In a patient with an estimated glomerular filtration rate (eGFR) of 47 mL/min/1.73 m², which imaging modality is safer, magnetic resonance angiography (MRA) or computed tomography angiography (CTA)?

MRA is Safer in a Patient with eGFR 47 mL/min/1.73 m²

In a patient with an eGFR of 47 mL/min/1.73 m², magnetic resonance angiography (MRA) without gadolinium-based contrast is safer than computed tomography angiography (CTA) with iodinated contrast, because this level of kidney function (CKD stage 3a) places the patient at risk for both contrast-induced nephropathy from iodinated contrast and nephrogenic systemic fibrosis from gadolinium-based agents—but non-contrast MRA techniques eliminate gadolinium exposure entirely while iodinated contrast carries unavoidable nephrotoxic risk. 1

Understanding the Renal Function Context

• An eGFR of 47 mL/min/1.73 m² represents CKD stage 3a (eGFR 45-59 mL/min/1.73 m²), which is moderate chronic kidney disease. 2
• This level of kidney function does not meet the threshold for severe renal impairment (eGFR <30 mL/min/1.73 m²) where gadolinium-based contrast agents carry the highest risk of nephrogenic systemic fibrosis. 1
• However, this patient is still at increased risk for contrast-related complications compared to those with normal kidney function. 2

Risks of CTA with Iodinated Contrast

• Iodinated contrast used in CTA is directly nephrotoxic and can precipitate acute kidney injury, particularly in patients with pre-existing CKD. 3
• The risk of contrast-induced nephropathy increases progressively as eGFR declines below 60 mL/min/1.73 m². 2
• When contrast imaging is necessary, adequate hydration and the lowest feasible contrast volume should be employed to minimize renal risk. 3
• There is no way to perform CTA without exposing the patient to nephrotoxic iodinated contrast, making this an unavoidable risk. 3

Risks of MRA with Gadolinium-Based Contrast

• Nephrogenic systemic fibrosis (NSF) occurs almost exclusively in patients with severe chronic kidney disease (eGFR <30 mL/min/1.73 m²), making the risk substantially lower at an eGFR of 47 mL/min/1.73 m². 1
• The American College of Cardiology states that gadolinium-based contrast agents are associated with NSF development in patients with severe renal dysfunction (GFR <30 mL/min/1.73 m²). 1
• NSF is a progressive, potentially fatal multiorgan fibrosing disease causing cutaneous sclerosis, subcutaneous edema, disabling joint contractures, and internal organ injury. 1
• Even newer macrocyclic gadolinium agents carry an exceedingly low but non-zero NSF risk (estimated <1%) in patients with eGFR <30 mL/min/1.73 m². 1

The Optimal Solution: Non-Contrast MRA

The safest approach is non-contrast MRA, which completely eliminates exposure to any contrast agent. 1

• Non-contrast MRA techniques have been specifically developed for patients with renal insufficiency, including flow-sensitive dephasing, quiescent-interval single shot, electrocardiogram-gated fresh-blood partial Fourier fast spin echo, balanced steady-state free precession, and arterial spin labeling. 1
• The American College of Radiology recommends that if imaging is absolutely necessary, non-contrast MRA should be considered as the first alternative in patients with severe renal insufficiency. 1
• These non-contrast techniques may have limitations including lower signal-to-noise ratio, limited spatial resolution, motion artifacts, long acquisition times, and unreliable visualization of lesions with high flow and turbulence. 1

When Contrast-Enhanced MRA Is Unavoidable

If non-contrast MRA is technically inadequate and contrast is absolutely necessary:

• Use macrocyclic gadolinium-based contrast agents (group II agents) at the lowest effective dose, as these have the lowest NSF risk profile. 1
• The ACR-National Kidney Foundation 2021 consensus states that when gadolinium must be used in patients with reduced kidney function, macrocyclic agents should be preferred. 1
• Obtain explicit informed consent discussing the residual NSF risk, even though it is low at this eGFR level. 1
• Do not initiate dialysis solely for the purpose of removing gadolinium, as no form of dialysis is considered prophylactic for NSF and the reduction in NSF risk from dialysis is only theoretical. 1

Clinical Decision Algorithm

1. First choice: Non-contrast MRA techniques to completely avoid contrast exposure 1
2. Second choice: If contrast is absolutely required and non-contrast MRA is inadequate, use gadolinium-based MRA with macrocyclic agents at the lowest dose 1
3. Avoid: CTA with iodinated contrast due to unavoidable direct nephrotoxic effects in a patient with already compromised kidney function 3

Common Pitfalls to Avoid

• Do not reflexively order contrast studies without first assessing renal function and the clinical necessity of contrast enhancement. 1
• Do not assume that gadolinium is "safe enough" at eGFR 47—while NSF risk is low, non-contrast alternatives should always be explored first. 1
• Do not use iodinated contrast thinking it is safer than gadolinium at this eGFR level—iodinated contrast carries direct nephrotoxic risk at all levels of renal impairment. 3
• Do not schedule prophylactic dialysis after gadolinium administration, as this has never been shown to prevent NSF in randomized controlled trials. 1