionising radiation, which carries a risk of teratogenicity and other harmful effects. Furthermore, the procedure itself could disrupt an early pregnancy. Other recognised contraindications are active pelvic infection, recent uterine or tubal surgery and allergy to iodinated contrast medium.

While a previous ectopic pregnancy warrants careful consideration and discussion of risks, it's not an absolute contraindication. Previous caesarean section, endometriosis, and fibroids are not contraindications to HSG; in fact, HSG can be used to assess the uterine cavity and fallopian tubes in the presence of these conditions.

Question 2

Biophysics

Which imaging investigation uses a radio-opaque dye injected into the cervical canal to visualise tubal patency?

  1. AHysterosalpingography
  2. BUltrasound
  3. CMRI pelvis
  4. DCT pelvis
  5. ELaparoscopy
Show answer and explanation

Correct answer: A. Hysterosalpingography

Key takeaway

In hysterosalpingography, a radio-opaque dye is injected into the cervical canal, allowing X-ray visualisation of the uterine cavity and, crucially, assessment of tubal patency.

A hysterosalpingography (HSG) is an X-ray based imaging technique used primarily to assess the patency of the fallopian tubes and the uterine cavity's shape.

Hysterosalpingography involves injecting a radio-opaque contrast medium (dye) through the cervix, allowing visualisation of the uterine cavity and fallopian tubes on a series of X-rays. If the tubes are patent, the dye will spill out into the peritoneal cavity. If there is an obstruction in the tubes, this flow is impeded. It is commonly used to assess for possible mechanical causes in cases of infertility.

Ultrasound can assess tubal patency when an echogenic contrast agent (saline, air–saline or foam) is injected through the cervix, as in hysterosalpingo-contrast-ultrasonography (HyCoSy), which NICE accepts as an alternative to HSG where expertise is available; however, it uses an ultrasound contrast agent rather than a radio-opaque dye and involves no X-rays. MRI pelvis and CT pelvis provide valuable information about pelvic anatomy and pathology but are not routine tests of tubal patency. Laparoscopy, while it can directly visualise the tubes and assess for patency using a dye (chromopertubation), is a surgical procedure and more invasive than a HSG.

8 more x-rays questions in the bank

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What the syllabus covers under x-rays

  • X-ray Production and Detection
  • Use of X-rays in Clinical Practice

Read the full section in the Biophysics guide, or check the next exam dates.