Spectral CT: Less contrast, less radiation, more information

A doctor’s job is not simply to obtain an answer. It is to obtain the best possible answer while causing the least possible harm.

In computer tomography (CT), that principle should apply to both exposures we control: ionising radiation and iodinated contrast.

The goal is not the lowest number for its own sake. It is the lowest radiation and iodine exposure that still allows us to answer the clinical question confidently.

We are becoming better at thinking about radiation. Terms such as low dose and ultra-low dose are now commonplace. But these remain relative descriptions, and the actual radiation exposure is still rarely stated clearly on the diagnostic report.

We have progressed even less with contrast. The contrast agent may be documented within the imaging record, but the volume of iodine administered is often invisible to the patient and referring doctor.

We should do better.

Less iodine does not have to mean less information

The nephrotoxicity of modern intravenous iodinated contrast has probably been overstated historically, partly because contrast-associated acute kidney injury was often conflated with kidney injury actually caused by contrast. But unnecessary iodine exposure remains relevant.

Approximately 2.7 million Australian adults have biomedical evidence of chronic kidney disease, and the number with moderate-to-severe loss of kidney function has increased by more than 60% since 2011–12. Many do not know they have it.

If we can answer the same diagnostic question using substantially less iodine, why would we routinely administer more than we need?

Spectral CT changes what is possible.

Low-energy virtual monoenergetic reconstructions increase iodine conspicuity, while iodine maps and material decomposition can provide additional information from the same acquisition. Less iodine does not necessarily mean less diagnostic information.

CT pulmonary angiography (CTPA) provides a useful example.

A recent multidisciplinary consensus statement describes routine CTPA protocols using approximately 80–100 mL of iodinated contrast.

In a randomised controlled trial, Ferrández-Ferrández and colleagues compared 40, 30 and 20 mL protocols using dual-layer spectral CT, demonstrating diagnostic pulmonary arterial enhancement with substantially reduced contrast volumes. In a comparative study, Li and colleagues subsequently compared spectral CTPA using only 10 mL with conventional CTPA using 30 mL. The spectral protocol reduced iodine intake by 66.7% while maintaining predefined diagnostic image quality, with iodine maps providing additional perfusion information.     

At our Gold Coast practice, our routine CTPA protocol for appropriately selected adults uses 30 mL of contrast with detector-based Philips iQon spectral CT, a 62.5–70% reduction compared with an 80–100 mL reference protocol. We have obtained diagnostically useful pulmonary angiographic imaging in selected patients with as little as 10 mL.

But the achievement is not the number 30.

Giving less contrast and accepting poorer imaging is not optimisation. The standard should be: use the least contrast required to answer the clinical question confidently.

Radiation should be treated the same way

We already think this way about radiation, at least conceptually.

ARPANSA now provides an objective diagnostic reference level for Australia’s lung cancer screening program: CTDIvol 3 mGy and DLP 90 mGy·cm.     

At our practice, a low-dose spectral chest CT is approximately 0.7 mSv in a standard-sized adult while retaining spectral data. Where appropriate, our ultra-low-dose non-contrast chest CT can be performed at less than 0.2 mSv (the equivalent to approximately one month of natural background radiation)     .     

Again, the smallest number does not automatically represent the best examination. A 0.2 mSv CT is not better than a 0.7 mSv CT if it fails to answer the question.

The aim is the best answer for the least necessary exposure.

The problem is that patients and their treating referrers rarely see the numbers

This is where radiology should be held to a higher standard.

Food comes with nutritional information. Pharmaceuticals are prescribed and documented by dose.

Yet we can administer ionising radiation and intravenous iodine as part of a medical procedure without clearly stating either exposure on the report.

That is particularly difficult to justify because we already measure them.

The scanner records CTDIvol and DLP. We know the contrast agent, its concentration and exactly how much was administered.

So put it on the report.

Every CT report should routinely state:

• contrast agent, concentration and volume; 
• CTDIvol; and 
• DLP. 

An estimated effective dose in mSv can also be useful where appropriate, provided it is identified as an estimate rather than an individually measured biological dose.

This is not about creating a competition for the lowest number. Sometimes more radiation or contrast is necessary, and if it is necessary to confidently answer the clinical question, it is justified.

But the exposure should be measurable, visible and defensible.

Without those numbers, terms such as low dose, ultra-low dose and low contrast risk becoming marketing labels rather than meaningful measures of optimisation. Two providers can use the same terminology while delivering very different exposures, with little ability for patients or referring doctors to compare them.

Patients should not have to take “low dose” on trust.

Advanced CT should ultimately be judged by how much reliable diagnostic information we obtain for the biological exposure we ask the patient to accept.

We measure these exposures already. The next step is simple: Measure it. Minimise it. Report it.

Dr Zane Sherif is a specialist radiologist, Fellow of the Royal Australian and New Zealand College of Radiologists (RANZCR), and co-founder of MBR Health and Whole Body MRI. With specialised training in emergency medicine and spinal orthopaedics, his clinical and research interests span advanced diagnostic imaging, whole-body MRI, spinal imaging, preventative health and regenerative medicine. A published researcher and educator, Dr Sherif has presented at scientific symposiums in Australia and internationally and is actively involved in research exploring emerging applications of advanced imaging technologies.